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+Project Gutenberg (https://www.gutenberg.org) public repository for
+eBook #65114 (https://www.gutenberg.org/ebooks/65114)
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-The Project Gutenberg eBook of The Wonders of Optics, by Fulgence Marion
-
-This eBook is for the use of anyone anywhere in the United States and
-most other parts of the world at no cost and with almost no restrictions
-whatsoever. You may copy it, give it away or re-use it under the terms
-of the Project Gutenberg License included with this eBook or online at
-www.gutenberg.org. If you are not located in the United States, you
-will have to check the laws of the country where you are located before
-using this eBook.
-
-Title: The Wonders of Optics
-
-Author: Fulgence Marion
-
-Translator: Charles W.  Quin
-
-Release Date: April 19, 2021 [eBook #65114]
-
-Language: English
-
-Character set encoding: UTF-8
-
-Produced by: deaurider, Barry Abrahamsen, and the Online Distributed
-             Proofreading Team at https://www.pgdp.net (This file was
-             produced from images generously made available by The Internet
-             Archive)
-
-*** START OF THE PROJECT GUTENBERG EBOOK THE WONDERS OF OPTICS ***
-
-
-
-
-                         THE WONDERS OF OPTICS.
-
-
-
-
-
-------------------------------------------------------------------------
-
-
-[Illustration:
-
-  Spectrum showing the absorptive power of Sodium vapour (Fig. 6).
-]
-
-[Illustration:
-
-  Solar Spectrum (Fig. 5).
-]
-
-[Illustration:
-
-  Action of a prism on a ray of light (Fig. 7).
-  Eng.^d by A. Robin N.Y.
-]
-
-
-------------------------------------------------------------------------
-
-
-                                  THE
-
-                           WONDERS OF OPTICS.
-
-
-
-
-                                   BY
-
-                               F. MARION.
-
-
-
-               TRANSLATED FROM THE FRENCH, AND EDITED BY
-
-                        CHARLES W. QUIN, F.C.S.
-
-
-
-
-      ILLUSTRATED WITH SEVENTY ENGRAVINGS ON WOOD, AND A COLOURED
-                             FRONTISPIECE.
-
-
-
-
-                             --------------
-
-
-
-                               NEW YORK:
-                        CHARLES SCRIBNER’S SONS,
-                             SUCCESSORS TO
-                       SCRIBNER, ARMSTRONG, & CO.
-
-
-------------------------------------------------------------------------
-
-
-
-
-                                PREFACE.
-
-                             --------------
-
-
-THE present work needs but little introduction to the English public.
-The author, M. F. Marion, who holds a high official scientific position
-in Paris, is well known, especially in Europe, as a popular writer on
-the “Wonders of Optics,” and kindred subjects. As a rule, the original
-text has been strictly adhered to by the Translator, but in a few
-instances certain anecdotes of a local character have been altered so as
-to be more generally applicable, or condensed to make room for the
-chapter on the Spectroscope, which is entirely original.
-
-
-------------------------------------------------------------------------
-
-
-
-
-                               CONTENTS.
-
-                             --------------
-
-
-                                PART I.
-
-                        THE PHENOMENA OF VISION.
-
-
-                               CHAPTER I.
-
-                                                       PAGE
-            THE EYE                                      15
-
-
-                              CHAPTER II.
-
-            THE STRUCTURE OF THE EYE                     22
-
-
-                              CHAPTER III.
-
-            THE ERRORS OF THE EYE                        30
-
-
-                              CHAPTER IV.
-
-            OPTICAL ILLUSIONS                            36
-
-
-                               CHAPTER V.
-
-            THE APPRECIATION OF COLOUR                   44
-
-
-                              CHAPTER VI.
-
-            ILLUSIONS CAUSED BY LIGHT ITSELF             53
-
-
-                              CHAPTER VII.
-
-            THE INFLUENCE OF THE IMAGINATION             60
-
-
-                             --------------
-
-
-                                PART II.
-
-                           THE LAWS OF LIGHT.
-
-
-                               CHAPTER I.
-
-            WHAT IS LIGHT?                               73
-
-
-                              CHAPTER II.
-
-            THE SOLAR SPECTRUM                           84
-
-
-                              CHAPTER III.
-
-            OTHER CAUSES OF COLOUR                       94
-
-
-                              CHAPTER IV.
-
-            LUMINOUS, CALORIFIC, CHEMICAL, AND          100
-              MAGNETIC PROPERTIES OF THE SPECTRUM
-
-
-                               CHAPTER V.
-
-            THE LAWS OF REFLECTION.—MIRRORS             106
-
-
-                              CHAPTER VI.
-
-            METALLIC BURNING MIRRORS                    117
-
-
-                              CHAPTER VII.
-
-            LENSES                                      127
-
-
-                             CHAPTER VIII.
-
-            OPTICAL INSTRUMENTS.—THE SIMPLE AND         141
-              COMPOUND MICROSCOPE. THE SOLAR AND
-              PHOTO-ELECTRIC MICROSCOPE
-
-
-                              CHAPTER IX.
-
-            THE TELESCOPES OF GALILEO, GREGORY,         150
-              NEWTON, HERSCHEL, LORD ROSSE, AND
-              FOUCAULT
-
-
-                             --------------
-
-
-                               PART III.
-
-                             NATURAL MAGIC.
-
-
-                               CHAPTER I.
-
-            THE MAGIC LANTERN                           173
-
-
-                              CHAPTER II.
-
-            THE PHANTASMAGORIA                          183
-
-
-                              CHAPTER III.
-
-            OTHER OPTICAL ILLUSIONS                     196
-
-
-                              CHAPTER IV.
-
-            THE PROPERTIES OF MIRRORS                   216
-
-
-                               CHAPTER V.
-
-            CHINESE SHADOWS                             223
-
-
-                              CHAPTER VI.
-
-            POLYORAMA—DISSOLVING VIEWS—DIORAMA          231
-
-
-                              CHAPTER VII.
-
-            THE STEREOSCOPE                             236
-
-
-                             CHAPTER VIII.
-
-            THE CAMERA OBSCURA AND CAMERA LUCIDA        242
-
-
-                              CHAPTER IX.
-
-            THE SPECTROSCOPE                            249
-
-
-                               CHAPTER X.
-
-            SPECTRES—THE GHOST ILLUSION                 264
-
-
-------------------------------------------------------------------------
-
-
-
-
-                         LIST OF ILLUSTRATIONS.
-
-                             --------------
-
-
-          FIG.                                            PAGE
-
-            1. Section of the Eye                           24
-
-            2. A Camera Obscura                             27
-
-            3. The Phenakistiscope                          54
-
-            4. Disc of the Phenakistiscope                  55
-
-            5. Solar Spectrum                           Frontispiece
-
-            6. Absorption of Light by Sodium Vapour      _ib._
-
-            7. Action of a Prism on the Solar Rays       _ib._
-
-            8. The Recomposition of Light                   86
-
-            9. Recomposition of Light by means of a         87
-                 Concave Mirror
-
-           10. Recomposition of Light by means of a         88
-                 number of Mirrors
-
-           11. Newton’s Disc                                89
-
-           12. Newton’s Rings                               95
-
-           13. Reflection from Plane Surfaces              107
-
-           14. Refraction                                  108
-
-           15. Experimental Proof of Refraction          _ib._
-
-           16. The Effects of Plane Mirrors                109
-
-           17. Reflection from the Surface of Water        110
-
-           18. Concave Mirror                              111
-
-           19. Conjugate Foci                              113
-
-           20. Virtual Focus                               114
-
-           21. Concave Mirror                            _ib._
-
-           22. Magnifying Effect of Concave Mirrors        115
-
-           23. The Reversal of real Images               _ib._
-
-           24. Diminishing Power of Convex Mirrors         116
-
-           25. Burning Mirror                              124
-
-           26. Double Convex Lens                          127
-
-           27. Forms of Lenses                             128
-
-           28. Path of a Ray through a Convex Lens         129
-
-           29. Path of Divergent Rays through a Convex   _ib._
-                 Lens
-
-           30. Conjugate Foci                              130
-
-           31. Images formed by Convex Lenses              131
-
-           32. Magnifying Property of Convex Lenses        132
-
-           33. Diminishing Effect of Concave Lenses      _ib._
-
-           34. Cannon of the Palais Royal                  134
-
-           35. Fresnel’s Lighthouse Apparatus              136
-
-           36. Lantern of a First-Class Lighthouse         140
-
-           37. The Compound Microscope                     143
-
-           38. The Theory of the Compound Microscope       144
-
-           39. Photo-Electric Microscope                   147
-
-           40. Solar Microscope                            148
-
-           41. The Galilean Telescope                      155
-
-           42. The Astronomical Telescope                  156
-
-           43. Section of an Astronomical Telescope        157
-
-           44. Section of the Gregorian Telescope          160
-
-           45. Gregorian Telescope                         161
-
-           46. Section of a Newtonian Telescope            162
-
-           47. Herschellian Telescope                      164
-
-           48. Foucault’s Large Telescope                  169
-
-           49. Foucault’s Small Telescope                  171
-
-           50. Section of the Magic Lantern                179
-
-           51. Magic Lantern                               182
-
-           52. The Phantasmagoria                          184
-
-           53. The Phantascope                             185
-
-           54. Phantasmagoria (ROBERTSON)                  194
-
-           55. Wizard Dance                                198
-
-           56. Nostradamus and Marie de Médicis            201
-
-           57. The Arrangement of the Reversing Prism      203
-
-           58. The Goat Trick                              205
-
-           59. How to see through a Brick                  207
-
-           60. The Polemoscope                             210
-
-           61. Protection against ill-natured People       213
-
-           62.                                             218
-
-           63. Anamorphosis                                220
-
-           64. Effect of Cut Paper-work                    225
-
-           65. Seditious Toys                              229
-
-           66. Diorama                                     234
-
-           67.                                             237
-
-           68. Stereoscope                                 238
-
-           69. The Principle of the Refracting             239
-                 Stereoscope
-
-           70. The Camera Obscura                          243
-
-           71. Section of Camera Lucida                    247
-
-           72. The Spectre—an Optical Illusion             269
-
-           73. How to produce Spectres                     271
-
-
-------------------------------------------------------------------------
-
-
-                         THE WONDERS OF OPTICS.
-
-
-                             --------------
-
-                                PART I.
-
-                        THE PHENOMENA OF VISION.
-
-                             --------------
-
-
-
-
-                               CHAPTER I.
-                                THE EYE.
-
-
-THE Eye is at once the most wonderful and the most useful of all our
-organs of sense. It is especially by means of the eye that we gain a
-knowledge of the exterior world. Our other senses are far more limited
-in their action: thus the sense of touch only extends to objects within
-our reach; the sense of taste is only a delicate and exquisite
-modification of the sense of touch; the sense of smell can only be
-exercised on substances that are close to us; and the use of our ears is
-limited by the distance at which the loudest sound ceases to impress
-them. But the eye has the privilege of extending its dominion, whether
-for mere enjoyment or for serious instruction, far beyond the limits of
-this little world. Not only is it the origin of all our ideas upon every
-object that comes within its ken; not only does it reveal to us our own
-position and that of our surroundings; but, thanks to the discoveries of
-modern science, it is able to admire, on the one hand, a world of
-infinite minuteness that remained unknown to us for centuries, and, on
-the other, the immeasurable immensity of the starry universe.
-
-Admirable as the eye undoubtedly is through the possession of the power
-of vision, it is also capable of enchanting us by its own particular
-beauties. Not to speak of its internal mechanism, which we shall
-consider very fully by and by, let us for a moment examine its outward
-appearance. Have you never, dear reader, been enchanted with a pair of
-soft and gentle eyes, or with a couple of black orbs veiled with long
-dark lashes, or with those wondrous eyes that rival the heavens in
-colour and depth, shedding on you rays of light whose mute eloquence was
-irresistible? If it be true that man’s face is the canvas upon which the
-affections and desires of his mind are depicted as soon as they are
-formed, the eyes are unquestionably the central point of the picture,
-and it is in them, as in a looking-glass, that every sentiment that
-passes across our brain is reflected.
-
-When the mind is undisturbed, says Buffon, all the parts of the face are
-in a state of repose; their proportion, unity, and general appearance
-indicate the pleasing harmony of our thoughts and the perfect calmness
-of our mind; but when we are agitated, the human face becomes a living
-picture, in which the passions that disturb us are depicted with equal
-force and delicacy, a picture in which every emotion is expressed by a
-stroke, every action by a letter, so to speak; in which the quickness of
-the impression outstrips the will, and reveals by the most sympathetic
-signs the image of our secret trouble.
-
-It is more especially in the eyes, adds this great naturalist, that
-these signs are manifested and recognised. The eye is connected with the
-mind more than any other organ: it seems almost to be in contact with it
-and to participate in all its movements; it expresses in obedience to it
-the strongest passions and the most tumultuous emotions, as well as the
-gentlest thoughts and most delicate sentiments, and reproduces them in
-all their force and purity just as they have sprung into existence; it
-transmits them with exquisite rapidity even to the minds of others,
-where they once more become impressed with all their original fire,
-movement, and reality. The eye both receives and reflects the light of
-thought and the warmth of sentiment, and is at once the sense of the
-mind and the tongue of the intellect. Persons who are short-sighted, or
-who squint, have much less of this external intelligence that dwells in
-the eye. It is only the stronger passions that can bring the other
-features of the face into play, that are depicted on their physiognomy;
-and the effects of fine thought and delicate feeling are rendered
-apparent with much greater difficulty.
-
-The elegant author of _L’Histoire Naturelle_ rightly thinks that we are
-so accustomed only to see things from the outside, that we are hardly
-aware how much this exterior view of everything influences the judgment
-of even the gravest and most thoughtful of us. Thus we are apt to set
-down a man as unintellectual whose physiognomy does not particularly
-strike us; and we allow his clothes, and even the manner in which he
-wears his hair, to influence our judgment of him. Hence, our author goes
-on to say, not wholly without some show of reason, that a man of sense
-ought to look upon his clothes as part of himself, because they really
-are so in the eyes of others, and play an important part in the general
-idea that is formed of him who wears them.
-
-The vivacity or languor of the movement of the eyes forms one of the
-chief characteristics of facial expression, and their colour helps to
-render this characteristic more striking. The different colours seen in
-the eye are dark hazel, or black, as it is generally called, light
-hazel, blue, greenish grey, dark grey, and light grey. The velvety
-substance which gives the colour to the iris is arranged in little
-ramifications and specks, the former being directed towards the centre
-of the eye, the latter filling up the gaps between the threads.
-Sometimes they are both arranged in so regular a manner that instances
-have been known in which the irises of different eyes have appeared to
-be so much alike that they seemed to have been copied from the same
-design. These little threads and specks are held together by a very fine
-network.
-
-The commonest colours seen in the eye are hazel and blue, and it mostly
-happens that both these colours are found in the same individual, giving
-rise to that peculiar greenish-grey hue that is far from being uncommon.
-Buffon thinks that blue and black eyes are the most beautiful, but this
-of course is a matter of taste. It is true that the vivacity and fire
-which play so important a part in giving character to the eye, are more
-perceptible in dark eyes than in those whose tints are lighter; black
-eyes, therefore, have greater force of expression, while in blue eyes
-there is more softness and delicacy. In the former we see a brilliant
-fire, which sparkles uniformly on account of the iris, which is of the
-same colour throughout, giving in all parts the same reflection; but a
-great difference may be perceived in the intensity of the light
-reflected from blue eyes, from the fact of the various tints of colour
-producing different reflections. There are some eyes that are remarkable
-for being almost destitute of colour, and appear to be constituted in an
-abnormal manner. The iris is tinted with shades of blue and grey of so
-light a hue that it appears quite white in some places. The shades of
-hazel in such eyes are so light that they are hardly distinguishable
-from grey and white, in spite even of the contrast of colour.
-
-For our part, we think that the beauty of the eye consists not so much
-in its colour, or even in its harmony with the rest of the face, but in
-its expression.
-
-There are also numerous instances of green eyes. This colour is, of
-course, much less frequent than blue, grey, or hazel. It often happens,
-too, that the two eyes vary in colour in the same individual. This
-defect is not confined to the human species, being shared by the horse
-and the cat. In most other animals the colour of the two eyes is always
-similar. The colour of the eye in most animals is either hazel or grey.
-Aristotle imagined that grey eyes were stronger than blue, that those
-persons whose eyes are prominent cannot see so far as others, and that
-brown eyes are less valuable in the dark than those of another tint; but
-modern investigations have failed to bear out the ancient philosopher’s
-ideas with regard to the human eye.
-
-Although the eye appears to move about in every direction, it has in
-reality only one movement, that of rotation round its centre, by means
-of which the eyeball rises or falls, or passes from side to side at
-will. In man the eyes are parallel with each other in relation to their
-axes; he can consequently direct them at pleasure upon the same object:
-but in most animals this parallelism is wanting. In some cases the eyes
-of animals are set almost back to back, rendering it impossible for them
-to see the same object with both eyes at once.
-
-Buffon makes the remark, that after the eyes, the eyebrows contribute
-more strongly than any other part of the face towards giving character
-to the physiognomy, being, inasmuch as they differ in their nature from
-the other features, more apparent by contrast, and hence strike us more
-than any other portion of the countenance. They are, in fact, a shadow
-in the picture, bringing its colour and drawing into strong relief. The
-eyelashes also contribute their effect; when they are long and thick,
-they overshadow the eye, making its glance appear softer and more
-beautiful. The ape is the only other animal besides man that possesses
-two eyelashes, the rest having them only on the upper eyelid. Even in
-man they are more abundant in the upper eyelid than in the lower. The
-eyebrows have but two movements, upward and downward, governed by the
-muscles of the forehead. In the action of frowning we not only lower
-them, but move them slightly towards each other. The eyelids serve to
-protect the eyeball, and keep the cornea from becoming dry. The upper
-eyelid has the power of raising and lowering itself, the lower one being
-almost destitute of movement. Although the motion of the eyelids is an
-effort of will, there are times when it is impossible to keep them open,
-as for instance when we are overpowered by sleep, or when the eyes are
-suddenly subjected to the effects of strong light. The eyelid is a most
-admirable arrangement for the protection of the eye, and it is almost
-impossible to admire this provision of nature too much, even when we
-confine ourselves to an outward examination of it. It is not merely the
-outward mechanism and motion of the eyelids, nor the colour of the eyes,
-that constitutes their beauty; we have already said that the leading
-characteristic of the eye was _expression_. It is this expression which
-causes the eye to appear to speak, to fire up suddenly, to sparkle with
-flashes of light, to languish or conceal itself underneath its lashes,
-to raise itself with inspiration, or to pierce the abyss of thought,
-just according to the particular sentiment governing the mind at the
-moment. Hence it is expression that constitutes the true beauty of the
-eye: every one knows instances of eyes which, while at rest, would never
-be noticed by anybody, but which, when once animated by intense
-eloquence, lend to the voice of their possessor an unexpected power,
-which moves and transports the listener to an extent infinitely beyond
-that resulting from the simple spoken words.
-
-Enough, however, has been said upon the external aspect of the human
-eye; we will, therefore, at once endeavour to penetrate the circle in
-which are contained the wonders that this little book is intended to
-describe. The object of these lines is not so much to describe the
-beauty of man’s glances, nor the value of his senses, but rather to make
-known those illusions to which the most sagacious of all his senses is
-apt to fall a prey. But before entering the temple it was but right to
-have bestowed a little admiration upon the façade. By the way, as we are
-about to describe many illusory wonders, do not let us commence by
-deceiving ourselves with regard to our first marvel—the eye itself. A
-great philosopher calls the eyes the windows of the soul, and, although
-meant as a poetical image, the saying is not far from the truth; for the
-optic nerve by which we see external objects, is an extension of the
-nerves of the brain, whose functions and actions are an unfathomable
-mystery.
-
-
-------------------------------------------------------------------------
-
-
-
-
-                              CHAPTER II.
-                       THE STRUCTURE OF THE EYE.
-
-
-OF all the senses, says an ardent admirer of nature, the sight is
-certainly that which furnishes the mind with the quickest and most
-widely-extended perceptions. It is the source of the richest treasures
-of the imagination, and of our ideas of the beauty, order, and unity of
-the world around us. How unhappy are those whom a hard fate has deprived
-of the sense of sight from their birth! Alas! the finest day and the
-darkest night differ in nothing as far as they are concerned; the light
-of heaven never brings joy into their hearts. The enamelled beauties of
-a bed of flowers, the varied plumage of the peacock, the glories of the
-rainbow are alike unknown to them. They cannot contemplate from the
-mountain height the beauties of the valley beneath; the fields golden
-with the harvest, the meadows smiling with verdure, and watered by
-winding rivers, and the habitations of man dotted about here and there
-over the surface of this magnificent picture. To them is unknown the
-sight of the mighty ocean; and the innumerable legions of the cloud army
-of Heaven are to them as if they did not exist. The impenetrable
-obscurity which surrounds them allows them neither the contemplation of
-what is grandest in man’s outward aspect, nor even the admiration of
-those qualities which they themselves would hold most dear.
-
-A strong sentiment of pity should, therefore, animate the breast of
-every right-thinking man, when he considers the unhappy condition of
-those who are born blind.
-
-The eye infinitely surpasses in its complexity and beauty of structure
-all the other organs of sense, and is most unquestionably the most
-marvellous object that the human mind is capable of examining and
-understanding. Let us first examine the external parts of this wonderful
-organ. With what a singular system of entrenchments and defences do we
-find the eye provided! It is itself placed in the head at a certain
-depth, and surrounded on all sides by solid bone, so that it is only
-with the greatest difficulty that it is hurt by accident from without.
-The eyebrows also play their part as protection to the eye, and prevent
-the perspiration from entering and irritating the organ. The eyelids too
-are always ready to rush to the rescue, whether to protect the eye from
-outward attacks, or to shade it from too strong a light during sleep.
-The eyelashes not only add to the beauty of the eye, but they shade it
-from the too brilliant light of the sun, and act as advanced guards to
-prevent the entrance of dust or any other foreign body with which the
-eyes might be injured.
-
-But its internal structure is still more admirable. The globe of the eye
-is almost spherical and measures nearly one inch in diameter. Fig. 1 is
-a view of the eyeball, showing the details of its structure; the various
-membranes surrounding it have been cut away in order that it may be
-better examined. If we commence our examination by the exterior portion
-of the front, we shall first find immediately beneath the eyelashes a
-perfectly transparent membrane (C), called the _cornea_. It is a
-prolongation of the hard opaque external coating of the eye, called the
-_sclerotic membrane_, and marked S in the figure. The cornea is
-sufficiently hard in its nature to present a strong resistance to any
-violence from without.
-
-Immediately beneath the cornea and in contact with it is the _aqueous
-humour_, a thin transparent liquid occupying a small portion of the
-front of the eye.
-
-Next comes the _iris_, a circular disc perforated with a round hole in
-the middle, and coloured with various shades of blue, brown, and grey.
-
-[Illustration:
-
-  Fig. 1.—Section of the Eye.
-]
-
-The opening in the centre, which appears like a black spot when the eye
-is examined, is not really an object, but simply an aperture, capable of
-changing its size according to the quantity of light striking the eye.
-This change of size in the opening, or pupil, as it is popularly called,
-is effected by the contraction or expansion of the iris, which thus
-possesses the peculiar property of exactly proportioning the amount of
-light that enters the eye, so that there is never too much or too
-little. It is through the pupil that the rays of light proceeding from
-the various objects around us pass into the interior of the eye, and
-form an image upon the retina, as will be afterwards explained.
-
-Immediately behind the pupil is O, a bi-convex lens to transmit the rays
-of light to the retina. It is generally called the _crystalline lens_.
-
-From the crystalline lens to the back of the eyeball, is a space more or
-less globular in form, containing a gelatinous diaphanous mass somewhat
-resembling white of egg in appearance, and called the _vitreous humour_.
-
-Behind the vitreous humour, and immediately opposite the pupil and lens,
-is the most delicate and important of all the membranes of the eye, the
-_retina_, which serves as a screen whereon are received the images of
-the objects around us. This membrane is an expansion of the optic nerve
-N leading from the brain, and lines the whole of the interior of the
-eye. The eye is also enveloped in a second membrane (C), called the
-_choroid_, which is impregnated with a black pigment. Round this is
-wrapped a third membrane, the _sclerotic_ (S), which unites with the
-cornea in front of the eyeball.
-
-The crystalline lens through which all the rays pass before they reach
-the retina, possesses the marvellous power of being able to modify its
-curvature in such a manner as to adapt itself to the distance of the
-object seen, and thus throw a distinct image on the retina. When we come
-to talk of the properties of lenses, we shall see that the focus of a
-lens differs for objects at different distances; if, therefore, the eye
-were not provided with some such means for altering the focus of the
-crystalline lens, we should only see objects distinctly at one
-particular point. The crystalline lens consists of infinite numbers of
-extremely thin transparent little plates, each of which is in itself
-composed of fine fibres so united together as to be capable of a small
-degree of compression or extension. Hence the power of the lens to alter
-its form according to circumstances. It is calculated that the human eye
-contains over five millions of the laminæ above referred to. With such
-wonders is the world of nature replete,—wonders that we daily and hourly
-pass by without examination.
-
-It is by means of this ingenious and inimitable structure of the eye
-that external objects pass from the domain of the material world into
-that of the mind, and become accessible to every faculty of our brain.
-Of its own accord, and without apparently any effort of our own will,
-does this marvellous mechanism adapt itself to all the variations of
-distance and intensity of light, a power possessed by no instrument as
-yet constructed by the hand of man—being capable, as it is, of
-distinguishing instantaneously between the distance of the remotest
-nebulæ and that of the letters forming this page. This wonderful organ,
-writes Brewster, may be considered as being the sentinel that guards the
-passage between the world of matter and that of mind, and as the medium
-through which they interchange all their communications. The optic nerve
-perceives the objects written on the retina by the hand of nature, and
-conveys them to the brain in all their integrity of form and colour.
-
-The path of the rays of light and the formation of images upon the
-retina are shown in the preceding figure. At first sight it will be
-perceived that the objects thereon depicted are in a reversed position,
-that is to say, when we look at a view similar to that shown in fig. 2,
-we should find, if we had any means of observing the positions of
-objects reflected on our retina, that the flock of sheep coming up the
-road were at the top of the eye, while the trees, the roof of the house,
-and the chimney were in the contrary position. Similar reversed images
-may be seen in dark rooms, by holding a screen before any little crack
-or pinhole in the door or shutter of the room. In fig. 2 the keyhole of
-the door is represented as playing the part of a lens. The author, in
-common with almost every other boy, observed this fact at a very early
-age, and the idea immediately struck him that it would be only necessary
-to fix these images to procure exact representations of natural scenery;
-but in making inquiries into the subject, he found that his juvenile
-observations had been made a little too late, photography having already
-gained the end he intended striving for.
-
-[Illustration:
-
-  FIG. 2.—A Camera Obscura.
-]
-
-Seeing that the images of all objects appear on our retina upside down,
-the student is naturally disposed to ask how it happens that we do not
-see them in that position. Physiologists and natural philosophers have
-advanced numerous theories on the subject. Some, with Buffon, admit at
-once that it is by habit and education of the eye that we see objects
-unreversed. Others, like the great physiologist Müller, imagine that as
-we see everything upside down, and not a single object only, we have no
-points of comparison, and practically ignore the reversal. The truth,
-however, appears to be that it is the brain, and not the eye, that
-possesses the power of determining the real position of what we see.
-That the eye alone has no power of determining the positions of objects
-by itself, may be easily proved by showing a person an astronomical
-object, such as the moon through a telescope. Unless the observer has
-been already familiarized with the appearance of our satellite, he will
-not know whether the image he sees is reversed or not. It is the brain,
-therefore, and the brain only, that has the power of determining the
-position of objects around us, without taking into consideration the
-reversed picture of them that is depicted on our retina. The student who
-takes an interest in the structure of this important organ, would do
-well to procure a sheep’s or bullock’s eye from the butchers, and
-dissect it carefully with a sharp penknife and pair of scissors. The
-image formed on the retina may be easily seen by cutting away the
-sclerotic and choroid coatings at the back of the eye.
-
-The ordinary distance of distinct vision for small objects, such as the
-letters of a book, is from ten to twelve inches. But possibly there do
-not exist two pairs of eyes in the world whose foci are the same. Even
-in the same individual it frequently happens that the focal length of
-the eyes differs considerably. In some persons the focus of the eye is
-so reduced that they are obliged to bring the object they are examining
-within six, and even four inches of their eyes, before they can see it.
-This defect is known ordinarily as _short sight_, and results from the
-too great convexity of the cornea and crystalline lens. It is corrected
-by wearing spectacles with concave glasses. Others again, on the
-contrary, place the book or object they are looking at, at a greater
-distance from the eye than that named. Such people are called
-long-sighted, and the defect results from the too great flatness of the
-cornea and the crystalline lens. The fault is of course corrected by the
-use of spectacles containing convex lenses.
-
-Long-sightedness is generally the result of old age, and it may be taken
-as a fact that the older we grow the flatter becomes the crystalline
-lens. Hence short-sighted people have been known to recover their sight
-perfectly as they advance in years through the natural process of the
-flattening of the crystalline lens. These matters, however, will be more
-fully treated of when we begin to speak of the properties of lenses of
-different forms and curvatures.
-
-
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-
-
-
-
-                              CHAPTER III.
-                         THE ERRORS OF THE EYE.
-
-
-IT is with our own organization that we shall commence our task of
-exposing the illusions that we shall meet with during our optical
-experiments,—in fact with that wonderful and important organ of our body
-that we are apt to look upon as sure and infallible, but which we shall
-find is deceiving us constantly, and hourly proving the fallacy of the
-popular saying, that “every one must believe his own eyes.” In ancient
-times there existed a school of sceptics who doubted everything
-beginning with Pyrrho, the great theorist, and ending with the follower
-of his school who doubted the existence of muscular force even after he
-had received a sound box on the ear from an opponent of his system of
-philosophy. If any of our readers were to become followers of Pyrrho,
-they might easily do so when considering the numberless illusions we
-shall describe to them, if they did not remember that if our senses are
-subject to error, we have a brain to set them right: our mind, if
-logical and well regulated, soon discovers errors of observation, and
-speedily places our judgment on the most solid basis. We shall find
-endless instances of this throughout our little book. If we are dazzled
-with illusions from time to time we shall as often recover ourselves;
-and no matter how beautiful or interesting these deceptions may appear,
-we shall speedily be able to convince ourselves that they are unreal. In
-this chapter we shall only speak of those errors of the eye of which we
-have actually lost all cognizance, so effectually has our judgment
-succeeded in counteracting their influence.
-
-We all know that the first thing a child does with its eyes, even when
-it is only five or six weeks old, is to turn them towards the most
-brilliant object within its reach. Instinctively and without being aware
-of it, the child’s eye seems to seek the light. The whole of nature,
-from the lowest plant to the baby in the cradle, appears more or less
-endowed with this instinct of turning towards the light.
-
-From the time that children begin to distinguish objects, their eyes are
-liable to be affected by two causes of error. Before being able to judge
-of the position of things surrounding them, they see everything upside
-down; they consequently acquire a false impression of the position of
-objects. The next cause of error that is likely to mislead them is the
-fact of their seeing everything double, a separate image of everything
-being formed on each eye; and it can only be by the experience gained
-through the sense of touch that they can acquire the knowledge necessary
-to rectify these errors, and see those objects single which appear to
-them double. This error of sight, as well as the first one, is set right
-so easily in the end, that although in reality we see everything double
-and upside down, we imagine that we see them single, and in their proper
-positions, a state of things brought about entirely through another
-sense exercising its power over our judgment; and it is hardly too much
-to say that, if that sense were deprived of the power of feeling, our
-eyes would deceive us, not only as to the number, but the position of
-the objects within our view.
-
-It is very easy to convince ourselves that we really see objects double,
-although we imagine them to be only single. We have only to look at the
-same object first with the right eye, and we shall see it directly
-against some portion of the wall of the room in which we are sitting;
-then looking at it with the left eye, we shall see that it covers a
-different part of the wall. This experiment is easily tried, and is very
-convincing. Thus we see that an image is formed on both eyes, and we
-consequently see the object, whatever it may be, repeated twice. By
-degrees, however, the eyes gain the power of converging their axes on
-objects at different distances, so that they fall on similar portions of
-each retina, and so convey a single impression to the brain. Thus, for
-instance, if we look at a pencil held up at arm’s length, and then,
-without changing the position of the eyeball, look at some distant
-object, we shall see it double. Let us, however, converge the eyes upon
-it, and the two images unite. Reverse the experiment by now looking at
-the pencil without converging the eyes upon it, and we shall see that
-object double in its turn. The same thing happens if we push aside one
-of the eyes with the finger while looking at any object. During severe
-illness it often happens that the patient from extreme weakness loses
-the power of convergence, and consequently sees every thing double, and
-we continually see children’s faces wearing a most distressing
-appearance through having temporarily lost the power of moving the
-muscles of the eye. It is a common expression to use in speaking of
-drunken people, that they see double, but the saying, unlike many
-others, is no metaphor; when a man gets drunk he loses his power over
-the muscles of his eye, just as he does over those that sustain his
-body, and the instinctive closing of one eyelid, in order that he may
-see objects single, is an effort of his weakened judgment to set things
-right once more.
-
-While on this subject we may mention the experiment made by the famous
-English surgeon Cheselden upon a boy who was born blind, and upon whom
-he operated successfully.
-
-This boy, who was thirteen years old at the time that Cheselden restored
-to him the sense of sight, was not born absolutely blind, his affliction
-having been caused by a cataract or film spread over the eyeball, which
-allowed him to distinguish night from day, or black from white or
-scarlet when placed in a very good light, although he was unable to
-perceive the form of things around him. At first Cheselden operated on a
-single eye, perfectly restoring its power; but so little idea of
-distance did the new sense convey to the boy’s mind that for a long time
-he imagined that everything touched his eyeball, just as those he felt
-touched his skin, and it was only by the sense of touch that he could
-persuade himself of the fallacy of his supposition. At first he had no
-perception of form whatever, and could only recognize objects he had
-already been familiar with after he had felt them all over. He was a
-long time, for instance, before he could distinguish between the dog and
-the cat without touching them, and was greatly surprised to find that
-the persons and things he had liked best when blind were not always the
-pleasantest to his newly acquired sense. His ideas of size, too, were
-all at fault, and he could not, for a long time, be made to understand
-how his father’s picture could be got into the back of his mother’s
-watch; even after he had possessed his sight for a comparatively long
-time, he could still only recognise people he had known during his
-blindness by touching their faces. Whenever he saw a new object he
-looked at it attentively for some time, in order, as it were, to learn
-its form by heart; but his memory was at first so overtaxed that he
-continually forgot his visual impressions, and mistook one thing for
-another. He was more than two months before he could appreciate form as
-depicted in a painting or drawing, having hitherto learned to consider
-pictures as flat objects. When, however, he began to understand the
-power of light and shade in producing the representations of solid
-objects, he was often extremely surprised to find the surface on which
-they were depicted quite flat when he touched it. The same thing
-frequently happens to ourselves, when looking at the photographs of
-bas-reliefs for instance. If these objects be well photographed, with
-the proper arrangement of light and shade, the illusion is so complete
-that the finger involuntarily touches the paper to feel if the surface
-is not really raised. In the Bourse at Paris there are some figures
-painted to represent bas-reliefs in so wonderful a manner, that
-numberless bets have been made, lost and won, over them. When feeling
-such representations of solid objects, the boy would often ask those
-around him which of his senses was deceiving him, his sight or his
-touch.
-
-At first he saw everything of an enormous size, but as he saw things
-larger than those around him, he found the latter diminish. He also
-imagined that there was nothing beyond the room he was in, and could not
-be brought to comprehend how the house could be larger. When the sight
-of the second eye was restored to him a year afterwards, he at first saw
-every object of an enormous size, just as in the case of the first eye;
-but as he had now the perfectly educated organ to help him as well as
-his sense of touch, he soon began to see things under their natural
-appearances.
-
-While he was in ignorance of what sight really meant, he was not
-particularly anxious to undergo the operation, saying that he did not
-think it possible to derive more pleasure from things that he liked than
-he did while he was blind. But now that his sight was restored he found
-every fresh object a new pleasure. When first he was shown the landscape
-from the top of a high hill, he was so delighted that he exclaimed that
-he had found another sense. When his second eye was operated upon, he
-saw things apparently twice as large with both eyes as with the one
-already restored to him. Even at first he seemed to have no difficulty
-in converging the eyes on any object.
-
-These extracts from the history of Cheselden’s patient show us how
-utterly incapable the eye must be of rightly understanding the number,
-position, size, and form of objects without frequently correcting our
-impressions by the aid of the sense of touch.
-
-
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-
-
-
-
-                              CHAPTER IV.
-                           OPTICAL ILLUSIONS.
-
-
-BESIDES the errors of sight already spoken of, there are other
-illusions, which are either common to all persons or confined to certain
-individuals, the knowledge of which will serve as a fitting prelude to
-the descriptions of those which are artificial.
-
-The following defect, for instance, is one which is little known, but
-notwithstanding our ignorance of its existence it is nevertheless true
-that we all suffer from it. There is in every one’s eye a blind spot,
-totally incapable of experiencing the effects of the rays of light when
-they impinge upon it. For objects situated opposite to this particular
-spot we are as completely blind as if we had no eyes at all. To convince
-yourself of the truth of this assertion it is only necessary to try the
-following simple experiment.
-
-Place upon a piece of white paper two small wafers, or two blots of ink
-about an inch and a half apart. Take the sheet in your right hand, and
-hold it up parallel to the lines of the eyes; shut the _left_ eye, and
-fix the _right_ eye on the centre of the _left_ wafer or ink-spot. Move
-the sheet of paper steadily towards the eye, until it is about two
-inches and a half or three inches’ distance from it, and you will find
-that in a certain position the _other_ wafer or ink-spot will disappear,
-although it is evidently still in the field of view. Having discovered
-this point which differs for different eyes, you will find that if you
-diminish or increase the distance of the paper you will once more see
-the missing object. The same thing happens if you move the eye from the
-centre of the wafer. The same experiment may be repeated with the left
-eye with a precisely similar result.
-
-It has been found by experiment that this particular blind space exists
-exactly over the base of the optic nerve, at the spot where it joins the
-eye. (Fig. 1). Thus we see that the nerve which actually conveys the
-impression of sight to the brain is in itself incapable of being excited
-by light. In such cases as these Nature seems to laugh at us, and
-escapes from our grasp just as we are most confident in our power of
-wresting her secrets from her; indeed we may compare her to a wise and
-good-natured mother, who, though always amiable and willing to instruct
-those about her, sometimes smiles when her children fancy they are as
-learned as she is.
-
-If we do not perceive the constant recurrence of the phenomenon just
-mentioned, it is because when both eyes are open the object whose image
-falls on the blind spot in one eye is seen by the other, the insensible
-portions of each eye being on opposite sides. Not only this: the spot
-being always situated on the outer and indistinct portion of the image
-reflected on the retina, we do not take notice of it; for as every one
-has no doubt observed, it is only the small portion of the object we are
-looking at exactly opposite the centre of the eye that is perfectly
-distinct and clear, all the rest being confused in its details, although
-quite visible.
-
-Again, we may account for our not noticing it by the fact of our seeing
-clearly only those things which specially attract our attention—a fact
-first noticed by Mariotte. We see only what we wish to see with our
-physical eyes, as well as those of our mind. If our attention is
-attracted by a particular portion of a landscape, we see only that, and
-nothing else. If it is fixed on some subject that we are contemplating
-inwardly, we see nothing at all, although our eyes may not only be wide
-open, but absolutely fixed on some particular object. For instance,
-suppose a sportsman is out in the fields preceded by his dogs, Bran and
-Ponto. If he follows the movements of Bran with attention, he becomes
-the only object animate or inanimate, that depicts itself on his retina.
-Ponto may jump and caper in vain: he is lost to his master’s eye as much
-as if he were not there at all; his mind is entirely fixed on the beauty
-of Bran’s coat, on the fit of his collar, or fifty other things, and he
-sees nothing else. But let the sportsman begin to think of the number of
-birds he shot yesterday, or how he will find time to get up to the
-grouse in Scotland, or of that fine stag he missed when he was last
-amongst the heather, and dogs, cover, and landscape will fade from his
-sight as effectually as if he had been struck with blindness. Let him,
-however, strike his foot against a stump, or let the dogs suddenly begin
-to point, and he instantly receives back his sight, which but a few
-moments before he had lost to all intents and purposes.
-
-The phenomena of _ocular spectra_ and _complementary colours_
-experienced by every one forms a curious chapter in the history of those
-illusions which take their origin in the eye itself. Every one has
-noticed that after looking fixedly at a bright light or a striking
-colour for a few moments, the eye preserves an impression of the object
-for a certain time. A very light window looked at intently for several
-seconds will leave the impression of its cross-bars on the retina for
-several minutes, the colour of the image changing at every movement of
-the eye. The same effect may be observed when looking at the setting
-sun, or a flaring gas light. If the light at which we look is coloured,
-we shall see the complementary colour in the impression left on the
-retina. Sir David Brewster was one of the first to notice and experiment
-upon these very interesting facts.
-
-If we cut out any simple figure, a small cross for instance, in scarlet
-paper, place it upon a white background and look at it fixedly for a
-minute or two, we shall find that its tint will gradually become duller.
-If we now suddenly look at a piece of white paper, we shall see the
-cross depicted upon it in green, which is the complementary colour to
-red. It should be explained, that the complementary of any colour is
-that which is necessary to make white light. Thus, blue, yellow, and red
-(as we shall find out when we come to speak of the prismatic spectrum),
-mixed in certain proportions, form white light; consequently the
-complementary of orange, which is composed of red and yellow, will be
-blue; of green, which is yellow and blue, red; of purple, which is blue
-and red, yellow, and _vice versâ_. The complementary of black is white,
-and of white, black as a rule; but if the white object be very
-brilliant, the black spectrum will speedily become coloured. The
-impression left by the setting sun is of this character. At first, while
-the eye is open, the image is black, then brownish red, with a light
-blue border; but if the eye be shut suddenly, it becomes green, with a
-red border, the brilliancy of colour being apparently in proportion to
-the strength of the impression. These spectra may be perceived for a
-long time, if the eye is gently rubbed with the finger now and then.
-Some eyes are more impressionable in this respect than others, and Beyle
-gives an instance of an individual who saw the spectrum of the sun for
-years, whenever he looked at a bright object. A modern instance of this
-occurred lately to an amateur astronomer who was looking at an eclipse
-of the sun. He unfortunately used a glass that was not sufficiently
-smoked, and the image of the sun’s disc, with the black space caused by
-the intervening moon, remained on his retina for months after. This
-gentleman’s case afforded an instance of the necessity of attention in
-order to see any object, for after the first few days he only became
-sensible of his unfortunate mishap when his attention was called to it
-by some accidental circumstance. These facts were so inexplicable to
-Locke, that he consulted Newton on the subject, and was surprised to
-learn that the great philosopher himself had suffered for several months
-from a sun-spectrum in the eye.
-
-Without affirming that optical illusions are the cause of all the
-supposed supernatural appearances of which we have heard so much, there
-is no doubt that in many instances the eye plays an important part in
-deluding the brain. The following example, also cited by Beyle, will
-show this clearly. A horseman dressed in black, and riding a white
-horse, was trotting along a portion of the road, which through a sudden
-break in the clouds was brilliantly illuminated by the rays of the sun.
-The black figure of the man was projected against a white cloud, and the
-horse appeared doubly brilliant from being seen against the
-dark-coloured road. A person who was greatly interested in the arrival
-of the horseman was watching them with great attention, when suddenly
-the horse and his rider disappeared behind a wood. An instant after the
-observer was terrified at seeing a _white_ cavalier on a _black_ horse
-projected on a white cloud at which he was accidentally looking. It may
-be readily imagined that such an occurrence, followed up by a succession
-of unusual events,—such as illness, death, or any other series of
-misfortunes,—might even in the present day add a chapter to the history
-of the marvellous.
-
-To the illusions to which, like the preceding, we are all subject, may
-be added those resulting from some abnormal conformation, or some
-disease of the eye, in those who labour under them. An example of this
-occurs in the case of double or triple vision, many remarkable instances
-of which are mentioned by Müller, the celebrated physiologist.
-
-Although, as before explained, the image of an object is depicted at the
-same time on both our eyes, still we only see one impression, in
-consequence of the two images being carried to the brain from
-corresponding portions of the retina. If this relation be disturbed by
-any cause, or if the eyes are not converged exactly upon the same point,
-a double image is the result. The first of these facts may be proved by
-looking at the moon, for instance, with the left eye shut; on suddenly
-opening it, two images will be seen for an instant. The second is
-instantly proved by pushing either of the eyes aside with the finger,
-when looking at any object.
-
-It is necessary, however, to distinguish between these effects and true
-double vision, as well as a certain defect which exists in the eyes of
-many people, consisting in the apparent multiplication of distant
-objects by the same eye. In these cases, there is a superposition of
-images upon the retina, each having its proper bounds. With the majority
-of individuals afflicted in this way, it only happens when they look at
-a very distant object, the moon or stars for instance. There are many,
-however, who suffer from it in the case of everything they look at,
-whether far or near. Stephenson, who was affected with it, made it the
-subject of many interesting experiments. When he looked at a clear mark
-on a white ground, and gradually walked away from it, not only did the
-image become indistinct, but it seemed to unfold itself into several,
-independently of many others much more indistinct, more especially two
-situated on each side, whose distance increased the farther he walked
-away. As these latter images became more and more separated, they also
-became more confused. The image seen by the right eye was a little
-higher than that seen by the left. Griffin states, that after having
-used the telescope for any length of time, the eye that he kept shut
-always saw objects triple and double for some hours afterwards. These
-phenomena are possibly connected in some way with the disposition of the
-plates and fibres of which the crystalline lens of the eye is composed.
-
-Semi-vision, or _hemiopia_ as it is called, is much more rare and more
-difficult to explain than the phenomena of double vision; and consists
-in the power of being able to see only the right or left half of the
-object looked at, the separation being vertical when the eyes of the
-observer are in the same horizontal line. Thus, in looking at the word
-NEWTON, the person so afflicted would only see either the letters NEW or
-TON according to which half of the eyes were defective.
-
-Wollaston was afflicted with hemiopia on two different occasions; the
-first time after violent exercise, during two or three hours, when he
-could see distinctly only the left-hand halves of the objects he was
-looking at. Both eyes were similarly affected, and the phenomenon only
-lasted about a quarter of an hour. Twenty years afterwards he suffered
-again from the same accident, but on this occasion in the contrary
-manner; that is to say, he only saw the right halves of the objects he
-was looking at—to use his own words, he could only see the right half of
-every friend he met. At certain distances from the eye, one of two
-persons would become invisible, and by simply changing his own position
-or that of the persons he was near, he could make one or other of them,
-or indeed both, disappear at will. It must be acknowledged that similar
-tricks of Dame Nature, due to an unconscious insensibility of the eye,
-are most singular, and at first sight appear to have a supernatural
-origin.
-
-Bartholin mentions the case of a hysterical woman who was afflicted with
-hemiopia horizontally, and saw all natural objects cut in two, the lower
-halves being invisible. In this instance it was only the left eye that
-was defective.
-
-Another interesting example of optical illusion is the luminous
-sensation produced internally when the eye, or the neighbouring parts,
-are struck or stimulated by friction or electricity. These appearances
-are experienced even by those who have lost their sight. Müller states
-that a case was submitted to a legal tribunal to decide whether the
-luminous sensations which are perceptible when we rub our eyes are
-really light. The matter in dispute was whether a man who was attacked
-by robbers in the dark, could see and recognise them by means of the
-light produced in his eyes by a violent blow on the head; but he does
-not tell us how the question was decided. With regard to internal
-causes, Humboldt tells us that a man whose eye had been extirpated, was
-sensible of luminous appearances whenever he was galvanized. Lincke
-states that a man whose eye had been removed by a surgical operation,
-saw next day all kinds of luminous phenomena, which tormented him
-cruelly with the idea that after all his eye had been saved. When he
-shut the perfect eye, he fancied he saw with the missing eye circles of
-fire, persons dancing, and similar appearances for several days. These
-facts are analogous to those told of persons who have had their legs and
-arms amputated, but who, notwithstanding, apparently feel pain in their
-lost limbs.
-
-
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-
-
-
-
-                               CHAPTER V.
-                      THE APPRECIATION OF COLOUR.
-
-
-MOST people understand each other sufficiently to agree in their ideas
-about various colours. Thus every one agrees in saying that poppies are
-red, that the sky is blue, and the leaves green; but if any one were to
-assert that the sky was red, that the leaves were blue, and poppies
-green, who could possibly contradict him?
-
-This statement may appear a paradox, and an absurdity to many of our
-readers, but it is really a problem that has engaged the attention of
-many of our greatest philosophers. Who can prove that what I see as
-yellow may not appear blue to you, or that what you see red is not green
-to me? You would possibly explain the doubt by saying that because we
-both agree in calling a buttercup yellow, that we see the same colour. I
-call a buttercup yellow, because I have learnt since my childhood to
-give this name to the particular sensation I experience when I look at
-one of these flowers; but that is no proof that the sensation I feel is
-similar to that felt by everybody else, and it is not merely possible,
-but probable, that our personal sensations of colour are essentially
-different, although the arbitrary words we use to designate them are the
-same.
-
-It may be remarked in parenthesis, that colour is not an entity, but is
-simply the effect of certain properties of surface or interior structure
-possessed by every substance with which we are acquainted. The old
-saying, that “all cats are black in the dark,” is really a profound
-philosophical truth, which is not only true of cats but of the reddest
-rose that ever grew in a garden, the bluest violet that ever was
-plucked, the prettiest girl that ever was kissed under the mistletoe. It
-is a sad thing to think of, that when we put the candle out, and step
-into bed, we become blacker than the blackest negro that was ever
-emancipated. But without light there can be no colour, for there is no
-material, so to speak, from which to manufacture it. White light, as we
-have said before, is made up of red, blue, and yellow, and it is by the
-absorption of one or all of these that all tints are formed. The surface
-of a poppy leaf has the power of absorbing all the blue and a little of
-the yellow, reflecting the whole of the red and the remainder of the
-yellow, the mixture of the two forming scarlet. The surface of a
-marigold acts differently; all the blue is absorbed, as in the case of
-the poppy, and a good deal of the red with it, leaving just a little to
-brighten up the yellow which is reflected with it. Some substances,
-white marble for instance, have no power of splitting the light into
-colours, absorbing some and reflecting others, but reflect the whole of
-it in its integrity. Others again, like black velvet, absorb nearly the
-whole, just reflecting sufficient to enable us to see its surface.
-
-We began this chapter by speculating on the probability of our seeing
-different colours to our neighbours, and we shall now proceed to show
-that our speculations in that direction are not so absurd as they appear
-to be at first sight.
-
-The phenomenon of colour-blindness, or the insensibility of the eye to
-certain colours, has been for many years past a puzzle both to the
-physiologist and the philosopher. Perhaps the most remarkable case of
-the sort is that mentioned first by Huddart, and quoted by Sir David
-Brewster, of a shoemaker named Harris, living at Maryport, in
-Cumberland, who was utterly incapable of distinguishing any colour at
-all, and saw everything white, grey or black. The first time that Harris
-noticed this defect, was when he was about four years old; having found
-the stocking of a playmate in the street, he returned it to him at his
-cottage, and noticed that every one said it was a red stocking, but he
-could not understand why they should call this particular stocking red,
-as it seemed to him to be like every other. This circumstance remained
-in his mind, and a few more similar observations confirmed his
-suspicions that he had some defect of sight that prevented him from
-seeing as others did. He also observed that other children pretended to
-distinguish cherries from their leaves by what they called their colour,
-whilst he could see no difference between them, except those of shape
-and size. He also noticed that by means of the difference of colour,
-others could distinguish cherries on a tree at a much greater distance
-than he could; whilst he, on the contrary, could see other things at
-greater distances than his companions. Harris had two brothers, whose
-eyes were similarly defective; one of these, that Huddart examined,
-mistook green for yellow constantly, and orange for light green.
-
-In the _Philosophical Transactions_ Scott describes a similar defect in
-his own powers of vision. He states that he was unable to distinguish
-green, and that the colours known as crimson and pale blue presented no
-difference of hue. He further confesses his inability to see any
-difference between bright green and bright red, although he could
-distinguish between red and yellow, dark blue, and almost every shade of
-blue, except sky-blue. He goes on to relate how he married his daughter
-to a worthy young man of his acquaintance, and that the day before the
-wedding the bridegroom came to his house in a full suit of black, as he
-thought. He was greatly displeased to see him appear in mourning on such
-an occasion, and took an opportunity to remonstrate with him on the
-subject. But what was his surprise to hear his daughter exclaim, in loud
-tones of counter remonstrance, that she had rarely seen her lover in a
-coat of such a pretty colour, and that her father’s eyes must deceive
-him on this as on many other occasions. Scott’s father, his maternal
-uncle, one of his sisters, and two of his sons had the same defect of
-sight. Dr. Mitchell mentions the case of a naval officer who for his
-ordinary uniform chose a blue coat and waistcoat and red trousers, fully
-believing that they were all of the same colour. A tailor of Plymouth,
-also mentioned by Dr. Mitchell, mended a black silk waistcoat with a
-piece of crimson, and another put a red cloth collar to a blue coat.
-Several celebrated men have suffered from colour-blindness. Amongst them
-may be mentioned Dugald Stewart, the great philosophical writer; John
-Dalton, the originator of the atomic theory; and Troughton, the
-philosophical instrument maker. Dugald Stewart first discovered the
-defect on hearing a member of his family admire the contrast of colour
-between the leaves and fruit of a Siberian crab-tree, while he could see
-no difference between them, except in point of form and size. John
-Dalton could not distinguish blue from crimson, and he could only see
-two colours, blue and yellow, in the prismatic spectrum. Troughton could
-see no difference between dark crimson, bright orange, and yellow—in
-fact, he could only distinguish blue from yellow.
-
-In an article on this subject, published in the _Magasin Pittoresque_
-for 1846, a Swiss physician gives some interesting examples, which are
-worth repeating. In the solar spectrum obtained by passing a ray of
-light through a triangular prism, and which is composed of the following
-colours,—red, orange, yellow, green, blue, indigo, and violet,—Dalton
-could only see yellow, blue, and violet. Rose-colour by day appeared to
-him a pale blue, but at night it seemed to take an orange hue. By day
-crimson seemed to be dirty blue, and red cloth dark blue. Dr. Whewell
-having asked him one day to describe the colour of the doctor’s scarlet
-gown, Dalton pointed to the trees around them, and declared he could
-distinguish no difference in their colour; and one day having dropped a
-stick of red sealing-wax in the grass, he had the greatest difficulty in
-finding it again. Since Dalton’s time over five hundred distinctly
-marked instances of this imperfection have been noticed, and Professor
-Prévost, of Geneva, has named it _Daltonism_, an extremely
-unphilosophical piece of pathological nomenclature, which has
-unfortunately received the sanction of too many great physiologists to
-be abolished. Blindness might just as well be called _Homerism_ or
-_Miltonism_.
-
-Colour-blindness is much more frequent than is generally supposed, for
-those who are afflicted with it are mostly ignorant of the defect, and
-frequently practise trades or professions in which perfect sensibility
-to the different hues of colour is quite indispensable. An instance of
-this occurred some time since in the case of an engine-driver, who
-allowed his engine to run into a luggage train, through not noticing the
-red danger signal. At his examination it was proved that he was
-colour-blind, and could not distinguish red from green. Partial
-colour-blindness is, no doubt, the cause of the frequent disputes that
-we hear about the tints of certain objects; to say nothing of the
-glaring instances of bad taste in the arrangement of colour that are
-now-a-days so common. Out of forty boys at a school at Berlin who were
-examined by Leebech, he found five who were quite confused in their
-notions of colour, and could not distinguish between ordinary shades of
-the same hue. This affliction is in many cases hereditary, descending
-from father to son. It is singular that instances of colour-blindness
-are much more common amongst men than amongst women, for out of over
-five hundred cases there were only four in which females were the
-sufferers. It seems also that persons with grey eyes are more frequently
-colour-blind than those whose eyes are blue or brown. To the list of
-great men who were colour-blind, we must not forget to add the
-celebrated Italian historian, Sismondi.
-
-Physiologists consider that there are two kinds of colour-blindness,—one
-where only two colours are seen, the other where more than two are
-perceptible. Daubeny Turberville, an oculist of Salisbury, mentions a
-case of the former, in which a young girl, like the Maryport shoemaker
-mentioned by Brewster, could only distinguish between black and white,
-everything between the two being of different shades of grey. This girl,
-singularly enough, could see to read in twilight a quarter of an hour
-after her companions. This sharpness of sight appears to be not at all
-uncommon amongst those who are colour-blind. Spurzheim mentions the ease
-of a whole family who were afflicted in the same way as Turberville’s
-patient. All the male members of Troughton’s family were equally
-incapable of distinguishing any colours but blue and yellow.
-
-The cases of colour-blindness where more than two colours are
-distinguishable, are much more common. Goethe, the great German poet,
-who dabbled a great deal in optics, knew two young men who, although
-they possessed powerful sight, and could distinguish between white,
-black, grey, yellow, and orange, were at a loss when the shades between
-dark red and rose-colour were in question. A piece of dried carmine
-appeared bright red to them, and a faint carmine hue on a white shell,
-and a rose-leaf, light blue; the leaves of trees and grass appeared
-yellow, and they confounded rose-colour, blue, and violet together.
-Goethe supposed them to be incapable of perceiving blue and its several
-hues, and called their defect by a high sounding Greek name,
-_akyanoblepsy_, or blue-blindness. Péclet mentions two other persons,
-also brothers, who likewise were incapable of distinguishing between
-blue, violet, and rose-colour. Like Professor Whewell, they confounded
-the dull scarlet of the trousers of the French infantry with the leaves
-of the trees. Yellow appeared to them more brilliant than any other
-colour. Doctor Sommer and his brother could not distinguish between red
-and its derivatives and other colours; they could only distinguish
-between yellow, blue, white, and black. Doctor Nicholl mentions a child
-that could only see red, yellow, and blue, in the spectrum. It could
-distinguish green, but called it brown when it was dark, and pink when
-it was pale. The same physiologist knew a man who called red green, and
-brown dark green. A young lady who was an amateur artist, could not
-perceive a piece of scarlet cloth hanging on a hedge that was close to
-her, although others could see it plainly half a mile off. One day she
-gathered, as a great curiosity, a lichen which she supposed to be of a
-bright scarlet hue, but which was in reality of a beautiful green.
-Another time she could see no difference between carmine and prussian
-blue. A gardener living at Clydesdale, who began life as a weaver, was
-compelled to give up his first trade because in daylight he confounded
-all light colours; yellow and its varieties he could distinguish
-perfectly, but he was incapable of seeing any difference between red,
-blue, pink, brown, and white. Another man, who was a silk-weaver, had to
-change his trade, because he could not distinguish between red, pink,
-and sky blue. A Genevese artist whom circumstances compelled to paint a
-portrait by candle-light, used yellow for pink in laying on his flesh
-tints, with a pleasing result that may be readily imagined. In fact, the
-instances of colour-blindness mentioned by physiological writers are
-almost innumerable, and I should only weary my readers if I related all
-the authentic cases of this singular affliction. One instance, however,
-which was very carefully observed by Wartmann, a distinguished German
-oculist, merits our attention. The afflicted person, whom Wartmann
-speaks of as D., was thirty-three years old. Those of his brothers and
-sisters whose hair was fair suffered from the same infirmity, but those
-whose hair was dark were exempt from it. Like so many others who are
-colour-blind, he could not distinguish between cherries and their
-leaves, and confounded a sea-green piece of paper with a scarlet ribbon
-placed near it. A rose of the ordinary hue appeared greenish-blue. Being
-anxious to see if reflected, refracted, and polarized light exercised a
-different action on his retina, Wartmann tried him first with the
-prismatic spectrum, but he could only distinguish four colours,—blue,
-green, yellow, and red. He could distinguish perfectly the peculiar
-black lines seen crossing the spectrum in certain places, and known by
-the name of Fraunhofer’s lines. He then placed in his hands thirty-seven
-pieces of differently coloured glass, but he could only distinguish four
-varieties. The colours produced by polarized light seemed to give the
-patient quite as much trouble as those produced in the ordinary way.
-Chocolate brown appeared reddish brown; purple, dark blue; and violet, a
-dirty blue. When colours were illuminated by sunlight, they seemed to
-him to be redder than usual, even green and blue appearing red.
-
-In considering cases of colour-blindness, it is very difficult not to be
-misled into using wrong terms, as applied to colour, for we have no
-possible means of knowing what colour it is that is really seen by the
-patient. Thus, for instance, Dr. Whewell could not distinguish between
-red and green. But what colour did he really see? Did he see the leaves
-and cherries both red or both green, or was it some colour between the
-two that was impressed upon his retina? Again, great care must be
-exercised in placing implicit reliance on the statement of persons who
-are colour-blind, for we must recollect that their only means of
-conveying the results of what they experience is by the use of an organ
-that is confessedly defective, and which is quite likely to deceive
-them, and us too, without their being parties to the deception.
-
-The cause of colour-blindness is completely unknown; philosophers and
-physiologists are still in the realms of hypothesis concerning this
-peculiar optical defect. As yet, the most careful observation has failed
-to detect any difference between the eyes of those who are colour-blind,
-and the eyes of ordinary persons, that could in any way account for this
-singular affection of the sense of sight.
-
-
-------------------------------------------------------------------------
-
-
-
-
-                              CHAPTER VI.
-                   ILLUSIONS CAUSED BY LIGHT ITSELF.
-
-
-WHEN playing about the Christmas fire, children frequently amuse
-themselves by whirling round and round a piece of wood, one end of which
-they have previously lighted and blown out. In proportion as the
-movement becomes more rapid, the path of the red-hot end becomes more
-and more connected, until at last a burning ring is formed, in every
-part of which the shining charcoal appears to be at the same time. The
-only way of accounting for this illusion is by supposing that the image
-formed by the burning stick upon the retina remains there for an
-appreciable period, the impression made by it at one part of its journey
-remaining until it returns to its former position. The power possessed
-by the retina of retaining impressions explains a large number of
-illusions of the same kind. The chord of a musical instrument, for
-instance, when struck, appears to occupy a longer space during the time
-it vibrates, than when it is at rest. A rapidly revolving wheel appears
-almost solid on account of the combined images of the spokes seeming to
-unite into one homogeneous mass.
-
-The persistence of luminous impressions upon the retina has given rise
-to the invention of a number of well-known optical toys, amongst which
-may be mentioned the _phenakistiscope_, the _thaumatrope_, the
-_phantascope_, and many others.
-
-[Illustration:
-
-  FIG. 3.—The Phenakistiscope.
-]
-
-The phenakistiscope may be described (figs. 3 and 4) as consisting of an
-iron pin _a b_ turning easily on its axis, and passing through two holes
-in a brass rod _t g_, bent twice at right angles. Attached to one end of
-the pin is a disc of cardboard, divided into several equal sectors, and
-pierced near its circumference with as many similar sized rectangular
-holes (fig. 4). In each sector the same scene is represented, with this
-difference, that the movements of the objects are so arranged as to be
-progressive from one extreme to the other. The disc being fastened to
-the pin _a b_ (fig. 3) by the screw _v_, with the figures facing
-outwards towards _a_, the whole apparatus is held before a looking-glass
-by the handle _m_. If the disc be now rotated by the button _b_, and the
-eye placed opposite one of the square holes in the card, the figures on
-the disc will appear to move more or less quickly according to the rate
-at which it is rotated. The three bricklayers in fig. 4 will be seen to
-pass their bricks from one to the other with perfect regularity if the
-drawing has been made carefully. Numberless other designs may be made
-for this little instrument, such as a windmill in full sail, a man
-working a pump, a conjurer swallowing knives—in fact, any scene with
-objects in motion may be drawn, and will cause infinite amusement for
-the long winter evenings.
-
-The time during which the impression of any object remains upon the
-retina appears to be in direct proportion to its brilliancy. For a
-burning coal it is stated to be about the tenth of a second;
-consequently, if the stick mentioned at the beginning of the chapter is
-rotated ten times in a second, a continuous luminous ring will appear to
-be formed. That the time necessary for producing a distinct impression
-varies with the brilliancy of the object, may be readily guessed from
-the fact that an electric spark is perfectly visible, although its
-duration can hardly be measured, while a cannon-ball in flight is only
-perceptible to the practised eye of the artilleryman, owing to its
-reflecting only a small quantity of very diffused light.
-
-[Illustration:
-
-  FIG. 4.—Disc of the Phenakistiscope.
-]
-
-The second instrument, the thaumatrope, is constructed on the same
-principle. It consists of a certain number of circular discs of card
-three or four inches across, which are capable of being turned on their
-axes with great rapidity by means of the finger and thumb and a couple
-of silk threads fixed at opposite sides of their circumference. On each
-of these discs a design is painted, one-half appearing on one side, and
-the other half on the other, in such a manner that the two parts form a
-single picture. You may have, for instance, Harlequin on one side and
-Columbine on the other, but on turning the card you will see them
-together. The body of a Turk may be drawn on one side and his head on
-the other, and, by rotating the card, the head suddenly finds a pair of
-shoulders to fit it. A sentence may be divided in the same way, or the
-words, or even the letters, may be divided between the opposite sides of
-the card: in fact, like the phenakistiscope, the designs applicable to
-this little instrument are endless.
-
-The third of these instruments, the phantascope, is constructed in
-accordance with the peculiar power possessed by the eyes of adapting
-themselves to the distance of the objects they are looking at. Everybody
-must have noticed that in order to see objects plainly that are placed
-at different distances we insensibly alter the position and focus of the
-eyes, and that, consequently, objects even in the same plane as those we
-are looking at are not perceived by us until something calls our
-attention to them, and causes us to alter the position and focus of our
-eyes and fix our gaze on them. For instance, in looking at a canary in a
-cage, we have but a confused idea of the wires, which we will suppose to
-be midway between the bird and the observer. But if anything attracts
-our attention to the wires we lose sight of the bird, or at any rate see
-it only as a confused mass. If this experiment is made with care, it
-will be perceived that the object seen confusedly is always double,—a
-fact that may be verified by interposing the finger between the eyes and
-any object. When we look at the finger, the distant object will seem to
-be doubled; if we look at the object, it is the finger that undergoes
-duplication.
-
-We know by experience that when we look at an object and press one of
-the eyeballs slightly with the finger, the image of it becomes doubled.
-The explanation of this phenomenon is not very easy, but it is generally
-supposed that in the case of ordinary vision the two eyes produce the
-sensation of a single image in consequence of the two impressions being
-formed at corresponding parts of each retina, and that habit causes us
-to see only a single object in such a case. But when the eyes are so
-disposed as to be capable of seeing distant objects distinctly, the two
-images formed by a near object are no longer found in the corresponding
-portions of each retina, and so produce the sensation of double vision.
-The same thing happens when either of the eyes is momentarily displaced.
-
-These phenomena have given rise to the construction of a very simple
-instrument, the phantascope, with which many interesting experiments may
-be performed, and which was invented some years since by Dr. Lake, an
-eminent physician of New York.
-
-In the middle of one of the edges of a thin piece of wood, say six
-inches or a foot in length, which serves as a base for the instrument,
-is fixed a rod fourteen or sixteen inches long, upon which slide a
-couple of ferules capable of being fixed at any height by means of
-thumb-screws. Each of these ferules holds a piece of cardboard five or
-six inches long, and of any convenient breadth, in a horizontal
-position. The upper card is pierced in a longitudinal direction with a
-slit rather less than a quarter of an inch broad, and about three inches
-long; that is to say, a little wider than the distance between the
-centres of two eyes. The second card has a similar slit of the same
-length, and corresponding vertically with the one above it; the width,
-however, in this instance being only about the eighth of an inch. In
-addition, the lower card should be marked with a fine line drawn across
-the centre, which we shall call the index.
-
-Things being thus arranged, if we place two similar objects—two A’s, for
-instance—upon the wooden stage of the instrument, about three inches
-apart, and look at them through the two slits, we shall see them as
-under ordinary circumstances; but on fixing our eyes intently on the
-index of the lower card, and gradually raising it, we shall see the two
-A’s become double, the two images of each letter separating themselves
-more and more the nearer the lower card approaches the upper one, until
-the last two of the images will coalesce, and appear to be placed on the
-lower cardboard, the other two remaining in their proper place. The eyes
-must be kept firmly fixed upon the index, otherwise the illusion
-disappears immediately, and two A’s only are seen in their true position
-on the base of the instrument. This is an instance of the production of
-an image in a place where it certainly does not exist. This illusion is
-seen best when the upper screen is about ten inches from the object, the
-lower screen being just half-way between; but, as in most of these
-cases, the distances will differ according to the focus of the
-observer’s eyes. The proper distances once being found, the experiment
-may be varied in a hundred different ways. For example, instead of two
-letters and a line we may have two flowers on the stage, and the figure
-of a flower-pot on the intermediate screen. If the two flowers are
-painted different colours, they will unite and form a mixed tint. Thus a
-red and yellow flower will give an orange image, a blue and yellow a
-green image, and so on. A perpendicular stroke and a horizontal one will
-give a cross. A few experiments with this little instrument will throw a
-light upon many of the obscurer points that exist amongst the phenomena
-of vision, and will show conclusively that the two eyes rarely see in
-the same manner, and that it is sometimes one, and sometimes the other,
-that sees most distinctly. A couple of pieces of cardboard, pierced with
-suitable slits and held in the hand may be substituted for the apparatus
-above described, but of course they will be much more difficult to use,
-and will give less satisfactory results.
-
-
-------------------------------------------------------------------------
-
-
-
-
-                              CHAPTER VII.
-                   THE INFLUENCE OF THE IMAGINATION.
-
-
-THE above facts show plainly that optical illusions find their source in
-the very mechanism of the organs of sight, and that without going
-farther than the eye itself we may discover numberless examples of these
-phenomena. We shall presently bring before our readers the innumerable
-means devised by art for deceiving the sense of sight and impressing us
-with sensations that are purely imaginary. But before describing these
-numerous pieces of apparatus we must still remain for a short time
-within the domain of man’s faculties, and describe some of the illusions
-that we are subjected to by those powers of the imagination that are
-supposed to hold in check the five senses of the body. Our imagination,
-however, plays us as many tricks as our eyes, and, like them, is
-alternately false and true. Touch, taste, smell, hearing, and sight, are
-all supposed to be under its powerful influence for good or evil; but
-they are all deceived by it in turn, more especially the sense of sight,
-which we generally boast of as being the most trustworthy of them all.
-Were we to describe all the labyrinths into which our imagination is
-continually leading us, we might easily extend this little volume to one
-of treble the size. But our purpose is not so much to write a history of
-all the hallucinations to which the imagination is subject, but to cull
-from those already existing the most interesting instances in which this
-great faculty is alternately the victim and the tyrant of the sense of
-sight.
-
-Amongst many works on this subject we may cite that of Brière de
-Boismont on “Hallucinations, Apparitions, Visions, &c.,” from which we
-shall draw largely in the following pages. The examples we shall give
-will be those only in which the victims of the hallucination were in the
-full enjoyment of their mental faculties, and could healthily analyze
-the sensations and impressions to which they were subjected.
-
-One of the first of these bears upon those diseases of the eye to which
-allusion was made in Chapter IV. Towards the end of 1833, a poor
-washerwoman who was tormented grievously with rheumatic pains gave up
-her business, and took to sewing for her livelihood. Being but little
-accustomed to this kind of work, she was compelled to sit over her
-needle late at night in order to save herself from starving. The
-unwonted strain upon the eyes soon brought on ophthalmia, which speedily
-became chronic. Nevertheless, she continued her work, and fell a prey to
-_diplopia_, or double sight in each eye. Instead of a single needle and
-thread, she saw four continually at work, everything else about her
-being similarly multiplied. At first she took no notice of the singular
-illusion, but at last both imagination and sight joined arms against the
-judgment, and the poor creature imagined that Providence had taken pity
-on her forlorn condition, and had worked a miracle in her favour by
-bestowing on her four pair of hands in order that she might do four
-times her usual amount of work.
-
-The following is another instance of the passage of illusion into
-hallucination. A man fifty-two years old, of a plethoric constitution,
-after having suffered from a defect in his visual functions that caused
-him to see objects sometimes double, and at others upside down, suddenly
-showed signs of cerebral congestion, and threatened apoplexy. By proper
-treatment, however, he was saved for a time from the latter catastrophe,
-but he became permanently afflicted with strabismus, or squinting, and
-he suffered from a singular hallucination. His eyelids would contract,
-and his eyeballs would roll from side to side at more or less distant
-intervals. On these occasions he imagined he saw the figures of
-different persons that he knew moving about, and would even follow them
-outside his door into the other rooms of the house. He was perfectly
-aware that these appearances were merely the effect of the imagination,
-but this did not in any way detract from their appearance of reality.
-The man afterwards died from an attack of apoplexy.
-
-The following examples are also cases of singular optical deception,
-some of them being so extraordinary as to trench upon the supernatural,
-and in the days of ignorance would have given those who were their
-victims the character of unearthly personages.
-
-A certain English painter, who in some sort inherited the palette of Sir
-Joshua Reynolds, and believed himself superior in many respects to the
-great master, used to boast that in one year he painted over three
-hundred portraits, large and small. This fact seemed to Wigan a physical
-impossibility, and he questioned him closely as to the secret of his
-astonishing rapidity of execution, for he never required more than one
-sitting from his patrons. Wigan states that he saw him paint a miniature
-of a well-known personage in eight hours, which was incomparable in its
-fidelity to nature and finished execution. Wigan asked him to give him
-some details of the method he adopted, and he gave him the following
-answer: “When a sitter presents himself, I look at him attentively for
-half an hour, sketching the outlines of his features on my canvass
-during the time. I have no occasion for a longer sitting, and I pass on
-to some one else. When I wish to continue the first portrait, I take the
-sitter in my imagination, and I seat him in the chair, where I see him
-as distinctly as if he were really there, and I can even heighten a
-tint, or soften down a clumsy form at will, without altering the
-likeness. I look from time to time at the imaginary figure, and I go on
-painting. I stop now and then to examine his position, absolutely as if
-the original were before me; for every time I look towards the chair I
-see the sitter. This method of proceeding has rendered me very popular;
-and as I have always succeeded in catching the likeness of my patrons,
-they have been simply enchanted at my sparing them the tedious sittings
-exacted by other painters. Little by little I have begun to lose the
-distinction between the real and imaginary sitter, and I have often
-maintained stoutly that my patrons had already sat to me on the previous
-day. At last I became convinced that it was the real sitters that I saw,
-and thenceforth all became confusion. I suppose my friends took alarm at
-my hallucinations, for I remember nothing of what happened during the
-thirty years that I remained in the madhouse. This long period has left
-no trace on my memory, except indeed the last six months of my
-confinement. It seems to me, however, that when my friends talk of
-having visited me I have some vague recollection of the fact; but it is
-a subject that I do not care to pursue.”
-
-The most remarkable feature of the case is, that this artist after a
-lapse of thirty years resumed his pencil, and painted almost as well as
-when he was forced by madness to abandon his art.
-
-This faculty of being able to evoke shadows, with which to people one’s
-solitude, may be carried so far as to transform real persons into
-phantoms. Hyacinth Langlois, a distinguished artist, living at Rouen,
-tells us that Talma, with whom he was extremely intimate, confided to
-him that, whenever he went upon the stage, he had the power, by mere
-force of will, to cause the clothes and flesh of his numerous auditory
-to disappear, and become transformed from living beings into so many
-skeletons. When his imagination had peopled the house with these
-singular phantoms, the emotion he felt was so great that it gave his
-dramatic powers still greater force, and enabled him to produce the
-wonderful effects that have made his name so famous.
-
-Wigan says, that he once knew a most intelligent and amiable man, who
-could at will evoke his own image. He often laughed at seeing his second
-self standing before him, the phantom appearing to laugh as heartily as
-himself. This illusion was for a long time a matter of amusement to him,
-but at last he became persuaded that he was haunted by his own double.
-His second self appeared to hold arguments with him continually, and
-beat him frequently on various points of dispute, a matter which
-mortified him excessively, as he was rather proud of his powers of
-reasoning. This gentleman, although always considered as being somewhat
-eccentric, was never put under the slightest restraint, and at last the
-creature of his imagination so tormented him, that he resolved not to
-live through another year. He consequently paid all his debts, arranged
-his affairs, and waited pistol in hand until the clock struck twelve on
-the 31st of December, and then deliberately blew out his brains.
-
-In _Abercromby on the Mind_ we read an account of the observations made
-by a gentleman who was the victim of illusions during the whole of a
-pretty long life. If he met a friend in the street, he was unable to
-tell at first whether he saw a real human being or only a phantom. By
-close examination he could detect a difference between the real person
-and the creature of his imagination, the features of the former being
-sharper and more defined than those of the phantom; but in general he
-was obliged to test the reality of the figure he saw by the senses of
-touch and hearing. He was able, by concentrating his thoughts upon the
-appearance of any friend, to call up his image; a power which extended
-even to scenes that he had witnessed. Although he could produce these
-hallucinations at will, he was powerless in making them disappear; and
-when once he succeeded in calling forth these creatures of his
-imagination, he never could tell how long the delusion would last. This
-gentleman was in the prime of life, a good man of business, and
-otherwise in a perfect state of mental and bodily health. A member of
-his family possessed the same faculty, but in a minor degree.
-
-In 1806, General Rapp, when returning from the siege of Dantzic, having
-occasion to speak to the Emperor Napoleon, walked into his private room
-without being announced, and found him in such a profound state of
-abstraction, that he remained for some time unperceived by his imperial
-master. The General, seeing him thus perfectly motionless, fancied he
-must be ill, and purposely made a slight noise. Napoleon instantly
-turned his head, seized the General by the arm, and pointing upwards,
-exclaimed, “Do you see it up there?” The General, hardly knowing what to
-say, remained silent; but the Emperor repeated his question, and he was
-obliged to reply, that he saw nothing. “What,” said the Emperor, “you
-don’t see anything? You don’t see my star shining before your eyes?” And
-becoming more and more animated, he went on to say, that the mysterious
-visitor had never abandoned him, that he saw it throughout all his great
-battles, that it always led him onward, and that he was never happy but
-when he was gazing at it.
-
-That such hallucinations have no real existence as far as the eye goes,
-is proved by the fact of many people who have lost their sight, being
-subject to them. It is hardly to be wondered at that those who by
-accident have been deprived of their sight, should wish so ardently to
-see once more the persons and sights they have taken pleasure in, that
-they should at last create for themselves illusions of this character.
-The same thing has frequently occurred with those whose sight is more or
-less weak. An old man of eighty, who was purblind, never sat down to a
-table during the last years of his life, without seeing around him a
-number of his friends who had long been dead, dressed in the costume of
-fifty years before. This old man had but one eye, which was extremely
-weak, and wore a pair of green preservers, in the glass of which he
-continually saw his own face reflected.
-
-Doctor Dewar, of Stirling, mentioned to Abercromby a very remarkable
-instance of this species of hallucination. The patient, who was quite
-blind, never walked in the street without seeing a little old woman
-hobbling on before him and leaning on a stick. This apparition always
-disappeared when he entered his house.
-
-Similar illusions frequently happen to every one, even the most healthy
-amongst us, but a little consideration soon puts them to flight. It
-would be useless to mention the numberless cases in which a square tower
-has appeared round, or where the landscape has suddenly seemed to recede
-from the sight. Such illusions as these have been long well known, and
-appreciated at their proper value; but there are others whose true cause
-has remained a mystery, until explained by the progress of science, such
-as the Spectre of the Brocken, the Fata Morgana, and the mirage.
-
-Analogous appearances have been seen in Westmoreland and other
-mountainous districts, the inhabitants imagining that the air was full
-of troops of cavalry, and whole armies even; such illusions resulting
-simply from the shadows of men and horses passing over an opposite
-mountain being thrown on the fog.
-
-A vast number of different circumstances give rise to these illusions,
-such as a strong impression, or the recollection of some striking event,
-which may easily cause them, by the association of ideas. Wigan relates,
-that being at a _soirée_ held at the house of M. Bellart, a few days
-after the execution of Marshal Ney, the groom of the chamber, instead of
-calling out the name of _M. Maréchal aîné_ (M. Maréchal, senior),
-announced the arrival of M. le Maréchal Ney. A shudder passed through
-the company, and many of them declared, that for an instant they saw the
-face and figure of the dead man in place of those of his involuntary
-representative.
-
-When the mind is thus prepared, the most familiar objects are
-transformed into phantoms. Ellis relates an anecdote of this kind, which
-he heard from an eye-witness, who was a ship’s captain of
-Newcastle-on-Tyne. During a voyage that he made, the ship’s cook died.
-Some days after the funeral, the chief mate came running to him in a
-great fright, with the news that the ship’s cook was walking on the
-water, astern of the vessel, and that all the crew were on deck looking
-at him. The captain, who was angry at being disturbed with so
-nonsensical a tale, answered sharply, that they had better put the ship
-about and race the ghost to Newcastle. His curiosity, however, was
-presently aroused, and he went upon deck and looked at the spectre. He
-frankly avowed that for some moments he saw what really appeared to be
-his old shipmate, just as he knew him in life, with his walk, clothes,
-cap and figure perfectly resembling those of the dead man. The panic
-became general, and every one was struck motionless for a time. He had
-the presence of mind, however, to seize the helm and put the ship about,
-when as they neared the ghost, they found the absurd cause of their
-fright was a broken mast from some wreck, which was floating after them
-in an upright position. If the captain had not boldly sailed up to the
-supposed ghost, the story of the dead cook walking upon the water would
-have continued to this day to terrify half the good inhabitants of
-Newcastle.
-
-Such facts as these are innumerable, and we shall mention a few more
-which will explain a host of stories found in various ancient and modern
-authors.
-
-Ajax was so angry at the arms of Achilles being awarded to Ulysses, that
-he became furious, and, seeing a herd of pigs, drew his sword and fell
-upon them, taking them for Greeks. He next seized a couple of them and
-beat them cruelly, loading them at the same time with insults, imagining
-one of them to be Agamemnon, his judge, and the other Ulysses, his
-enemy. When he came to himself, he was so ashamed at what he had done,
-that he stabbed himself with his sword.
-
-Theodoric, blinded by jealousy and yielding to the base solicitations of
-his courtiers, ordered that Symmachus, one of the most upright men of
-his time, should be put to death. The cruel order had hardly been
-executed, when the king was seized with remorse, and bitterly reproached
-himself with his crime. One day a new kind of fish was put upon the
-table, when the king suddenly cried out that he saw in the head of the
-fish the absolute resemblance of that of his victim. This vision had the
-effect of plunging the king into a state of melancholy that lasted his
-whole life.
-
-Bessus once, when surrounded by his guests and giving himself up to the
-enjoyment of the feast, ceased suddenly to listen to the flattering
-speeches of his courtiers. He apparently listened with great attention
-to some sound that was heard by no one else, and suddenly leaping from
-his couch, mad with rage, he seized his sword and rushing at a swallow’s
-nest that was near, beat it down, killing the poor birds inside it,
-crying out that these insolent birds dared to reproach him with the
-murder of his father. Surprised at such a sight, his courtiers gradually
-disappeared, and it became known some time afterwards that Bessus was
-really guilty, and that the senseless action he had performed simply
-resulted from the voice of conscience.
-
-The illusions of sight and hearing are often found to take an epidemic
-form, and historians relate an immense number of anecdotes bearing on
-this particular phase of self-delusion. One of the commonest of them is
-that which transforms the clouds into armies and figures of all kinds.
-Religious prejudices, optical phenomena, physical laws that are still
-unknown, dangerous fevers, derangements of the brain, afford a natural
-explanation of these hallucinations.
-
-We have borrowed most of these examples from Brière de Boismont’s works,
-for the special purpose of showing how easy it is to deceive the
-imagination, and to demonstrate the facility with which the sense of
-sight is led astray without the intervention of complicated apparatus.
-In addition, we may quote instances from Brewster, showing the ease with
-which the imagination enables us to see distinct forms in a confused
-mass of flames, or in a number of shadows superposed upon each other.
-This great philosopher gives us an anecdote of Peter Heamann, a Swedish
-pirate and murderer. One day that his crew were repairing some
-unimportant portion of the ship, after having pitched the place well he
-took the brush in order to tar the other parts of the vessel, which were
-much in want of such treatment; but as soon as he spread the pitch over
-the timbers of the ship, he was thunderstruck at seeing apparently
-reflected in its shining surface the image of a gallows with a headless
-man beneath. The head belonging to the body was lying before it, and the
-body itself was depicted with every limb—legs, thighs, and arms—perfect.
-He frequently told his crew of these illusions, adding that it was
-evidently a prediction of the fate in store for them. He was often in
-such a state of terror, that on calm days he would drop down into the
-hold and wrap himself up in a spare sail in order not to catch sight of
-the horrible image that he constantly saw in the shining surface of the
-tar.
-
-The imagination really seems to create for itself a sort of mental
-visual organ which is in intimate relation with that of the body, and
-which often takes its place so efficiently—as in the case of dreams—that
-the mind is utterly unable to perceive the substitution. It is on
-account of this that practical opticians are so unsparing in their
-endeavours to predispose their spectators to being deceived.
-
-When both the body and mind are healthy, the relative intensity of the
-two kinds of impressions is very unequally divided, mental images being
-more evanescent and comparatively weak, and with persons of ordinary
-temperament incapable of effacing or disturbing the reflections of
-visible objects. The affairs of life could not go on if the memory
-introduced amongst them brilliant representations of the past in the
-midst of ordinary domestic scenes or the objects familiar to us. We may
-account for this by supposing that the set of nerves which carries the
-efforts of the memory to the brain cannot execute their functions at the
-same time as those which take cognizance of the images reflected on the
-retina. In other words, the mind cannot accomplish two separate
-functions at one and the same time, and the mere act of directing the
-attention to one class of subjects causes all others to become instantly
-imperceptible. The exercise of the mind in these instances is, however,
-so rapid that the alternate appearance and disappearance of the two
-different impressions is completely unnoticed. Thus, for instance, while
-looking at the dome of St. Paul’s, if our memory suddenly evokes the
-image of some other object, Mont Blanc for instance, the picture of the
-cathedral, although still depicted on our retina, is momentarily effaced
-by the effort of the will, although we may not change the position of
-our eyes during the time. While the memory continues to dwell on the
-picture it has called up, it is seen with sufficient distinctness,
-although its details may be somewhat misty and its colours confused; but
-as soon as the wish to see it passes away the whole disappears, and the
-cathedral is seen in all its former distinctness.
-
-In darkness and solitude, when surrounding objects produce no images
-that can interfere with those of the mind, these latter are more lively
-and distinct: and when in addition we are half asleep and half awake,
-the intensity of mental impressions approaches that of visible objects.
-In the case of persons of studious habits who are continually employed
-in mental effort, these images are more distinct than with those who
-follow the ordinary avocations of life, and during their working hours
-rarely see the objects round them. The earnest thinker, absorbed by
-meditation, is in a manner deprived for the time of the use of his
-senses. His children and servants pass in and out of his study without
-his seeing them, they speak to him without his hearing them and they may
-even try to rouse him from his reverie without success; and yet his
-eyes, ears, and nerves received the impression of light, sound, and
-touch. In such instances, the mind of the philosopher is voluntarily
-occupied in following out an idea which interests him profoundly; but
-even the most unlearned and thoughtless of us sees the images of dead or
-absent friends with his mind’s eye, or even fantastic figures which have
-nothing to do with the train of thought he may be pursuing. It is with
-these involuntary apparitions as with spectres of the imagination:
-although they are intimately connected with some thought that has passed
-through our mind unperceived, it is impossible to trace a single link of
-the chain connecting them together.
-
-
-------------------------------------------------------------------------
-
-
-                                PART II.
-
-                           THE LAWS OF LIGHT.
-
-                             --------------
-
-
-
-
-                               CHAPTER I.
-                             WHAT IS LIGHT?
-
-
-EVERYBODY knows the effects of the action of light, without, however,
-understanding precisely what constitutes light itself. Any formal
-definition would rather puzzle than help the student; we must therefore
-content ourselves with saying that light is that effect of force which
-causes us to perceive external objects.
-
-A man who was blind from his birth, and upon whom the operation for
-cataract had been successfully performed, had accustomed himself for a
-long time to imagine the nature of those unknown phenomena that his
-affliction had prevented him from observing. He had arranged in his mind
-the various definitions that had been given to him as to the nature of
-light, and having combined them, he fancied he had acquired some notion
-of what the sense of vision really meant. But what was the astonishment
-of the surgeon who had restored to him his fifth sense, when he asked
-him to give his opinion upon the effects of light, to see him take up a
-lump of sugar and reply that it was under that form that he had imagined
-it to himself.
-
-As for us who have the happiness of possessing the sense of sight, we
-know this mysterious agent more by the enjoyment that we have derived
-from it, than from any analysis we have made of its nature. It is an
-endless chain that connects us with the entire universe; a bond that
-laughs at distance and spans the abysses of space. By means of light we
-can appreciate the beauties of hue and form, and by its power we touch
-as it were the inaccessible. It constitutes the most intimate connexion
-between ourselves and external objects—a connexion that seems even to
-alter our temper, disposition, and character, according to the
-variations of its intensity. The dull and foggy days of winter, those
-days when sleet and rain struggle in the atmosphere, spread like a veil
-over us, and throw a shadow upon our life. The return of the bright
-spring sun, the reappearance of light and blue sky, on the contrary,
-open up our hearts and minds, gay nature enchants us once more, and a
-feeling of fresh happiness prepares us for the coming glories of the
-newly risen year.
-
-This intimate connexion between the light of heaven and the human mind,
-hallowed as it is by our desire to rise towards the Source of all light,
-might be made the subject of many eloquent pages; and it would be an
-interesting and useful task to show the gradual progress of mankind from
-those ancient people who trembled at the approach of darkness, and who
-fervently saluted the dawn with prayers and praises, down to the
-philosophers of the present age, who investigate its effects with so
-much reverential joy. But we must cease paying any more attention to the
-superficial action of this marvellous force which in these latter days
-has become, in the hands of man, the source of so many illusions and the
-origin of a complete world of rich and brilliant pictures, but which
-after all only exist in the imagination.
-
-It was believed for a long time that light was a compact mass of tiny
-particles emitted by luminous bodies, which struck our eyes and so
-produced the phenomenon of vision. These particles or molecules were
-naturally thought to be extremely minute, and the objects illuminated by
-them were supposed to throw them off as if they were endowed with
-elasticity. Under this hypothesis, light was a material body. The
-illustrious Newton was the first propagator of this theory; the last was
-M. Biot, a French philosopher, lately dead.
-
-The undulatory theory has now-a-days completely superseded the
-corpuscular hypothesis. It was first started about the year 1660 by the
-Dutch philosopher Huyghens, who has left behind him numerous treatises
-on optics, and the properties of light, as well as a curious account of
-the inhabitants of the other members of the solar system, including a
-minute description of the various planetary manners and customs. At the
-beginning of the present century, Fresnel showed, by the most brilliant
-discoveries the superiority of this theory, and shortly after Arago
-confirmed him in his demonstrations. According to the undulatory
-hypothesis, light is not a mass of molecules emitted by a luminous body,
-but simply the vibration of an elastic fluid which is conceived to fill
-the whole of space. A comparative example may assist you in
-understanding this theory more clearly. If you throw a stone into a
-smooth piece of water, there will form around the point where the stone
-fell, a series of circular undulations, starting from the centre and
-gradually enlarging themselves. If a loud noise is suddenly heard, the
-same effect is produced round the point from whence the sound proceeds.
-A series of waves are formed which spread not only horizontally, as on
-the surface of the water disturbed by the stone, but in every direction.
-In fact, in the case of sounds, the waves are so many gradually
-increasing spheres. In the case of light, when a luminous body is placed
-in space, the ether which surrounds it is thrown into a state of
-vibration, and the motion is immediately propagated in all directions,
-with extreme velocity. It is these undulations that produce upon our
-eyes the sensation of light. We may therefore say that light, like
-sound, is movement, while darkness, like silence, is absolute rest.
-
-Many people still believe that light is propagated instantaneously, and
-cannot bring themselves to imagine that we do not see a flame the moment
-we light it, but only an instant after. I have myself spoken to
-well-educated people possessed of good judgment and a certain amount of
-elementary knowledge, who could never bring themselves to believe that
-we see the stars, not as they now exist, but as they appeared at the
-particular moment when the luminous wave by which we are enabled to
-perceive them left their surface, and which only reaches us after
-travelling through space a certain number of years, days, or hours,
-according to their distance. It is extremely useful and interesting to
-form a correct idea upon the way in which light is propagated.
-
-The determination of the prodigious quickness with which the waves of
-light move through space, says Arago, is undoubtedly one of the happiest
-results of modern astronomy. The ancients believed that it moved with
-infinite velocity, and their view of the subject was not, like so many
-of the questions relating to physics, a mere opinion without proof; for
-Aristotle, in mentioning it, brings forward the apparently instantaneous
-transmission of daylight. This notion was disputed by Alhazen, in his
-_Treatise on Optics_, but only by meta-physical weapons, which were
-again opposed by several very worthless arguments, by his commentator,
-Porta, although he admitted the immateriality of light. Galileo seems to
-have been the first amongst modern philosophers who endeavoured to
-determine the velocity of light by experiment. In the first of his
-dialogues, _Delle Scienze Nuove_, he announces by the mouth of Salviati,
-one of the speakers present, the ingenious means he had employed, and
-which he thought quite sufficient to solve the question. Two observers
-with lights were placed at the distance of one mile from each other; one
-of them extinguished his light, and the other as soon as he perceived it
-extinguished his. But as the first observer saw the second light
-disappear the instant he had extinguished his own, Galileo concluded
-that light was propagated instantaneously through a distance double that
-which separated the two observers. Certain analogous experiments that
-were made by the members of the Academy _Del Cimento_, but at three
-times the distance, led to precisely the same conclusions.
-
-These attempted proofs seem at first sight to be absurd, when we think
-of the vastness of the problem to be solved; but we must judge these
-experiments with less severity, when we consider that almost at the same
-epoch, men of such well-deserved repute as Lord Bacon believed that the
-velocity of light, like that of sound, was sensibly altered by the force
-and direction of the wind.
-
-Descartes, whose theories upon light had so much analogy with those
-known under the name of the undulatory hypothesis, believed that light
-was transmitted instantaneously throughout any distance, and endeavours
-to prove his position by proofs that he thought he had obtained whilst
-observing an eclipse of the moon. It must be acknowledged, however, that
-his very ingenious train of reasoning proves that whether the
-transmission of light is instantaneous or not, it is at least too
-considerable to be determined by experiments made on the earth, like
-those of Galileo, and which he vainly hoped would have solved the
-question.
-
-The frequent occultations of the first satellite of Jupiter, the
-discovery of which was almost consequent upon that of lenses, furnished
-Römer with the first means of demonstrating that light was propagated by
-perceptible degrees.
-
-In tracing out the history of human knowledge, says Dr. Lardner, we have
-frequently to point out with some little surprise, joined to a feeling
-of profound humility, the important part played by chance in the
-advancement of science. In searching zealously after mere trifles which,
-when found, are of no consequence, we frequently lay our hands on
-inestimable treasures. The frequency of this fact impresses the mind
-with the notion that some secret and unceasing power exists, in
-accordance with which human knowledge and science are continually
-progressing. It is in physical, as in moral philosophy. In our
-ignorance—like the dog mentioned by Æsop, which, seeing in the water the
-reflection of the prey it held in its mouth, dropped the substance and
-tried to seize the shadow—we are continually searching after trifles;
-but, more fortunate than the animal of whom we have been speaking, the
-shadow that we try to seize is often transformed into a rich treasure.
-We can say with every confidence that “the Providence which shapes our
-ends,” knows our wants better than we do ourselves, and bestows on us
-the things we _ought to have_ asked for instead of those we _have_ asked
-for. We shall find a very simple proof of this in the history of the
-discovery of the velocity of light.
-
-A short time after the invention of the telescope and the consequent
-discovery of Jupiter’s satellites, Römer, a celebrated Danish
-astronomer, was engaged in a series of observations, the object of which
-was to determine the time which one of these bodies took to revolve
-round its planet. The method employed by Römer was to observe the
-successive occultations of the satellite, and to notice the interval
-that elapsed between each of them. But it at last happened that the
-interval between the two occultations, which was about forty-five hours,
-became prolonged by periods of 8, 13, and 16 minutes, during that half
-of the year when the earth was receding from the planet, while it became
-proportionally cut short during the rest of the year. Römer was struck
-by a happy idea; he suspected instantly that the moment when he remarked
-the disappearance of the satellite was not always coincident with the
-instant when it really took place, but that it sometimes appeared to
-happen later—that is to say, after an interval of time sufficiently long
-to allow the light that had left the satellite immediately after its
-disappearance, to reach the eye of the observer. Hence it became evident
-that the farther off the earth was from the satellite, the longer was
-the interval of time between its disappearance and that of the arrival
-of the last portions of its light upon the earth; but that the moment of
-the disappearance of the satellite is that of the commencement of the
-occultation, and that the moment of the arrival of the last portions of
-light is that when the commencement of the occultation is observed.
-
-It was thus that Römer explained the difference between the calculated
-and observed time of the occultation, and he saw that he was on the
-threshold of a great discovery. In a word, he saw that light propagated
-itself through space with a certain velocity, and that the fact we have
-just mentioned furnished the precise means of measuring it.
-
-Thus the occultation of the satellite was retarded one second for every
-185,000 miles that the earth is distant from Jupiter; the reason being,
-that a ray of light takes a second to travel this distance, or, in other
-words, because the velocity of light is at the rate of 185,000 miles per
-second.
-
-It must be remembered when considering this subject, that in any system
-of undulations or vibrations, no matter through what medium they are
-propagated, their movement is simply a change of form, and not a
-transmission of matter. The waves which spread round a central point
-when a stone is thrown into the water, give one the idea that the water
-which forms the wave really moves towards the observer. But it is not
-so, as may be readily proved by placing on the surface a floating body,
-which we shall find is but little, if at all, influenced by the
-undulations of the water. The appearance of rolling waves given on the
-stage by means of a painted cloth, to which an undulatory motion is
-given, is an instance of this apparent movement. In the case of the
-floating body, which would follow the movements of the water, we shall
-find that wave after wave rolls to the shore, in the same way as the
-painted marks on the imitation sea keep their place, although the cloth
-itself undulates. The waves of the sea even appear to the eye to be
-endowed with a progressive motion, but an instant’s observation will
-convince us of our error; for if such were the case, every object
-floating on the ocean would be gradually carried on shore. A vessel
-floating on the waves is not carried along by them, at least not until
-it reaches within a few yards of the shore, where the water is really in
-motion; but out in the open sea a floating body will alternately rise on
-their crests, and fall into the valleys that separate them. The same
-effect may be observed with any object floating on the water. If,
-however, in addition to being in a state of undulation the sea is really
-in motion from the effects of a current, or from any other cause, the
-floating object will of course be carried along by it—in fact, the two
-movements are quite independent of each other, and may take place in
-similar or contrary directions. It is very important that we should be
-able to distinguish at an early period the exact difference between true
-movement and mere undulation; and we must remember that although the
-waves of light are propagated at the rate of 185,000 miles a second,
-still there is no transmission of any material substance at this
-marvellous rate. The same observation applies to sonorous vibrations
-transmitted through the air.
-
-Thus we are constrained to admit peaceably the truth of the undulatory
-hypothesis as compared with the corpuscular theory. I say _peaceably_,
-because I am forcibly reminded by the contrast I have made between the
-two theories of an anecdote related of one of the greatest monsters who
-ever walked this earth, but who was afterwards struck down in the midst
-of his power by the hand of a weak girl. I allude to the infamous Marat,
-who one day presented himself at the house of Dr. Charles, a celebrated
-natural philosopher, of the time of the first French Republic, in order
-to advance certain notions of his own against the optical principles
-that Newton has left behind in his _Principia_, and other works—also, to
-oppose certain theories connected with electrical science. Dr. Charles,
-who did not approve of Marat’s wild notions, undertook to convince him
-of his errors. But instead of discussing the matter peaceably, Marat
-allowed himself to be carried away by his temper, which was naturally
-very violent. Every argument advanced by his antagonist seemed to
-increase his rage, until at last he lost all control over himself, drew
-his small sword, and rushed upon his opponent. The doctor, who was
-unarmed, had to exercise all his powers to prevent himself from being
-wounded, and being much more stoutly built than Marat, he at last
-succeeded in throwing him down, and wresting his sword from him, which
-he immediately took care to break. Whether it was the violence of the
-fall, the shame he felt at being doubly beaten, or the effects of his
-fit of passion, does not appear, but Marat fainted. Assistance was
-called, and he was carried home to his house, his offence against all
-the laws of propriety being forgiven by his more talented and
-better-tempered adversary.
-
-There are many persons, no doubt, whom we should astonish, and possibly
-enrage, by asserting positively that we could cause darkness by means of
-light, that silence could be produced by sound, or cold by heat. These
-are daring paradoxes, and at first sight appear almost as reasonable as
-that of Anaxagoras, a Greek philosopher, who asserted that snow was
-black. But as I hope that most of my readers do not possess the
-passionate temper of the French tribune, I will confide to them a little
-secret that will make these paradoxes plain. It is called by natural
-philosophers the theory of interference.
-
-The experiments connected with this subject are exceedingly difficult to
-perform, and require the aid of apparatus far beyond the reach of the
-ordinary student. It is a case where theory and description are much
-easier than practice.
-
-If a ray of electric light is thrown upon a screen, it is possible to
-direct another ray upon the same spot in such a manner that they will
-extinguish each other mutually. The reason of this phenomenon may be
-understood, if we remember that light is caused by undulatory movement,
-and that by opposing two series of waves to each other in such a manner
-that their vibrations coming in contact produce rest, we can easily see
-how the waves of light of one ray may be stopped by those of a second.
-
-Going back to our illustration of the eddies on a pool of water, it is
-easy to prove that by throwing a second stone into the water we form
-another series of undulations; which are mutually destroyed when they
-encounter each other. It is the same with the peculiar fluid which,
-existing throughout space, is thrown in a state of undulation by
-incandescent bodies; by opposing one set of waves to another we obtain
-rest as a result.
-
-This fact was first observed by Grimaldi in 1665, and Dr. Thomas Young
-was the first to offer an explanation. Fresnel used it with great
-success at the beginning of the century to demonstrate the truth of the
-undulatory theory, by showing that it could not be explained by any
-other.
-
-
-------------------------------------------------------------------------
-
-
-
-
-                              CHAPTER II.
-                          THE SOLAR SPECTRUM.
-
-
-THE white light that the glorious orb of day spreads over the face of
-nature is the original source of all those brilliant and sombre colours
-with which the works of the Creator are beautified. To the rays of the
-sun we owe not only the whiteness of the lily, but the scarlet of the
-field poppy, the modest blue of the timid violet, the splendour of the
-peacock’s plumage, the cool green of the meadows, and the purple and
-gold of the distant mountains. For, as we have hinted before, this white
-light, which seems of itself so destitute of colour, is productive of
-every hue that the eye of man is capable of appreciating.
-
-It may seem that I am bestowing too much praise upon our own sun; but if
-you are surprised that I should seek to exalt this brilliant globe of
-ever-burning fire, I must ask you to recollect, that though the starry
-heavens are full of suns as vast and important as ours, and possibly
-affording brilliant colourless light to worlds full of inhabitants,
-there are others that give forth rays that are far from being white.
-Some are as green as emeralds, others are as blue as sapphires, while
-others give out a warm light like a ruby or topaz. The worlds which
-surround these can only receive light of a certain colour, or at any
-rate they are restricted to a few shades and hues. Imagine living in a
-world where everything was always _couleur de rose_, or in which the
-inhabitants were continually looking blue! A residence in either of them
-for a short time would undoubtedly cause us to appreciate the relative
-value of our own little sun, small as it is in comparison with some of
-the mighty orbs floating about in space.
-
-The fact that the light of the sun is the source of all the changing
-hues to be found on the surface of the earth season after season was
-first discovered by Newton, and his experiments are easily repeated with
-a very few and inexpensive appliances.
-
-A small round hole is made in the window-shutter of a room, facing the
-sun, and the pencil of light proceeding from it is allowed to fall upon
-the surface of a three-sided prism, held in a horizontal position, and
-placed at a distance of a few inches from the aperture (fig. 5,
-Frontispiece). The pencil of light does not pass through the prism as if
-it were a plate of glass with parallel sides, but in virtue of the laws
-of refraction, of which we have already spoken, it is turned out of its
-natural course, and is thrown upon the wall in the direction indicated
-in the figure. The pencil of light is not only turned aside, but it is
-also widened out into a band which is truly painted with all the colours
-of the rainbow, every tone and hue being of the most marvellous
-brilliancy. This long coloured stripe, which constitutes one of the most
-beautiful sights that the science of optics can afford us, is known to
-scientific men by the name of the solar spectrum.
-
-Before going into the causes that produce these colours, let us first
-examine their number and position. Beginning at the top, we shall find
-that they run in the following order:—Violet, indigo, blue, green,
-yellow, orange, red. The red being lowest is called the least
-refrangible of them all; or, in other words, in passing through the
-prism it was bent less out of its course than its companions. Violet,
-being at the top, is of course the most refrangible. The cause of the
-separation of the colours of white light is consequently only the effect
-of their individual character. They were, so to speak, so many streams
-flowing together until an unexpected deviation in their course caused
-them to separate. This change in the direction of their flow brought out
-their personal individuality, and they at once became completely
-disunited.
-
-Every single tint in the prismatic spectrum is simple, and cannot be
-decomposed. This may be shown by passing any of them through another
-prism, when it will be found that no change will take place in the
-colour or size of the pencil. Hence those worlds already spoken of,
-whose light of day is red, blue, or green, never see any colours but
-these. (Fig. 6, Frontispiece).
-
-It is just as easy to reunite the colours into which white light is
-decomposed, by applying a second prism in a reversed position to the
-pencil of coloured light, as it is to separate them in the first
-instance. The method of accomplishing this is shown in fig. 7,
-Frontispiece.
-
-[Illustration:
-
-  FIG. 8.—The Recomposition of Light.
-]
-
-Another experiment in the same direction consists in reuniting the
-colours by causing them to pass through a double convex lens, behind
-which is placed a screen of ground glass, or a card (fig. 8). By
-advancing and withdrawing this screen we can easily find the exact spot
-where the rays reunite, and form a dazzling spot of white light. This
-point is called the focus, from a Latin word, signifying “fire-place,” a
-term which will put the student in mind of the frequently repeated
-experiment of burning a piece of paper with an ordinary
-magnifying-glass.
-
-Instead of using a lens, you can, if you please, employ a concave
-mirror, using the ground glass or cardboard screen, as before. The
-colours reflected by the mirror unite at its focus, and produce a
-brilliant white spot in just as conclusive a manner as in the other
-experiment.
-
-[Illustration:
-
-  Fig. 9.—Recomposition of Light by means of a Concave Mirror.
-]
-
-A fourth experiment, which is somewhat more difficult for the student to
-accomplish, consists in causing every one of the seven different colours
-to be reflected from a separate mirror.
-
-The mirrors in this case are concave, and are so mounted as to be
-capable of being moved in any direction. By directing each of the seven
-rays, one by one, upon the same point, you may observe the gradual
-decomposition of the coloured light. The effect obtained by adding the
-last colour to the mixture is quite magical, the white circle being
-produced from two brilliantly-coloured spots.
-
-[Illustration:
-
-  Fig. 10.—Recomposition of Light by means of a number of Mirrors.
-]
-
-A fifth experiment, first devised by Newton, is also within the reach of
-the student. On a disc of cardboard the centre and border of which have
-been previously painted black, are pasted seven strips of paper, painted
-as nearly as possible of the same colour as the components of the
-spectrum—or if the student is anything of an artist he may paint the
-disc in imitation of the spectrum, carefully shading off the tints into
-each other. If the disc be now rapidly rotated the colours will
-disappear, and a greyish hue will be seen, which will approach more
-closely to white, the nearer the colours on the disc are to those of the
-spectrum. This experiment is not precisely the same in principle as the
-preceding ones, for it is evident that the colours on the disc do not
-mix, but only the impressions they form upon the retina. We have already
-said that such impressions remain on the eye for one-tenth of a second
-or there-abouts; the disc must therefore revolve at least ten times a
-second, or the effect will not be perceived.
-
-[Illustration:
-
-  FIG. 11.—Newton’s Disc.
-]
-
-From these experiments it follows that the colours with which all
-natural substances are clothed, ought not to be looked upon as belonging
-to them absolutely, but only as a property dependent on the reflection
-and absorption of light from their surfaces. The leaves of plants, for
-instance, must not be regarded as being really green in themselves, but
-as being capable of absorbing certain portions of light, and reflecting
-others. Grown in the dark, the green substance contained in the plant
-and its leaves becomes white, and no longer possesses the property of
-absorbing red light, and reflecting green. A green leaf placed in red
-light becomes almost black, from its power of absorbing light of that
-colour; in the blue it reflects a much greater proportion of the
-coloured ray. A very striking experiment may be performed with a
-substance known to chemists as the iodide of mercury. If a little of
-this salt, which is of a brilliant red, be placed in a watch-glass, and
-heated over a spirit-lamp, it will gradually sublime, and a card held
-over it will be covered with a number of light yellow crystals. In this
-case no change of composition has taken place, but simply a change in
-the power the salt possesses of reflecting some rays and absorbing
-others. By simply scratching the surface of the card with a pointed
-piece of wood, the yellow crystals become transformed once more into the
-red variety; not only this, the transformation gradually spreads, like a
-red cloud, over the whole of the deposit. There are some other salts
-known to chemists which possess the property of dichroism, or double
-colour. The double cyanide of platinum and barium, for instance, appears
-violet when viewed in one direction, and yellow in another. Change of
-temperature is often sufficient to change the colour of bodies—white
-oxide of zinc, for example, becomes bright yellow when heated. Such
-instances might be supplied _ad infinitum_, but enough has been said to
-prove that colour, after all, is only an appearance, and not an
-essential property of bodies.
-
-We have already spoken of complementary colours, or those which it is
-necessary to add together in order to produce white light. Blue, for
-instance, is complementary to orange, red to green, violet to yellow,
-and _vice versa_. But it is not by the aid of the palette that this can
-be proved, for in the case of coloured pigments the arrangement of their
-atoms interferes in some way with the success of the experiment, and it
-is only by means of the colours of the spectrum that such recompositions
-can be effected.
-
-Although most philosophers consider that there are seven colours in the
-spectrum, there are others who do not admit it, but assert that there
-are really only three, red, yellow and blue—which by the superposition
-of their edges produce the intermediate hues of green and orange.
-Perhaps it would be nearer to the truth to say that the spectrum is
-composed of an infinite number of colours of different hues.
-
-We have already stated that every one of these colours is
-indecomposable, and that there are certain worlds illuminated by a
-single colour only, instead of possessing the infinite number of tints
-enjoyed by the inhabitants of the solar system. An idea of this effect
-can easily be gained in a very simple but surprising manner by inserting
-panes of glass of different colours in the hole of the shutter of a dark
-room. If the light is yellow, you will find that all those objects that
-are capable of reflecting yellow light are coloured by it, while those
-which are bright red or blue become almost black by absorbing the only
-light present. If we could procure an object which was perfectly
-complementary in colour to the yellow glass, it would appear perfectly
-black. The same experiment may be repeated with the other colours. After
-remaining in this coloured light for some time, if you suddenly pass out
-into daylight the complementary colour will tinge everything around you.
-
-Instead of using a room into which coloured light only is admitted,
-lamps burning with a coloured flame may be employed. Brewster mentions
-the following experiment, which is a very striking one:—Fill a
-spirit-lamp with alcohol in which has been dissolved as much common salt
-as the spirit will take up; on being lit it will be found to burn with a
-livid yellow flame. A room lighted entirely with one or two lamps of
-this kind will form a laboratory for some very singular experiments. It
-should, if possible, be hung with pictures in water and oil colours, and
-the persons present ought to wear nothing but the brightest colours, and
-the table be ornamented with the gayest of flowers. The room being first
-lighted with ordinary daylight, the lamps above mentioned should be
-brought in, and the daylight carefully excluded, when an astonishing
-metamorphosis will take place. The spectators will be hardly able to
-recognise each other; the furniture of the room, and every other object
-contained in it, will reflect but a single colour. The flowers will lose
-their brilliant tints, the paintings will appear as if they were drawn
-in Indian ink. The brightest purple, the purest lilac, the richest blue,
-the liveliest green, will be converted into a monotonous yellow. The
-same change will take place in the countenances of those present; a
-livid paleness will spread over their faces, whether young or old, and
-those who are naturally of an olive complexion will hardly appear
-changed at all. Every one will laugh at the appearance of his
-neighbour’s face, without thinking that he is just as great a subject of
-laughter to them. If, in the midst of the amusement caused by this
-experiment, the light of day is admitted at one end of the room, the
-other end being still lighted with the salt-lamp, every one will appear
-to be half-illuminated with the livid colour which has caused so much
-surprise, the other portion of their figure and clothes being of the
-natural hue. One cheek, for instance, will appear animated with its
-usual brilliancy, while the other will be that of a corpse; one side of
-a lady’s dress will be brilliant blue or green, as the case may be, the
-other a colour that it would puzzle an artist to give a name to. The
-experiment may be varied by admitting the white light through several
-small holes in the shutter of the room, every luminous spot painting the
-place where it falls in its natural colours, and the yellow spectators
-will become spotted with the most singular tints and hues. If a magic
-lantern is used to throw on the walls of the room and the clothes of the
-company any luminous figures, such as those of flowers or animals, they
-will be coloured with these figures in the tint of the wall or fabric
-upon which they fall, yellowish colours of course escaping the
-transformation. If nitrate of strontia be substituted for the salt, a
-crimson tint will be spread over everything. In fact, a lamp prepared in
-this way will form a source of endless amusement. It is not necessary to
-use alcohol for the purpose; wood-spirit or methylated alcohol will
-serve the purpose equally well. If a lamp is not to be had, a few pieces
-of cotton-wool, tied on wires and dipped in the salted spirit, will do
-almost as well.
-
-
-------------------------------------------------------------------------
-
-
-
-
-                              CHAPTER III.
-                        OTHER CAUSES OF COLOUR.
-
-
-THE colours of the spectrum are to the sense of sight what the tones of
-the gamut are to the sense of hearing. On the one hand, the differences
-in the lengths of the sonorous waves constitute the variety of note
-perceptible by the ear; on the other, the differences in the lengths of
-the luminous waves constitute the variety of colour perceptible by the
-eye. By and by, we shall learn both the length and rapidity of these
-vibrations, but it will be as well first to describe the experiments
-made in this direction by the immortal Newton himself.
-
-Every one has, doubtless, at one period of his life, amused himself with
-blowing soap-bubbles by means of a tobacco-pipe and a little lather—a
-sufficiently childish amusement, you will possibly say, but one narrowly
-connected with the most intricate secrets of the science of optics.
-These little globes, so fragile that they disappear in a breath, hardly
-seem worthy of the attention of a thinker, and still less the
-examination of a philosopher; but it is nevertheless true that Newton
-made experiments on the colours shown on the surface of these apparently
-insignificant objects which ended in the most brilliant discoveries,
-just as on seeing an apple fall he began a train of thought which only
-terminated in the enunciation of the hypothesis of the earth’s power of
-gravity.
-
-All transparent substances, whether liquid, solid, or gaseous, become
-coloured with the most brilliant hues as soon as they are reduced to
-plates of extreme thinness. In the soap-bubble it is the oleaginous
-particles floating on the surface which thus become coloured, but Newton
-showed that thin plates of air were similarly capable of showing colour,
-and that the thinner the plates were the more brilliant were the tints.
-We may see this in the soap-bubble, which becomes more beautiful as it
-gets larger and thinner. By placing a convex lens of large size on a
-flat plate of glass, Newton observed that rings of different colours
-were formed round the spot where the two pieces of glass touched.
-
-[Illustration:
-
-  Fig. 12.—Newton’s Rings.
-]
-
-By measuring the convexity of the lens and the diameter of the various
-rings, Newton was enabled to tell to a minute fraction the exact
-thickness of the plate of air corresponding to the different colours.
-The glasses being placed in position, a ray of a particular colour—red,
-for instance—was thrown upon the surface. The result was a black spot at
-the point where the two surfaces touched, and surrounding it at various
-distances were several rings alternately red and black. Calculating the
-thickness of the plates of air at the part where the dark rings made
-their appearance, Newton found that their dimensions were in the
-proportion of the even numbers two, four, six, eight, &c.; while the red
-rings showed figures corresponding to the odd numbers. Although
-trammelled by the corpuscular theory, Newton’s deductions from these
-experiments show that they can only be accounted for by the undulatory
-hypothesis. Thus the thickness of the plate of air at the first red ring
-is that of the red wave, the thickness at the second that of two red
-waves, and so on; so that in order to arrive at the thickness of the red
-wave we need only measure the distance between the portions of the
-glasses where the first red ring occurs.
-
-This experiment, was applied to the measurement of all the waves.
-Whenever they were reflected on the glasses a parallel series of rings
-was formed, but it was found that the first ring was more or less
-distant from the central spot, according to the colour used. The red
-ring was the largest; the orange, yellow, green, blue, indigo, and
-violet, following in the same sequence as in the spectrum. The word
-“thickness” seems hardly fit to apply to dimensions arrived at by Newton
-in his experiments, so infinitely small do they appear to be, yet their
-correctness has never been impugned, although the experiments have been
-repeated by the philosophers of all countries. The waves of red light
-are so small that 40,000 of them go to an inch, and those of violet
-light situated at the other end of the spectrum are still smaller,
-measuring only the 60,000th part of an inch.
-
-The waves of the other colours are between these two, while the wave of
-white light, which is a mixture of them all, is just half-way between
-the two.
-
-Thus was the physical cause of the various hues of colour discovered by
-this great man, revealing as it does the singular and mysterious analogy
-between sound and light. The rays of light, like the waves of sound,
-produce a different effect, according to their length, by causing
-quicker or slower pulsations in the nerves of sight, just as musical
-sounds vibrate upon the drum of the ear with different velocities.
-
-This is not all, for the relationship between sound and light does not
-cease here: we have as yet only spoken of the size of the undulations,
-and have only shown how their dimensions are connected with the
-sensation of colour; but there are other things to be considered, for on
-investigation we find that not only do the different coloured waves vary
-in the length of their undulations, but also in the number that take
-place in a given time.
-
-The perception of sound is produced by the action of the drum of the
-ear, which vibrates sympathetically with the pulsations of the air that
-have been originated by the vibrations of the sounding body; and the
-perception of light is produced in a similar manner by the vibrations
-originating in a luminous body, and propagating themselves through the
-luminous ether until they reach the nerves of sight. The number of these
-pulsations taking place in the eye has been accurately determined in the
-following manner. Let us suppose that we are looking at a coloured
-object—let us say, a red railway signal-lamp; from the lamp to our eye
-there flows a continuous line of luminous undulations; these undulations
-enter the eye and become depicted on the retina. For every wave that
-passes through the pupil, there is a separate and corresponding
-vibration of the optic nerve, and the number of these vibrations that
-take place in the course of a second can be easily calculated if we know
-the velocity of light and the breadth of the waves. We have before found
-that light travels at the rate of 185,000 miles per second; it therefore
-follows, that a series of undulations 185,000 miles long pass through
-the pupil every second; consequently the number of vibrations per second
-is arrived at by calculating how many waves measuring the 40,000th of an
-inch—that being the length of a wave of red light—are contained in
-185,000 miles. The following table, showing the number of waves passing
-into the eye per second for the different colours, will interest the
-student:—
-
-             Extreme red     458,000,000,000,000 waves per
-                             second.
-
-             Red             477,000,000,000,000      "
-
-             Orange          506,000,000,000,000      "
-
-             Yellow          535,000,000,000,000      "
-
-             Green           577,000,000,000,000      "
-
-             Blue            622,000,000,000,000      "
-
-             Indigo          658,000,000,000,000      "
-
-             Violet          699,000,000,000,000      "
-
-             Extreme violet  727,000,000,000,000      "
-
-Whatever theory we may adopt to explain the phenomena of light, we
-arrive at conclusions that strike the mind with astonishment and
-admiration. According to the corpuscular hypothesis, it was supposed
-that the molecules of light were endowed with the power of attraction
-and repulsion, that they possessed poles and centres of gravity like the
-earth, and that they had other physical properties that could only be
-given to ponderable matter. Starting with these notions, it is difficult
-to divest oneself of the idea of sensible size, or to induce the mind to
-conceive particles so extremely small as those of light would
-necessarily be if the theory of emission were accepted. If a particle of
-light weighed a grain, it would produce by means of its enormous
-velocity the effects of a cannon-ball weighing 120 lbs., travelling at
-the rate of 300 yards per second. How infinitely small would be these
-particles, seeing that the most delicate optical instruments are
-submitted to their action for years without being injured!
-
-If we are astonished at the extreme smallness and prodigious rapidity of
-the luminous molecules whose existence is necessitated by the
-corpuscular theory, the numerical results of the undulatory hypothesis
-are not less surprising. The extreme smallness of the distance between
-the waves, and the inconceivable quickness of their undulations,
-although both are easily calculated, must raise in the mind of the
-student feelings of the utmost wonder and admiration.
-
-Colour, then, simply results from the difference in the rate of
-vibration of the rays, as Professor Tyndall observes in his lectures on
-the “Analogy between Sight and Sound,” the impression of red being
-produced by waves that undulate a third less rapidly than those which
-produce the sensation of violet.
-
-
-------------------------------------------------------------------------
-
-
-
-
-                              CHAPTER IV.
-LUMINOUS, CALORIFIC, CHEMICAL, AND MAGNETIC PROPERTIES OF THE SPECTRUM.
-
-
-THE solar spectrum may be compared to a battle-field with an army drawn
-up upon it ready for action. In the centre we find the luminous rays, on
-one side the light troops which produce chemical effect, and on the
-other the heating rays, which may be compared to squadrons of heavy
-cavalry. Close by the light brigade are the magnetic rays, which are a
-corps of skirmishers, sometimes appearing, and at others hiding
-themselves from view in a very mysterious manner.
-
-But to drop metaphor, we shall find on examination of the spectrum that
-the three great forces—heat, light, and chemical effect—are regularly
-distributed over three different portions of this wonderful band of
-colour.
-
-Before Fraunhofer the intensity of the light of different parts of the
-spectrum remained undetermined with any degree of accuracy; but this
-philosopher, by the use of a very delicate photometer, obtained the
-results given below.
-
-The maximum of luminous effect is situated just at the junction of the
-yellow and orange. Taking this spot as its starting-point, it gradually
-decreases on each side until it ceases altogether at the extreme red and
-violet.
-
-With respect to the calorific portion of the spectrum it was for a long
-time supposed that the heat-giving properties of any part were in direct
-proportion to the amount of its luminous effect; but Sir John Herschel
-proved by a long series of experiments that the heat of the spectrum
-gradually increased from the extreme violet to the extreme red, and that
-passing this point it still further increased until it attained its
-maximum at a point where not a single ray of light existed. From these
-grand experiments he adduced the important conclusion, that in solar
-light there existed invisible rays, which produced heat, and which
-possessed even a less degree of refrangibility than the extreme red
-rays. Sir John Herschel then tried, but unsuccessfully, to determine the
-exact refrangibility of the invisible heat rays.
-
-Sir Henry Englefield compared these results, and obtained the following
-figures:——
-
-                      Blue         56 deg. Fahr.
-
-                      Green        58     "
-
-                      Yellow       62     "
-
-                      Red          72     "
-
-                      Beyond the   79     "
-                      red
-
-Bérard obtained similar results, but he at first found that the maximum
-of heat was just at the end of the extreme red, and that beyond it the
-air was only about one-fifth warmer than the ordinary temperature. Sir
-John Herschel attributed these discordant results to Bérard having used
-a thermometer with too large a bulb; he accordingly repeated his
-experiments with other instruments with long narrow bulbs, and arrived
-at similar results to those obtained by the English philosopher.
-
-We will now pass on to the physical properties of the other end of the
-spectrum. Towards the end of the last century, Scheele, a Swedish
-philosopher, remarked that chloride of silver was blackened more quickly
-by the violet portion of the spectrum than by any other. In 1801, Ritter
-of Genoa, in repeating certain experiments made by Herschel, found that
-a much stronger blackening effect was produced at a point beyond the
-violet, and that the discoloration was produced with less intensity by
-the violet and still less so by the blue, the change gradually
-decreasing till the red ray was reached. He also found that when
-slightly blackened chloride of silver was exposed to the effects of the
-red rays, or even in the space beyond, its colour was restored to it.
-From these facts he drew the conclusion that in the solar spectrum there
-existed two kinds of rays, one at the red extremity, which favoured
-oxygenation; the other, at the blue end, which possessed the contrary
-properties. He also found that when phosphorus was placed in the
-invisible rays beyond the red, it gave off fumes of oxide, which were
-immediately extinguished when it was transferred to the other end.
-
-On repeating the experiment with chloride of silver, Lubeck found that
-the tint varied according to the colour in which it was placed. Beyond
-or in the violet ray it became brownish red, in the blue it became
-bluish or bluish grey, in the yellow it remained white, or became
-slightly yellow and reddish in or beyond the red ray. When he used
-prisms of flint glass, the chloride of silver was discoloured beyond the
-visible limits of the spectrum.
-
-Without being aware of Ritter’s experiments, Dr. Wollaston obtained the
-same results by acting on chloride of silver with violet light. In
-continuing his researches he discovered that gum guaiacum was also
-influenced by the chemical rays of light.
-
-The magnetic influence supposed to be exerted by the solar rays still
-remains without positive proof, although numbers of philosophers have
-experimented in this direction. More than fifty years ago Dr. Morichini
-announced that the violet rays of the solar spectrum possessed the
-property of magnetizing steel needles that were previously free from
-magnetism. He produced this effect by concentrating the violet rays upon
-one-half of each needle with a convex lens, taking care to keep the
-other half concealed beneath a screen. After having continued this
-experiment for more than an hour, the needles were found to be quite
-magnetic.
-
-Dr. Somerville tested Morichini’s experiments by covering one-half of an
-unmagnetized needle an inch long with a piece of paper, and exposing the
-uncovered half to the violet rays of the spectrum, and found that the
-needle became magnetic in the course of a couple of hours, the exposed
-end being the north pole. The indigo rays produced almost the same
-effect, but the blue and green rays were much less powerful. When the
-needle was exposed to the yellow, orange, red, and invisible rays beyond
-the red, no magnetic effect was produced, although the experiment was
-continued for three days. Pieces of chronometer and watch springs were
-submitted to the same influences with a similar result; but when the
-violet rays were concentrated upon the needles and pieces of spring with
-a lens, the time necessary for magnetizing them was greatly reduced.
-
-Baumgartner of Vienna and Christie of Woolwich also repeated these
-experiments. The latter philosopher found that when a needle of
-magnetized steel, copper, or even glass, vibrated by force of torsion in
-the rays of the sun, the arc of vibration diminished much more quickly
-than when the experiment was conducted in the shade. The sun’s rays
-appeared to have the greatest effect upon the magnetized needle. From
-these results Christie concluded that the solar rays were capable of
-exerting a certain amount of magnetic influence.
-
-These experiments were afterwards fully confirmed by those of Barlocci
-and Zantedeschi. The former found that a natural magnet which was
-capable of supporting a pound weight, had its power almost doubled by
-exposure to strong sunlight for four-and-twenty hours. Zantedeschi
-exposed a magnet which would carry fifteen ounces to the sun for three
-days, and increased its power two and a half times. These experiments
-seem almost to decide the fact of the power of white and violet light to
-induce magnetic force; but a series of researches by a philosopher who
-without doubt is greater than any of those already mentioned, seems to
-throw some doubt on the facts we have related above.
-
-Before concluding, we must add a few more facts relating to the
-existence of invisible rays at both ends of the spectrum. “The visible
-portion of the spectrum,” says Dr. Tyndall, in one of his Royal
-Institution lectures, “simply marks an interval of radiant action, the
-rays existing in which bear such a relation to our visual organs, as to
-be capable of exciting in them the sensation of light. Beyond this
-interval, in both directions, right and left, the radiant action
-continues to exercise itself, but the rays emitted are dark, in
-consequence of their exerting no influence on our eye. Those that exist
-beyond the red ray are capable of producing heat, while those that are
-beyond the violet excite chemical action. These invisible violet rays
-can be actually made perceptible to the eye, or, in other words, the
-undulations or waves proceeding from this end of the spectrum can be
-made to strike against certain substances and induce luminous
-vibrations, so as to connect the dark space beyond the violet with a
-brilliantly illuminated band. I have here a substance capable of
-effecting this change. The lower half of this sheet of paper has been
-moistened with a solution of sulphate of quinine, the other half being
-left in its ordinary condition. I will now hold the paper in such a
-manner that the line that separates the prepared half from the other
-shall cut the spectrum in two halves horizontally. The upper half will
-remain unaltered and may be readily compared with the lower half, upon
-which you will see the spectrum prolonged beyond its ordinary limits.
-The effect produced is the addition of a splendid band of fluorescent
-light, which extends over a space of several inches, which but an
-instant before was a dark mass. I withdraw the prepared paper, and the
-light disappears; I replace it, and the light shines forth once more;
-showing us in the most brilliant way that the visible limits of the
-ordinary spectrum are not the limits of radiant action.
-
-“I plunge a pencil into the solution of sulphate of quinine, and I pass
-it over the paper. You see that wherever the solution falls, the light
-bursts forth. The existence of these rays has been known for a long
-time. Young was familiar with them, and subjected them to experiment;
-but it is to Professor Stokes that we are indebted for a complete series
-of researches on this subject. It was he who first made those invisible
-rays visible, as we have done.”
-
-In the same way the Professor proceeded to show that the heat rays were
-invisible by passing a beam of sunlight through a solution of iodine in
-spirits of wine, which, although it completely stopped all light,
-allowed the heat rays to pass uninterruptedly. By collecting these
-invisible rays into a focus by means of a lens, Dr. Tyndall was enabled
-to ignite various combustible bodies.
-
-Thus we see the reason why certain rays produce certain effects on the
-eye, each particular degree of refraction causing a different set of
-vibrations, resulting in a different sensation for every part of the
-spectrum, and reproducing the effect of various colours on the optic
-nerve. In the following chapters we shall conclude our account of the
-different colours in the spectrum and of the laws of light.
-
-
-------------------------------------------------------------------------
-
-
-
-
-                               CHAPTER V.
-                    THE LAWS OF REFLECTION.—MIRRORS.
-
-
-WHEN a ray of light falls obliquely on any polished surface, as that of
-a mirror, a piece of water, a plate of burnished metal, or any other
-reflecting substance, the ray, like an elastic ball, is immediately
-projected in a contrary direction to that in which it fell. Moreover,
-the direction in which it is reflected is at right angles to the
-surface, and in the same plane as that of the ray in the first instance.
-This experiment may be tried very easily, and will show the reason for
-the two following laws.
-
-1. The angle of incidence is equal to the angle of reflection, and _vice
-versâ_.
-
-2. Reflection can only take place in one direction—in that of the
-incident rays, both of which are always in a plane perpendicular to the
-reflecting surface.
-
-The following figure will assist the student in performing experiments
-on the reflection of light from flat surfaces.
-
-The ray A B falling obliquely on the horizontal mirror, is reflected
-upwards at the same angle in the direction B C. This may be proved
-geometrically by placing a graduated circle in a vertical position in
-the plane A B C, when we shall find that the angle A B D formed by A B
-(the incident ray) with the perpendicular D B is equal to the angle
-formed by this perpendicular line and the reflecting ray B C. You may
-also prove in the same way that these three lines are all in the same
-vertical plane.
-
-[Illustration:
-
-  FIG. 13.—Reflection from Plane Surfaces.
-]
-
-Let us now examine the effects of light reflected from plane surfaces.
-We must first, however, notice a certain optical illusion to which we
-are continually falling a prey, almost without our knowledge. We always
-fancy objects to be in reality in the place where we see them, and, in
-spite of our having already enumerated a large number of these
-deceptions, we must still add one more to the list. In reality we rarely
-see objects in the place where they really are; for if by the effect of
-reflection, refraction, or any other cause, the rays of light are made
-to deviate from their course, we no longer see the object from which
-they proceed in its real position, but in the direction taken by the
-luminous pencil at the moment of entering the eye.
-
-[Illustration:
-
-  FIG. 14.—Refraction.
-]
-
-[Illustration:
-
-  FIG. 15.—Experimental Proof of Refraction.
-]
-
-For instance, if the ray A B is bent during its passage to the eye at B,
-and consequently reaches it in the direction B C, it is at A´, and not
-at A, that we shall see the object from which it proceeds. Every ray of
-light which passes out of a medium of a certain density into another of
-a different density is bent from its primary course, or, in scientific
-language, it is refracted. The experiments we made in a former chapter
-on the properties of the prism are founded on this principle. The law
-may be easily illustrated by allowing a ray of light to fall upon the
-surface of a vessel of water, as shown in the preceding figure.
-
-[Illustration:
-
-  FIG. 16.—The Effects of Plane Mirrors.
-]
-
-The light of the stars and planets undergoes a similar deviation when
-passing in its course through the earth’s atmosphere; and at the moment
-we see the rising of the sun, the moon, or a star, they are in reality
-still below the horizon. Our eyes consequently are still deceiving us,
-no matter what part of the domain of optics we may enter.
-
-There are two kinds of mirrors—plane and curved. We will first examine
-the properties of the former sort, being those which are ordinarily
-applied to the usages of every-day life.
-
-[Illustration:
-
-  FIG. 17.—Reflection from the Surface of Water.
-]
-
-In the figure in the preceding page we have a young lady looking at her
-reflection in a tall cheval glass. Every point upon the surface of her
-clothes and face is reflected back to her eye from the surface of the
-tin amalgam which has been applied to the back of the mirror by the
-looking-glass maker, for the purpose of rendering the image of the
-object more brilliant than if the glass alone were used. The rays which
-proceed from every one of these points strike upon the surface of this
-metallic layer, are stopped by its opacity, and are reflected back to
-the eye at an angle equal to that at which they strike the surface. The
-image seen by the eye is formed, consequently, by the reflection of
-every one of these rays; and as we always see objects in the direction
-taken by the luminous ray at the moment it enters the eye, we fancy we
-see objects before us that are really behind, or on each side of us. For
-instance, the ray starting from the left foot of the young lady in the
-figure is reflected from the point indicated on the surface of the
-glass, but the eye does not stop here, but sees the foot at an equal
-distance beyond the mirror.
-
-The same thing takes place, not only with glass, but with all substances
-having polished surfaces. Still water, which to all intents and purposes
-has a polished surface, reflects the objects within its range as
-perfectly as a mirror.
-
-The preceding observations apply to all plane reflecting surfaces; but
-there are other sorts of mirrors, whose effects are of a more
-interesting nature, and which we must hasten to describe—we allude to
-those whose surfaces are either convex or concave.
-
-Curved mirrors are made of a great variety of shapes, but for the
-present we shall only describe those which are spherical. Spherical
-mirrors may of course be either concave or convex.
-
-[Illustration:
-
-  FIG. 18.—Concave Mirror.
-]
-
-Suppose the arc M N (fig. 18) to be movable round the point O, this
-revolution will describe the surface of the mirror. The central point C
-of the hollow sphere of which the mirror forms part, is called the
-centre of curvature, the line O L the principal axis. By remembering
-these very simple definitions, we shall be able to understand the action
-of these mirrors without the slightest difficulty.
-
-To understand how the rays of light are reflected from the surface of
-the mirror N M at the point F, which is called the focus, we have only
-to consider the mirror as consisting of an infinite number of facets,
-all inclined towards that particular point, and forming by reason of
-their immense numbers a regular spherical surface. In considering the
-mirror from this point of view, we can immediately see that, on account
-of the inclination of the supposed facets, the rays that they receive
-are all reflected back again at the same point; and it may be proved
-geometrically, that when the incident rays are parallel the focus will
-be situated somewhere on the line O C, its position depending on the
-curvature of the mirror.
-
-If, therefore, we receive on a spherical mirror a pencil of sunlight,
-the rays which compose it may be regarded as parallel, the sun being at
-so great a distance from the earth; it follows that these rays will all
-be reflected together in a particular point, viz., at F, and if any
-object be placed there it will be illuminated with great brilliancy. The
-laws governing the reflection of heat being nearly similar to those
-regulating the action of light, the rays reflected from a burning body
-will ignite any inflammable substance placed at the point F. The focus
-for parallel rays is called the principal focus of a mirror. Having
-described the effects of parallel rays, let us now see what happens when
-the source of light is close to the mirror. If it is placed at a very
-small distance, the luminous rays are divergent instead of parallel, and
-their meeting point becomes changed in accordance with the laws laid
-down at the beginning of this chapter. That is to say, the focus will
-approach more or less to the centre of curvature C, according as the
-source of light is placed nearer to or further from the mirror;
-consequently, in the case of the candle in fig. 19, instead of uniting
-at F, the rays will meet at _f_, a point situated somewhat nearer the
-mirror than the principal focus. If, instead of placing the light at A,
-we place it at _f_, we shall find the rays will be concentrated at the
-point A. Thus the foci are consequently related to each other, and are
-hence called _conjugate foci_. It will be readily seen that a spherical
-mirror may have an infinite number of conjugate foci, according to the
-distance of the source of light. It is also clear, that if we cause the
-light to approach the mirror, the focus will also approach it.
-
-[Illustration:
-
-  FIG. 19.—Conjugate Foci.
-]
-
-Continuing our experiment, we shall find that when the candle passes the
-principal focus so as to be between it and the mirror, the reflected
-rays first become parallel and then divergent, and cannot consequently
-produce any focus beyond the mirror, but are reflected in the way shown
-in fig. 20.
-
-In experimenting on the plane mirror, we imagined we saw the object at a
-certain distance behind it; the same thing happens when we see ourselves
-reflected in a concave mirror, and the particular point at which we
-suppose we see our reflection is called the virtual focus.
-
-[Illustration:
-
-  FIG. 20.—Virtual Focus.
-]
-
-If instead of a candle we place our head before a concave mirror, we
-shall see ourselves magnified as in fig. 21.
-
-[Illustration:
-
-  Fig. 21.—Concave Mirror.
-]
-
-We shall easily see how this happens by tracing the paths of the rays in
-fig. 22.
-
-[Illustration:
-
-  Fig. 22.—Magnifying effect of Concave Mirrors.
-]
-
-[Illustration:
-
-  Fig. 23.—The Reversal of real Images.
-]
-
-The rays, for instance, which proceed from the forehead at the point _a_
-are reflected from the point _o_ to the eye in such a way as to appear
-to proceed from a point beyond the mirror, A. In the same manner the
-rays reflected from the chin appear to take their origin from the point
-B. If, on the other hand, we place ourselves at a distance from the
-principal focus, we shall produce a reversed and diminished image of our
-face. This image is not illusory, like the preceding ones, but is real,
-and may be received upon a screen, as shown in fig. 23.
-
-We may easily follow the path of the rays as shown in the figure, and we
-shall see that the rays forming the images of the church-tower and the
-terrace below, cross at a certain point.
-
-Convex mirrors produce precisely opposite effects, and give a diminished
-image instead of a magnified one, as may be perceived on examining fig.
-24.
-
-
-[Illustration:
-
-  FIG. 24.—Diminishing power of Convex Mirrors.
-]
-
-
-------------------------------------------------------------------------
-
-
-
-
-                              CHAPTER VI.
-                       METALLIC BURNING MIRRORS.
-
-
-THE classical student will remember that Archimedes burned the fleet of
-Marcellus, by means of burning-glasses, from the heights of the
-fortifications of his native city of Syracuse. Unfortunately, any
-account of the system of catoptrics, or the science of reflections,
-employed by the ancient Syracusan in their construction is lost to us,
-and many modern writers have gone so far as to doubt the fact
-altogether. The knowledge of the properties, however, of concave mirrors
-which we have just been acquiring, will enable us to form a pretty good
-guess as to the means adopted by Archimedes for the destruction of the
-enemy’s fleet. The ancients, not having the means of either casting or
-grinding such enormous mirrors, must have constructed them of a large
-number of small ones, so arranged that the images of the sun reflected
-by them would all fall in the same place, or nearly so. In this case,
-the larger the number of mirrors, the greater would be the burning
-effect. In order to explain the reflection of rays incident upon the
-surface of concave mirrors, we supposed them to consist of an immense
-number of plane mirrors placed in a curve, so that the reflected rays
-might all meet in one point; but on examining into the history of
-burning mirrors, we find that the plan has been adopted in reality in a
-great number of instances. We have also said, that the reflection of the
-heating rays was governed by similar laws to those influencing the rays
-of light; consequently, by directing a pencil of sunlight upon the
-surface of a concave mirror, we obtain the maximum of light and heat at
-the focal point.
-
-Many modern writers give the ancients too little credit for their
-knowledge of optical principles, and late investigations seem to prove
-that the old school of philosophers were much more learned in these
-matters than has been generally supposed. The discovery of a rock
-crystal double convex lens in an Egyptian tomb of great antiquity is an
-instance of this. Descartes wrote a little treatise to prove that the
-stories related of the burning mirrors of Archimedes were pure
-fabrications, although many Latin authors have described them both as
-being used by that philosopher and in more modern times; Dion, for
-instance, who lived in the early part of the sixth century, states that
-at the siege of Constantinople, Proclus burnt the fleet of Vitalian with
-mirrors of brass; but the opinion of Descartes seemed to outweigh all
-other testimony. Buffon, who wished to sift the matter thoroughly,
-constructed for himself, after many previous experiments on the laws of
-reflection, a series of mirrors that closely imitated those ascribed to
-Archimedes. His first memoir, “On the Invention of Mirrors capable of
-burning at a great Distance,” was published in the Transactions of the
-French Academy of Sciences for 1747. A few years later he combated both
-theoretically and practically the opinion of Descartes, in a memoir
-containing an account of an immense number of experiments. Before
-speaking of the extraordinary effects of burning mirrors, it will be as
-well to do justice to the predecessors of the learned naturalist we have
-just mentioned, by quoting a passage from the works of Father Kircher,
-who, 128 years previously, experimented in this direction with great
-patience and perseverance, and tried to prove that the stories related
-of Archimedes were true. “The larger the surface of a mirror,” says this
-philosopher (who, like Huyghens, was a practised astronomer), “the more
-light it reflects from the objects opposite to it. If it is only a foot
-square, it will throw a square foot of light upon any wall or screen
-placed before it. Experiment shows that this light is composed of an
-infinite number of rays reflected from different points on the surface
-of the mirror. Direct the rays from a second mirror upon the same place
-as those from the first, and the light and heat will clearly be doubled.
-They will become trebled if you direct the rays from a third mirror upon
-the same spot, and so on _ad infinitum_. In order to prove that the
-intensity of the light and heat is in direct proportion to the number of
-reflecting surfaces employed, I took five mirrors, and found that on
-exposing them to the sun I obtained with only one, less heat and light
-than if I used direct sunlight. With two the light and heat increased
-considerably; three gave as much heat as an ordinary fire, and four gave
-me a still greater effect. I therefore concluded that by multiplying
-these plane mirrors, I not only obtained greater effects than those got
-by using parabolic, hyperbolic or elliptical mirrors, but that I could
-use them upon objects at a much greater distance. With five mirrors I
-could obtain these effects at a distance of 100 feet, but what terrible
-phenomena would have taken place had I used one thousand instead of
-five?” He ends by begging mathematicians to experiment in this direction
-with greater care than they had hitherto done.
-
-After Kircher we may cite as an experimentalist with these terrible
-instruments the French philosopher Villette, who constructed several
-mirrors, in direct imitation of those of Archimedes, for Louis XIV. and
-other sovereigns. The _Journal des Savants_ for 1679 gives an account of
-his principal metallic burning mirror in the most eulogistic terms,
-adding an instance of ignorance which is singularly quaint and curious.
-It is of the fourth and most perfect of Villette’s mirrors that the
-_Journal des Savants_ speaks, the first having been bought by Tavernier,
-and presented to the Shah of Persia, who considered it as one of the
-rarest and most precious curiosities that he possessed: the second was
-sold to the King of Denmark, and the third was given by M. Villette to
-Louis XIV., from whom he received the praises and rewards that were due
-to his talent and perseverance. “It was thirty-four inches in diameter,
-and vitrified flints and bricks almost instantaneously, no matter how
-large they were. It consumed the greenest wood, burning it to ashes in
-an instant, and fused the most refractory metals with equal ease and
-quickness. Steel, no matter how hard, resisted its power no more than
-other metals, and melted so quickly that one part burnt away in
-inconceivably brilliant sparks, some of them forming stars as large as a
-franc piece, leaving a flowing mass of metal behind. The last made by
-Villette was still more powerful, being larger and more carefully made.
-It was forty-four inches in diameter, and three inches and a line deep.
-Its burning point, or focus, was situated at a distance of three feet
-seven inches from the surface, and was apparently as large as a five-sou
-piece; and it was at this spot, where the rays of light and heat were
-concentrated into so small a space, that the wonderful effects of its
-violent power became manifest, the spot of light being of such
-brilliancy that the eyes could no more withstand its brightness than
-that of the sun. Besides the property of burning which it possessed in
-so wonderful a degree, it was capable of exhibiting other effects just
-as curious as those already related. It had the power of sending the
-images of objects to a distance of fifteen feet or more, so that a man
-looking at himself in this mirror with a stick or sword in his hand, saw
-the image of them suspended in the air, apparently ready to strike the
-observer. On seeing such an effect for the first time, the observer
-could hardly fail to experience the greatest surprise, and even fear;
-and it is stated that the king having placed himself, sword in hand,
-before one of these mirrors, in order to observe the effect, was
-surprised to find himself face to face with an armed hand apparently
-directed against him. When he advanced, the hand seemed to spring
-forward to meet him. The king could not conceal his surprise and fright,
-and afterwards felt so ashamed at being terrified with a mere shadow
-that he ordered the mirror to be taken away, and could never be
-prevailed upon to look into it again.” The _Journal des Savants_ then
-goes on quaintly to remark on the various startling effects produced by
-these mirrors, winding up by stating that its powers of reflection were
-so great, that at night the light of a torch or flambeau was reflected
-so perfectly that an observer placed at four hundred feet distant could
-read the smallest print.
-
-It also mentions a curious piece of superstition on the authority of a
-scientific writer of the name of Robertson, who states that it happened
-at Liége. In reading the accounts of these experiments we can see how
-easily the minds of individuals were affected in those days by the
-wonderful. It happened while one of Villette’s mirrors was at Liége,
-that the latter end of the summer was somewhat rainy, and great fears
-were entertained that a bad harvest and dear bread would be the result.
-Certain evil-minded people, who had taken a fancy to the mirror and
-wished to possess it by unfair means, spread the report that the
-continual rain was entirely caused by its action on the clouds and sun,
-and that the coming famine must be laid upon the shoulders of its owner
-and inventor. This absurd idea took such forcible possession of the
-minds of the populace of Liége, that great mobs collected together,
-uttering all kinds of maledictions against the mirror and its inventor,
-and at last became so violent that they attacked Villette’s house with
-the intention of smashing his great work, and administering to the
-unfortunate philosopher the chastisement they supposed he deserved.
-Happily, however, for M. Villette and his mirror, Liége was governed in
-those days by the Prince Bishop of Cologne, who was a man of great
-enlightenment. He put the crowds round M. Villette’s to flight by armed
-force, but he found that the conviction that all the coming mischief
-would result from the unlucky mirror was so strong, that he was obliged
-to issue a pastoral peremptorily declaring that the idea had originated
-with a number of malicious people, who spared no pains to propagate it
-for their own bad purposes, and that it was a mischievous and dangerous
-error to ascribe to a mirror a power which only belonged to the
-Almighty.
-
-In 1747, Buffon performed many extraordinary experiments with burning
-mirrors, which were more surprising than any that had hitherto been
-described. They were mostly performed at the _Jardin des Plantes_, at
-Paris, of which institution Buffon was director; and many of them are
-worth describing.
-
-[Illustration:
-
-  FIG. 25.—Burning Mirror.
-]
-
-On the 3rd of April, at about two o’clock in the afternoon, the great
-mirror was mounted on its stand, and was found to be capable of setting
-a plank of wood on fire at a distance of 138 feet, when 128 glasses were
-used, although the light was weak at the time, and the sun was covered
-with mist. In pursuing these experiments great care had to be taken to
-prevent the by-standers placing themselves within range of its terrible
-power, for several were nearly blinded by looking at the brilliant focal
-point of the instrument. The next day, at eleven in the forenoon,
-although the sun was still covered with mist and fleecy clouds they were
-able to produce such a heat at 150 feet distant, with 154 glasses, that
-a pitched plank began to smoulder and would have burnt into flame had
-not the sun disappeared at that particular moment. On the fifth of
-April, at three in the afternoon, with the light much in the same weak
-condition as it was on the other days, they succeeded in igniting at 150
-feet distant, a heap of shavings of deal mixed with charcoal and
-sulphur, in less than a minute and a half, with 154 glasses. When,
-however, the sun shone with its natural power, a few seconds were
-sufficient to effect these results.
-
-On the 10th, when the sun was shining pretty powerfully, a pitched pine
-plank was easily fired with 128 glasses, at 150 feet distant. In this
-case the ignition was very sudden, and extended over the whole of the
-radiant spot forming the focus, which at the distance named measured 16
-inches in diameter. The same day at half-past two, a pitched elm plank
-covered in some places with chopped wood, was set fire to with extreme
-rapidity, and burnt with such violence that it had to be dipped in water
-before it could be put out. In this experiment 148 glasses were used, at
-a distance of 150 feet.
-
-On the 11th of April, the burning point was fixed at 20 feet distant
-from the mirror, and combustible substances were easily burnt with only
-12 glasses. With 21 glasses a half-burnt elm plank was set fire to, and
-with 45 a piece of tin weighing six pounds was almost immediately
-melted. Silver sheet was fused, and an iron plate was made red-hot with
-117 glasses. In giving an account of these interesting experiments,
-Buffon expresses his conviction that at 50 feet it would have been easy
-to have melted metals if all the glasses of the mirrors had been used.
-When used at that distance, the burning spot was six to seven inches in
-diameter. He also noticed that when metals were melted, part of them
-were dissipated in brilliant vapour, which was so thick as to cast a
-shadow on the ground, although it seemed to be as bright as the sun
-itself. When the sun was at its full strength, and all the glasses were
-brought into requisition, wood was set on fire at a distance of over 200
-feet, and metals and minerals were fused at 40 and 50 feet. Hence the
-possibility of making and using these mirrors as Archimedes was said to
-have done, was proved practically by the great naturalist. Fig. 25
-represents a burning mirror in action.
-
-Robertson, an English philosopher, residing in France during the days of
-the first Republic, reconstructed the mirrors described by historians as
-being used by Archimedes, and the results he obtained were thought
-sufficiently important by the Council of the Department of Ourthe to
-merit an attentive examination by two members of their body, who
-reported in favour of their being used as instruments of war.
-
-It would be possible to pursue this subject still further, and give an
-account of numerous experiments made on burning mirrors by various
-philosophers, but we must not forget that it is light and heat that we
-have more especially to deal with in the present work. Already we have
-possibly strayed from our path a little too far, but the two influences
-are so closely connected with each other that it is almost impossible to
-speak of them separately when reflection is in question.
-
-
-------------------------------------------------------------------------
-
-
-
-
-                              CHAPTER VII.
-                                LENSES.
-
-
-THE word lens is derived from the Latin name of the seed of the _Ervum
-lens_, or ordinary lentil. When eating this wholesome vegetable, almost
-every one has noticed that its shape is exactly that of a double convex
-lens, as represented in the following figure:—
-
-[Illustration:
-
-  FIG. 26.—Double Convex Lens.
-]
-
-Perhaps it would be more correct if we were to say that a double convex
-lens is like a lentil, rather than turn the comparison the other way,
-seeing that this little seed has given its name not only to the
-particular-shaped glass depicted above, but also to some five others
-more or less analogous to it.
-
-In fig. 27 we have the different forms of lenses shown in section. The
-first is the _double convex lens_, the second the _plano-convex_, the
-third and sixth the _concavo-convex_, the fourth the _double concave_,
-and the fifth the _plano-concave_. A _crossed lens_ is a double convex
-lens whose one side is more convex than the other. The third lens is
-also called _meniscus_.
-
-[Illustration:
-
-  FIG. 27.—Forms of Lenses.
-]
-
-The properties of the first, second, and third are similar; that is to
-say, they cause parallel rays of light passing through them to converge
-at a certain point, called their focus; while the three others have a
-divergent action on rays passing through them. By examining the path of
-the rays through these lenses, we shall find that the first three
-magnify objects seen through them, while the latter have the contrary
-effect.
-
-As in the case of the curved mirrors, the rays falling on the surface of
-a convex lens may be either parallel, divergent, or convergent. In the
-case of parallel rays, as depicted in the following figure, they are
-represented as meeting at a point beyond the lens, which is called the
-sidereal focus, or the focus for parallel rays. It is generally found by
-causing the image of the sun or of some distant object to be thrown by
-the lens upon a screen, or by knowing the curvature of the faces, and
-the refractive power of the glass.
-
-Every ray on striking the surface of the lens is refracted inwards,
-until it meets with its companions at the focus F, in accordance with
-the law of refraction, by which a ray of light passing from one
-transparent medium, such as air, to another which in this instance is
-glass, becomes refracted or bent in proportion to the relative density
-of the two mediæ. The nearer the ray passes to the edge of the lens, the
-more it is refracted, the angle of incidence being greater; the ray
-through the exact centre being uninfluenced by the form of the glass.
-Hence they all meet in a single point. Figs. 29 and 30 show the path of
-the rays when they are divergent and convergent.
-
-[Illustration:
-
-  FIG. 28.—Path of a Ray through a Convex Lens.
-]
-
-[Illustration:
-
-  FIG. 29.—Path of divergent Rays through a Convex Lens.
-]
-
-If the rays of light are not parallel, as in the case of the source of
-light being near the lens, they do not converge so rapidly as when they
-proceed from a distant object, consequently the focus for near objects
-is longer in proportion to their distance. In fig. 29 for instance, if a
-candle be placed as shown, and a screen on the other side of the lens, a
-point will be found where the image of the candle is seen upon it in a
-reversed position. The distance between these two points is always
-relative, and they are called conjugate foci. Thus, the candle may
-change places with the screen with a similar effect, as long as the
-exact position of the two points is preserved. If the candle is placed
-farther off, we must diminish the distance between the screen and the
-lens, and _vice versâ_. In fact, the nearer the object, the longer the
-focus; the farther it is off, the shorter the focus. Half an hour’s
-experiment with a double convex lens, a piece of white cardboard, and a
-small candle, will teach the student more about the properties of convex
-lenses than a chapter of explanation. A common magnifying-glass, or even
-an old spectacle lens, will serve the purpose of more expensive
-instruments.
-
-[Illustration:
-
-  FIG. 30.—Conjugate Foci.
-]
-
-We now proceed to speak of the images formed by lenses. In fig. 31 we
-have a flower placed on one side of a lens. As it is not at an infinite
-distance, the rays sent out by its various parts are convergent, and not
-parallel, consequently they do not meet at the sidereal focus, but at a
-point beyond it, according to the rule already laid down. The rays
-proceeding from the exact centre of the flower striking the lens exactly
-in the middle at right angles, suffer no change, the others being
-refracted in proportion to their angles of incidence.
-
-[Illustration:
-
-  FIG. 31.—Images formed by Convex Lenses.
-]
-
-The rays proceeding from the flower cross each other at a certain point:
-hence the image on the screen is reversed. The dimensions of the image
-will depend on the distance of the object from the lens. This is a fact
-we meet with every day, when using an opera-glass or a telescope. Images
-formed by convex lenses upon a screen are called by opticians _real
-images_, in contradistinction to those which are the result of mere
-reflection, as in the case of plane mirrors. These latter are known as
-_virtual images_ and are produced by convex lenses as well as by plain
-reflecting surfaces. In fig. 32, for instance, the unreversed image of
-the insect seen by the eye is not a real image, but a virtual one,—a
-fact that might be easily proved by placing a screen in the position of
-the eye, when it would be found that no image would be formed.
-
-When using an ordinary magnifying-glass we see the virtual image of the
-object we are looking at, but in the case of a telescope or opera-glass
-we see the real image of the object, formed by the large lens in front,
-and reversed again by the arrangement of small lenses next to the eye.
-
-[Illustration:
-
-  FIG. 32.—Magnifying Property of Convex Lenses.
-]
-
-Double concave lenses produce effects which are just the reverse of
-those we have been considering. Instead of increasing in thickness from
-the edges to the centre, they follow the contrary plan, and increase
-from the centre to the edges. Consequently, instead of the rays meeting
-at the focus, they diverge from each other, and gradually spread out, as
-shown in fig. 33.
-
-[Illustration:
-
-  FIG. 33.—Diminishing Effect of Concave Lenses.
-]
-
-The above figure shows the path of the rays proceeding from the vase,
-and meeting the eye at such an angle that the virtual image is greatly
-diminished. Concave lenses, as the student has no doubt already guessed,
-do not give real images.
-
-[Illustration:
-
-  FIG. 34.—Cannon of the Palais Royal.
-]
-
-The effects produced by the action of concave mirrors may be produced
-with just as much facility by convex lenses. If a body is placed in a
-focus of a lens which receives the direct rays of the sun, the heat as
-well as the light will be concentrated at one point; and if the object
-is combustible, it will take fire sooner or later, according to the size
-of the lens. All the experiments mentioned by Buffon as being produced
-by a concave mirror are equally obtainable with a concave lens. When of
-sufficient diameter, the most refractory metals, such as platinum or
-iridium, may be melted and dissipated into vapour. Before lucifer
-matches and vesuvians were as common as they are now, it was not at all
-unusual to find smokers carrying a small burning-glass and a piece of
-tinder, for the purpose of lighting their pipes or cigars; and there
-hardly exists a boy who has not lighted a bonfire in the fields or
-playground by means of an old spectacle lens or telescope glass.
-
-Amongst other applications of this property of lenses may be mentioned
-that of causing guns to fire at a certain time, by arranging a small
-burning-glass above the touch-hole. In the Gardens of the Palais Royal,
-at Paris, there is such a gun, so arranged that on sunny days it fires
-exactly at noon, or, in other words, at the moment the sun comes to the
-meridian. Every fine day towards twelve o’clock, crowds of Parisians who
-have nothing to do may be seen bending their steps towards the Palais
-Royal to set their watches by the gun, which they believe to be superior
-as a time-keeper to the finest chronometer in the world. There they
-stand, most of them old fellows with a scar or two about their faces,
-showing that they have nobly won the rest they appear to enjoy so
-innocently and calmly with watch in hand, leaning against the railings,
-and waiting with impatience the moment when true solar noon is indicated
-by the sharp report of the little piece. Their belief in the correctness
-of solar time is something astonishing; and if a bystander were to
-insinuate, no matter how delicately, that solar time varied slightly
-every now and then, he would either receive a smile of pitying contempt,
-or else he would be called out upon the spot. Fig. 34 gives a pretty
-view of the celebrated cannon of the Palais Royal.
-
-[Illustration:
-
-  FIG. 35.—Fresnel’s Lighthouse Apparatus.
-]
-
-We now come to another application of the refracting power of lenses, in
-the way of concentrating rays, which is infinitely more valuable to
-humanity than either of those we have just mentioned; we mean the
-construction of enormous refracting apparatuses for lighthouse purposes.
-The first lighthouse of which we have any record is that which was
-erected on the island of Pharos, by Ptolemy Philadelphus, in the year
-470 of the foundation of Rome. This was merely a tower, upon the top of
-which fires were kept burning at night; but as the world progressed, the
-blazing tar-barrel or wood fire gave place to the carefully-constructed
-lamp and silvered reflector apparatus, which are fast disappearing in
-their turn before the electric or Drummond light and the refracting
-apparatus constructed by Fresnel, who was the first to endeavour to
-abolish the old-fashioned and inefficient metallic mirror from the
-lanterns of lighthouses. Fig. 35 shows a section of Fresnel’s apparatus.
-A is a plano-convex lens of about a foot in diameter, whose focus
-corresponds with those of the concentric lenticular rings of glass which
-surround it, and which are seen more plainly in fig. 36. These rings,
-which are ground and polished with the greatest accuracy, are somewhat
-in the shape of an ordinary quoit, and are equivalent to a plano-convex
-lens with the centre portion cut out. This arrangement is so powerful
-that the distance at which a light provided with it can be seen is only
-limited by bad weather, the state of the atmosphere and the distance of
-the horizon. It is common for such lights to be seen at a distance of
-between fifty and sixty miles. The apparatus is mostly arranged in the
-form of an octagon, and is generally provided with additional reflecting
-mirrors at those parts above the light which are out of the range of the
-lenses. The light shining fully in eight directions at one time, can
-scarcely be missed by any ship within range; but in order to guard
-against any possibility of accident, the optical apparatus is often made
-to revolve by clockwork, so that every point of the ocean is illuminated
-in turn. By using coloured glasses, or by causing the light to disappear
-at distinct intervals, different lighthouses may be identified by ships
-that are out of their reckoning. Fig. 36 represents the interior of the
-lantern of a first-class lighthouse, showing the arrangement of the
-lenticular rings round the central lens. If ever the student should pass
-through Havre, he should not miss the opportunity of seeing this noble
-apparatus, which is one of the finest ever manufactured.
-
-
-[Illustration:
-
-  FIG. 36.—Lantern of a First-class Lighthouse.
-]
-
-
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-
-
-
-
-                             CHAPTER VIII.
- OPTICAL INSTRUMENTS.—THE SIMPLE AND COMPOUND MICROSCOPE. THE SOLAR AND
-                       PHOTO-ELECTRIC MICROSCOPE.
-
-
-THE lenses and mirrors whose properties we have been considering in the
-previous chapters, have been combined in different ways for the purpose
-of examining objects too small or too distant to be perceived by the
-human eye. To instruments used for the former purpose the name of
-microscope has been given, from two Greek words signifying _small_ and
-_to see_. In like manner the name of telescope is also derived from two
-Greek words, meaning _distant_ and _to see_. Besides these two classes
-of optical instruments, others have been devised to facilitate the
-depicting of natural objects, either by means of the pencil or of
-photography, or to amuse the eye by optical illusions. Thus we have the
-camera obscura, the camera lucida, the magic lantern, the
-phantasmagoria, and numberless other instruments of the same sort, most
-of which will be described in the latter part of this book.
-
-There are two sorts of microscopes, the simple and the compound; the one
-consisting of a single convex lens, and the other of several
-combinations of both convex and concave lenses.
-
-When speaking of convex lenses, we described the properties of the
-ordinary magnifying-glass, or simple microscope. The uses of this
-instrument are almost too well known to need description. It is used by
-old people, the lenses of whose eyes have become flattened by old age,
-by watchmakers for examining the minute portions of their work, by
-jewellers for the same purpose, and by most people for examining maps,
-engravings, and photographs. Simple microscopes are generally mounted in
-horn, ivory, or metal handles for convenience’ sake. Some simple
-microscopes consist of two or more lenses mounted together in order to
-increase the magnifying power. The student must distinguish between
-several lenses mounted together in this way, and the true compound
-microscope, which is a comparatively complicated optical arrangement, as
-we shall see presently. When two single lenses are thus mounted
-together, the power of the combination is equal to the powers of each
-added together.
-
-There is good reason for supposing that the simple microscope is a
-comparatively ancient invention. Seneca, who lived in the first century,
-declares that in his time it was well known that, when writing was
-looked at through a globe full of water, it appeared larger and blacker.
-In the eighth century we find the use of magnifying spectacles for old
-people common in most countries, and yet it was only at the beginning of
-the seventeenth century that a true optical instrument, in the form of a
-telescope, was invented. It only needed the placing of two magnifying
-glasses in a line to discover the principle of the telescope, but nearly
-a thousand years elapsed after the first introduction of these glasses
-before an accident rendered the principle evident.
-
-In fig. 37 we see the commonest form of microscope in the hands of an
-observer; and by examining the following figure and tracing out the path
-of the rays, we shall easily discover the principles on which its action
-depends.
-
-[Illustration:
-
-  FIG. 37.—The Compound Microscope.
-]
-
-The object to be looked at is placed at _a_ (fig. 38), on a piece of
-thin glass usually called a _slide_. A small converging lens placed at
-_b_ collects the rays proceeding from the object, and transmits them as
-far as _c d_, where they come under the influence of a second converging
-lens B, which causes them to spread out still more before they reach the
-eye. Consequently we not only see the image of the object magnified by
-the lens _b_, but still more enlarged by the action of the lens B, and
-appearing considerably enlarged at C D. The lens placed in front of the
-object is called the _objective_ or _object-glass_; that placed nearest
-the eye, the _eye-piece_. These names apply equally to the similar
-lenses used in telescopes and other optical instruments. The instrument
-shown in fig. 38 is the simplest possible compound microscope, and is
-very rarely used. The eye-piece is generally constructed of two lenses,
-and the object-glass of as many as eight; the object in multiplying the
-lenses being, not only to increase the magnifying power, but to decrease
-certain defects inherent in all lenses whose surfaces are parts of
-spheres.
-
-[Illustration:
-
-  FIG. 38.—The Theory of the Compound Microscope.
-]
-
-The amplification depends mainly upon the power of the objective, but
-different eye-pieces are also used to increase the apparent size of the
-objects to be examined. Thanks to the investigations of modern
-philosophers, we are enabled to magnify objects to 2,000 times their
-diameter with perfect distinctness; that is to say, the surface of the
-object appears to occupy 4,000,000 times its natural extent. Under such
-a power a hair would appear about six inches thick, a fine needle would
-look like a street post, and a grain of sand like a mass of rock.
-Although it is possible to employ compound microscopes of such a high
-magnifying power in the investigation of certain classes of objects, all
-ordinary preparations are best seen under a power of 500 or 600
-diameters. It would be utterly impossible to give our readers the
-slightest idea of the benefits conferred on the human race by this
-marvellous instrument. Suffice it to say, that no naturalist or surgeon
-ever attempts the most simple investigation into the structure of any
-body without the aid of the microscope. It has already shown us that a
-world of creatures exists which, although invisible to the eye of man,
-are possessed of wonderful forms, colour, and beauty of structure, and
-is daily adding to our knowledge in this direction. We can hardly submit
-any substances to this marvellous instrument without discovering animal
-or vegetable life of the most vivid character. A drop of scum from the
-surface of a stagnant pool is instantly seen to be peopled with animal
-and vegetable life, when submitted to microscopic examination. At one
-moment a rolling ball glistening like glass slowly revolves past our
-view; then a little fellow like a piece of spiral spring screws his way
-along, backing when he meets with an obstacle; or a shuttle-shaped
-vegetable, apparently made of glass, with green balls inside him, slowly
-works his way from side to side, or, possibly, a mad battledore-shaped
-being dashes past at an inconceivable rate.
-
-As it is indispensable that the object should be well lighted, a concave
-mirror is placed below it to reflect the rays of light from a lamp or
-white cloud, through the object when it is transparent. When it is
-opaque, it is illuminated by the rays of light being concentrated upon
-it by means of a convex lens. The name microscope appears by common
-consent to be applied more particularly to the compound instrument, the
-epithet of magnifier or magnifying-glass being kept for simple
-microscopes, although they are all, strictly speaking, _microscopes_.
-
-In the ordinary compound microscope, it is only possible for one person
-to see the object to be examined at once; for popular exhibitions of
-microscopic objects the reflecting microscope has been devised, by means
-of which the images of the objects to be looked at are thrown upon a
-screen. The principle of this instrument is the same as that of the
-magic lantern and phantasmagoria, of which we shall speak presently.
-Fig. 39 (see next page) represents the photo-electric microscope, so
-called from the objects being reflected by the electric light.
-
-The jars seen on the ground are the cells of a voltaic battery, by which
-the electricity is generated. The luminous rays starting from the
-incandescent charcoal points are reflected through the tube and its
-lenses by the reflector placed at the back of the instrument, and are
-concentrated upon the object to be magnified. The image thus produced
-passes through a second system of converging lenses, and is projected
-upon the screen magnified some millions of times according to the power
-of the object-glass employed.
-
-[Illustration:
-
-  FIG. 39.—Photo-Electric Microscope.
-]
-
-“The experiments made with the photo-electric microscopes,” says M.
-Ganot, “are amongst the most curious and pleasing to be found in the
-whole range of physical science. With this instrument it is possible to
-show the smallest objects magnified almost indefinitely to an unlimited
-number of spectators. A human hair will appear as large as a broomstick,
-an ordinary flea will look the size of a sheep, and the tiny cheese
-mite, as well as the smallest animalcules, will be visible in all their
-beauty of form and colour as clearly as if they were seen with the naked
-eye. One of the most remarkable experiments to be made with this
-instrument is that which shows the circulation of the blood. The tail of
-a live tadpole is inserted between two plates of glass, or on an
-instrument specially made for the purpose, and placed in the microscope
-armed with a somewhat low power. The spectator immediately perceives
-upon the screen a mass of rivers and rivulets, all flowing with the red
-corpuscles forming the blood of the animal, and rushing through its
-veins and arteries with inconceivable rapidity. Another interesting
-experiment consists in dissolving a small quantity of sal-ammoniac in
-warm water, and passing a small portion of the solution across a warm
-glass slide. When placed in the microscope the water gradually
-evaporates, leaving behind a mass of feathery crystals, whose growth may
-be watched atom by atom, each crystalline molecule grouping itself
-around the others in forms resembling a mass of fern-leaves.”
-
-The apparatus we have been describing is sometimes illuminated with the
-rays of the sun, as in the following figure.
-
-[Illustration:
-
-  FIG. 40.—Solar Microscope.
-]
-
-It is then called the solar microscope, and exhibits objects with great
-beauty and clearness. The use of the sun’s rays, however, has, in our
-own country at least, been entirely superseded by the electric and lime
-light. The latter method of illumination, which consists in projecting a
-stream of oxygen and hydrogen upon a ball of lime, is cheaper and more
-certain than the electric light, although the latter is possibly the
-more brilliant of the two. The construction of the solar microscope
-differs but little from the instrument already described, and may be
-readily understood from the foregoing figure. The large mirror is placed
-outside the window of the room in which the microscope stands, so that
-the solar rays are reflected upon the surface of a series of convergent
-lenses, and from thence on to another mirror, from which it is again
-reflected through the microscope. As the position of the sun is
-constantly changing, it is necessary to connect the outside mirror with
-a train of clockwork. It may be mentioned that an instrument of this
-kind, for reflecting the sun’s rays, is called a heliostat.
-
-The student will, no doubt, at once perceive that if we concentrate the
-light of the sun upon an object, we shall also concentrate the heat, and
-either melt or consume it. A screen is therefore used in such cases,
-which will allow the light to pass while holding back the rays of heat.
-A solution of alum is found to answer the purpose admirably.
-
-
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-
-
-
-
-                              CHAPTER IX.
- THE TELESCOPES OF GALILEO, GREGORY, NEWTON, HERSCHEL, LORD ROSSE, AND
-                               FOUCAULT.
-
-
-IF history has failed to furnish us with the name of the inventor of the
-microscope, we have very exact information as to the first experimenters
-upon the powers of the telescope.
-
-“In the archives of the Hague,” says Arago, “we find documents, by the
-aid of which Van Swieten and Moll have come to a decisive conclusion as
-to the first and true inventor of the telescope.”
-
-We read in these documents that a spectacle-maker of Middleburg, named
-John Lippershey, addressed a petition to the States-General on October
-2, 1606, in which he asked leave to take out a patent, which should
-constitute him the only maker of this instrument, or which should confer
-upon him an annual pension, on the condition of not manufacturing them
-for other nations. The petition qualifies the instrument as serving to
-see distinct objects, as had already been explained to the members of
-the States-General.
-
-On the 4th of October, 1608, the States-General appointed a deputy from
-each province to experiment on the new instrument, which was placed on a
-tower of the palace belonging to the Stadtholder. Huggard says that the
-first telescopes experimented on were a foot and a half in length.
-
-On the 6th of October, the commission declared the instrument of
-Lippershey to be useful to the nation, but demanded that it should be
-made for two eyes instead of one.
-
-On the 9th of December, Lippershey, having announced that he had solved
-the problem, Van Dorth, Magnus, and Van der Au were ordered to verify
-the fact, which they did by making a very favourable report on the 11th
-of the same month. The binocular instrument was therefore found to
-answer.
-
-In reading the extracts from the archives of the Hague, given by Moll,
-we may remark with great pleasure the promptitude with which the
-commissioners of the States-General examined Lippershey’s instruments.
-But their satisfaction soon gave way to displeasure, when they found a
-large number of opticians making these instruments, and selling them to
-foreigners, like so much spice from the East Indies. Later on one feels
-indignant at finding the commissioners of the States-General to be so
-wanting in proper feeling as to decide that the telescope must be
-considered imperfect until it could be used with both eyes, without
-either winking or seeing the reflection of the pupils in the eye-pieces.
-Consequently, instead of being permitted to expend his talent on
-perfecting the optical powers of the single telescope, Lippershey saw
-himself condemned to waste his time upon the double instrument. The
-States-General finished by giving Lippershey 900 florins; but they
-refused him a patent, on the ground that it was already notorious that
-other opticians had commenced the manufacture of similar instruments.
-
-Amongst others who were rivals of Lippershey, we must mention John
-Adrian Metius, the son of Adrian Metius, of Amsterdam, who discovered
-that the nearest relation of the circumference of a circle to its
-diameter was 355 to 113. He addressed a letter to the States-General on
-the 17th of October, 1608, conceived in the following terms:—
-
-    “After two years’ labour and thought I have succeeded in making
-    an instrument, by the aid of which objects which are too distant
-    to be visible by the eye, are seen plainly. The one I show,
-    although constructed out of bad materials, and simply as an
-    experiment, is, in the judgment of the Stadtholder and of
-    several other persons, as good as the one lately presented to
-    the States-General by a citizen of Middleburg. I am sure of
-    improving it still further in the course of time, and I beg to
-    ask for a patent by which any person who is not already in
-    possession of this invention will be forbidden, under pain of a
-    heavy fine and confiscation, to make or sell similar instruments
-    for twenty-two years.”
-
-The States-General refused to grant the patent in this case also, but
-enjoined Metius to perfect his instrument, reserving to themselves the
-power to reward him in the future if they thought fit.
-
-In Italy, Galileo is generally supposed to have discovered independently
-the method of making a telescope on the principle of the Dutch
-philosophers, about the beginning of 1609, having received a very
-imperfect account of these instruments somewhere about that time. It may
-be remarked that in his letter to the chiefs of the Venetian Republic,
-giving an account of the properties of these new instruments, Galileo
-states that, if necessary, they could be made specially for the use of
-the navy and army belonging to the state. But secrecy was useless, for
-telescopes were already made and sold in Holland at a cheap rate.
-Besides, Galileo makes no allusion to the labours of his Dutch
-predecessors, either in a prior letter handed down to us by Venturi, or
-in the decree of the Venetian Senate, dated August 5, 1609.
-
-The Italian commentators are in error when they attribute the second
-discovery of the telescope to the knowledge that Galileo possessed of
-the laws of refraction, and that it was by deductions therefrom that he
-was enabled to construct his first instruments.
-
-Huyghens says, in his _Treatise on Dioptrics_, “I will unhesitatingly
-place that man above all mortals, who, by the aid of his own reflections
-and without the aid of accident, first succeeded in constructing a
-telescope.”
-
-“Let us see,” says Arago, when speaking on this subject, “if Lippershey
-and John Adrian Metius were men of unparalleled powers.”
-
-Hieronymus Saturnus tells us that an unknown man of genius called upon
-Lippershey, and ordered from him a number of convex and concave lenses.
-At the time agreed upon the man returned, and chose two, one convex and
-the other concave, and, placing them one before his eye and the other at
-some distance from it, drew them backwards and forwards, without giving
-any explanation of his manœuvres, paid the optician, and left the place.
-As soon as he was gone, Lippershey began immediately to imitate the
-experiments of the stranger, and soon found that distant objects were
-brought apparently nearer, when the lenses were placed in certain
-positions. He next fastened them to the ends of a tube, and lost no time
-in presenting the new instrument to Prince Maurice of Nassau.
-
-According to another version, Lippershey’s children were playing in
-their father’s shop, and were looking through two lenses, one convex and
-the other concave, when they found to their surprise that the vane on
-the clock-tower of Middleburg Church was greatly magnified and
-apparently brought nearer. The surprise expressed by the children having
-awakened the attention of Lippershey, he tried the experiment of fixing
-the lenses on a piece of board; afterwards he tried it again by fixing
-them at the ends of two pieces of tube, sliding in each other, and
-succeeded in making the first telescope on record.
-
-The principal documents from which the above facts touching Lippershey
-have been extracted, are to be found in a memoir on the subject by
-Olbers, printed in Schumacher’s _Astronomical Annual_ for 1843.
-
-It was said in the time of Galileo that he had in his possession a
-telescope by the aid of which he could see the birds flying at Fiesole
-from the window of his palace in Florence. This story does not in the
-least detract from the merit of the illustrious astronomer, who not only
-constructed a telescope for himself, but was the first to direct it
-heavenwards, and that too by purely theoretical researches; for in spite
-of all the documents adduced above, there is little or no proof that he
-had ever seen or heard of the Dutchman’s telescope. It is only right,
-therefore, that the instrument constructed on this principle should be
-called the Galilean telescope. He afterwards increased its power from
-four to thirty times, beyond which he could not get with the means at
-his command. With his imperfect instruments Galileo discovered the
-satellites of Jupiter, the mountains of the moon, and the spots on the
-sun, and earned for himself the name of Lynceus, who according to the
-ancients was one of the Argonauts, possessed of the power of seeing
-through a wall. Towards the end of his life, when the old man was blind,
-and the Academy of the Lincei treated his hypotheses with disdain, he
-would laugh sadly at the name bestowed on him, and the obstinate
-Academy. Fig. 41 (see next page) shows the path of the rays in a
-Galilean telescope. The object-glass _O_ is double convex, and the
-eye-piece o bi-concave. The image is formed between these lenses, and
-the eye appears to see it at that point. The States-General complained
-of being obliged to shut one eye when looking through a telescope, but
-in 1671 a good Capuchin monk, whose name was Cherubino, placed two
-telescopes together, little thinking that the moderns would imitate him
-in that very worldly instrument, the opera-glass.
-
-[Illustration:
-
-  Fig. 41.—The Galilean Telescope.
-]
-
-Everybody has noticed that when objects are close to us they appear
-larger than when they are at a distance; it accordingly amounts to the
-same thing whether, in speaking of the power of telescopes, we say they
-magnify twice, four times, or a hundred times, or that they are brought
-within half, a quarter, or a hundredth of their distance. Thus there is
-a telescope at Lord Rosse’s Observatory, at Parsonstown in Ireland,
-which is the finest yet constructed. Its highest magnifying power is
-6,000, therefore every object we look at with it is brought within the
-6,000th of its distance from us. Looking at the moon, for instance, we
-know that our satellite is distant some 240,000 miles from us; we have,
-therefore, only to divide that number by 6,000 to find that by means of
-this wonderful instrument the moon is brought within 40 miles of the
-earth. This statement, however, is not strictly true, for it supposes
-the whole of the apparatus used to be theoretically perfect.
-
-Kepler, whose great name is now-a-days always associated with that of
-Galileo, but who during their life-time was somewhat his rival,
-substituted for the single lens forming the eye-piece a combination
-consisting of two convex lenses, in order to obtain a larger field for
-observation than that given by the single bi-concave. This combination
-is commonly known as the astronomical eye-piece. It reverses the object
-looked at, but for astronomical purposes this defect is of no
-consequence.
-
-[Illustration:
-
-  FIG. 42.—The Astronomical Telescope.
-]
-
-The instrument shown in the above figure represents an astronomical
-telescope reduced to its simplest form.
-
-Fixed parallel to the axis of the larger telescope is the finder, a
-small telescope of low power and large field, used for finding celestial
-objects not easily visible to the naked eye. It is so arranged, that
-when the object is found and carried to its centre, it is also in the
-centre of the field of the larger instrument. The handle and the two
-toothed wheels serve to raise or lower the telescope, which is movable
-on the horizontal axis, which supports it in front, so that it may be
-directed to any part of the heavens the observer may desire.
-
-The following figure shows the arrangement of the lenses, and the path
-of the rays through them, in telescopes of this form.
-
-[Illustration:
-
-  FIG. 43.—Section of an Astronomical Telescope.
-]
-
-The convex lens which serves as an object-glass, gives at _a b_ a
-reversed image of the star A B. The small convex lens which acts the
-part of an eye-piece, enlarged this reversed image without changing its
-position, and causes it to be seen in the line A´ B´. This eye-piece is
-fixed at the extremity of a tube, which is smaller than that containing
-the object-glass, and slides easily backwards and forwards from the spot
-where the image _a b_ is found. The latter is an indispensable
-condition, for it is rare to meet two persons whose eyes are of the same
-focus; besides, the image _a b_ will fall at a different spot for
-objects at different distances: thus, if you are looking at the moon,
-and suddenly turn the instrument on to a distant nebula, you will find
-that the eye-piece requires adjusting. In showing ordinary observers an
-object in the telescope, it is well to insist on their moving the
-eye-piece backwards and forwards until distinct vision is obtained, for
-it often happens that people will say they see an object quite
-distinctly, when it is in reality misty, and will generally refuse to
-allow the focus to be altered. It is very singular how human vanity or
-complaisance will step in when some persons are looking through a
-telescope. They seem to think that there is some disgrace or rudeness
-involved in their not being able to see what their predecessors at the
-instrument have seen. Poor John Leech leaves us an amusing instance of
-this in a comic cut inserted in one of the early numbers of our old
-friend _Punch_. A gentleman is endeavouring to show a lady a distant
-steamboat through a telescope, but she has it accidentally pointed at
-two swans that are swimming on the margin of the lake below;
-consequently when he asks her if she sees the steamer, she replies that
-“she sees it most distinctly, and there are two of them,” a pretty good
-proof that the instrument was not only pointed at the wrong object, but
-was out of focus as well.
-
-In constructing a telescope similar to the one described above, the
-object-glass ought to be of considerable diameter and of long focus; the
-eye-piece, on the contrary, should be comparatively small and of short
-focus. A little consideration will show the reason of this. An
-object-glass of long focus will form a large image at the point _a b_,
-and the eye-piece of short focus will magnify this image more than
-another lens of less convexity. It is, however, on the size, length of
-focus, and perfection of workmanship of the objective that the
-excellence of the telescope depends; large object-glasses are
-consequently rare, and are only to be found in observatories of the
-first class. The object-glass of the large telescope at the Observatory
-at Paris is nearly fifteen inches in diameter, and the highest
-magnifying power capable of being employed with it is 3,000. The
-Observatory of Pulkowa, near St. Petersburg, possesses a similar
-instrument, and the Observatory at Chicago, United States, a still
-larger one, measuring between eighteen and nineteen inches in diameter.
-But the largest of all is an objective in the possession of Mr.
-Buckingham, an amateur astronomer, who has an observatory near London,
-which is twenty inches in diameter, and twenty-eight feet in focal
-length.
-
-The eye-pieces of astronomical telescopes are of different powers, and
-are changed according to the class of object to be observed. Thus, in
-taking a general view of the moon, a low power would be used. If you
-wished to examine any particular mountain, you would raise the
-magnifying power by inserting a stronger eye-piece. The power used also
-depends on the state of the atmosphere. For instance, on warm evenings,
-when the air is charged with moisture, the tremulousness of the
-atmosphere is so great, that it is often only possible to use the very
-lowest power. By combining four convex lenses together, we obtain what
-is called a terrestrial or erecting eye-piece, which has the property of
-_re-reversing_ the image formed by the objective. The eye-pieces of all
-telescopes for use on land or at sea are made on this principle. The
-same effect may be obtained, as we have already shown in fig. 41, by
-using a concave lens, but in this the field of view is much diminished.
-
-Hitherto we have only spoken of refracting telescopes or those
-instruments provided with a convex object-glass, to collect and refract
-the rays of light given off by the object we are desirous of examining;
-but there is another and very important class of instruments, in which
-the object-glass is replaced by a reflecting mirror. The first
-reflecting telescope was invented by Dr. Gregory, an English
-philosopher, about 1650. It consisted of a brass tube, at the lower
-extremity of which was fixed a concave mirror made of metal, and
-provided with a hole in its centre for the insertion of the small tube
-containing the eye-glass. Towards the other end of the telescope was
-placed a second and smaller mirror, which reflected the image formed by
-the large mirror, through the eye-piece to the eye. The following figure
-will show the path of the rays in the Gregorian telescope.
-
-[Illustration:
-
-  FIG. 44.—Section of the Gregorian Telescope.
-]
-
-The rays A B, proceeding from the object at which the instrument is
-pointed, are first reflected from the surface of the principal mirror M
-M on to the small mirror _m_, whence they proceed to form a magnified
-image at _a b_, which is then again enlarged by the eye-piece appearing
-to the eye as if placed at A´ B´. The focus in the Gregorian is altered,
-not by sliding the eye-piece backwards and forwards but by moving the
-mirror _m_, which is provided with a long screw, to which is attached a
-handle. At first sight a reflecting telescope has the appearance of a
-very stumpy-looking refracting instrument, but one instant’s examination
-will show the observer that the usual object-glass is absent at the end
-of the tube. In fig. 45 we have a Gregorian telescope, mounted on a
-tripod stand.
-
-[Illustration:
-
-  FIG. 45.—Gregorian Telescope.
-]
-
-Whilst experimenting on the Gregorian telescope, Newton made certain
-improvements in its construction, which we shall proceed to describe. A
-glance at fig. 46 will show that the path of the rays is much more
-simple than in the instrument we have just noticed.
-
-[Illustration:
-
-  FIG. 46.—Section of a Newtonian Telescope.
-]
-
-The rays of light A B are first reflected from the concave mirror M on
-to the surface of the small plane mirror _m_, which is placed at an
-angle of 45°, and reflects them as far as the point A´ B´, where they
-form the image to be magnified by the eye-glass. It is therefore at the
-_side_ of the instrument, and not at the end, as hitherto, that the
-observer is placed, and at right angles to the path of the rays.
-Observers looking at an object through a Newtonian telescope for the
-first time are generally sufficiently astonished to find that there is
-really no difficulty after all in seeing round a corner. We shall
-presently return to the subject of Newtonian telescopes, which were
-abandoned by astronomers for many years, until they were brought into
-use again by M. Foucault, a distinguished French philosopher.
-
-Towards the end of the last century Sir William Herschel invented and
-constructed the reflecting telescope which bears his name. His great
-object was to avoid the loss of light consequent on the double
-reflection which took place in all instruments constructed up to that
-time, and he succeeded at last in making a telescope in which the
-observer looked directly through the eye-piece at the image formed by
-the mirror, which was inclined in such a manner that the rays were
-reflected to the lower edge of the open end of the tube. In using this
-kind of telescope the observer is placed with his back to the object he
-wishes to examine, a position that is even more astonishing to those
-unaccustomed to the use of a Herschellian telescope than the one assumed
-when employing an instrument of the Newtonian construction. This
-position has the defect of causing a small portion of the rays
-proceeding from the object to be intercepted by the head of the
-observer, but the amount of light lost is so small in comparison to the
-size of the mirror that in practice it amounts to nothing.
-
-The dimensions of the telescope constructed by Herschel were enormous
-for that day. It measured 40 feet long, and the mirror was 4 feet in
-diameter. It was supported by a complicated system of scaffolding,
-pulleys, and cords, and was capable of magnifying an object 6,000 times.
-It was by means of this splendid instrument that Sir William Herschel
-made those wonderful discoveries in astronomy which are inseparably
-associated with his name. With it he discovered the planet Uranus, many
-of the double stars, and a large number of nebulæ, which up to that time
-were unknown. His son, Sir John Herschel, inherits his father’s talents
-as an astronomer, and has enriched science with numberless observations
-and discoveries of the greatest importance made with this fine
-instrument. Fig. 47 shows the construction of the Herschellian
-telescope, and the path of the rays may be easily followed by the
-student without any help from us.
-
-The vulgar, ever prone to make mountains out of molehills, magnified the
-power of Sir William Herschel’s telescope beyond all bounds. Stories
-were circulated about his having given a dinner in the interior of the
-tube to a select party of friends, but as the diameter of the telescope
-was only a little more than 4 feet, the entertainment, to say the least
-of it, would have proved somewhat inconvenient to the guests. Another
-story, which was credited by great numbers of people, was that he had
-discovered inhabitants in the moon, but that he hesitated to make the
-matter public for fear he should be prosecuted for spreading atheistical
-notions. In fact, the tales told of Sir William Herschel’s telescope
-were endless, and caused the astronomer great inconvenience by
-attracting crowds of idle people to the neighbourhood of Slough, where
-he vainly endeavoured to carry on his investigations in peace and
-quietness. It was in vain that these silly assertions were disproved
-again and again. Having once believed them, people were slow to reject
-them, and the story of the dinner was told over and over again for many
-years.
-
-[Illustration:
-
-  FIG. 47.—The Herschellian Telescope.
-]
-
-The instrument above described is one of those known as _front view
-telescopes_, on account of the image of the star being reflected from
-the surface of the mirror, which was placed obliquely at the bottom of
-the tube in front of the observer, who examined it by means of the
-eye-piece without any other reflection taking place, thereby effecting a
-saving of light, which fully compensated for any loss caused by the
-mirror being placed askew. The concave mirror made by Herschel alone
-weighed a ton, to say nothing of the enormous tube and its fittings.
-Herschel had consequently to invent a special apparatus for holding and
-moving this gigantic instrument. The moving gear consisted of a mass of
-beams, pulleys and cords, reminding one more of the rigging of a ship
-than of a philosophical instrument. The apparatus for moving the
-telescope appeared so complicated to the casual observer, although in
-reality it was very simple, that it doubtless contributed in no small
-degree to the propagation of the fanciful stories we have already spoken
-of.
-
-The performances of this splendid instrument hardly came up to the
-expectations of those who saw it in progress. Herschel, it is true, was
-enabled by its means to use a power of from 3 to 6,000, but he could
-only use these amplifications on a few objects—the planets, for
-instance, giving so little light under a high power as to become
-indistinct and misty. In 1802 Baron von Zach, in his _Monthly
-Astronomical Compendium_, went so far as to say that this colossal
-instrument was not of the slightest utility, that no discovery had ever
-been made with it, and that it ought to be considered merely as an
-optical curiosity. Subsequent events, however, proved very conclusively
-that Baron von Zach was utterly wrong in his statements and prophecies.
-
-The telescope constructed by Herschel, although very wonderful for the
-day in which it was made, has long since been eclipsed by that belonging
-to Lord Rosse, and erected by his late father at Birr Castle, near
-Parsonstown in Ireland. It is superior to Herschel’s instrument both in
-point of size, and workmanship. The late Lord Rosse, not fearing that
-his dignity would be compromised by such an act, went boldly to work,
-and learned to polish mirrors like an ordinary workman, the consequence
-of which was that he could bestow unusual pains upon the finishing of
-the speculum. His Lordship not only learnt the mere handicraft of
-speculum polishing, but went deeply into the engineering difficulties of
-the operation, and succeeded in inventing many improvements for
-diminishing labour and rendering the form of the surface more perfect.
-The specula ground and polished under Lord Rosse’s method are almost
-entirely free from what is called spherical aberration,—that is to say,
-all rays proceeding from a single point of light, such as a star, are
-collected into a single point instead of being scattered in a round
-mass. This freedom from spherical aberration is of course necessary to
-produce perfectly distinct images. In his _Life of Newton_ Sir David
-Brewster calls it one of the most marvellous combinations of art and
-science yet seen in the world.
-
-The tube of Lord Rosse’s instrument is 55 feet long, and weighs 6½ tons.
-In form it may be compared to the chimney of a steamboat of enormous
-size. At one end it terminates in a kind of square box, within which is
-contained the mirror, whose diameter is 6 feet, and which weighs nearly
-4 tons. The weight of the whole apparatus is consequently nearly 10½
-tons, or four times as much as Herschel’s. It is erected on an oblong
-mass of masonry, 75 feet in length from north to south, between two
-solid walls nearly 50 feet high, which serve as supports for the
-mechanism intended to move this enormous tube in all directions. To the
-walls are also fixed movable staircases with platforms that can be
-brought up to the eye-piece with the greatest facility, no matter in
-what position the telescope may be placed. This noble instrument has
-penetrated space to a distance perfectly unattempted before its
-existence, and has resolved numerous nebulæ into masses of stars that
-until then were supposed to be mere clouds of luminous matter. The exact
-forms of other nebulæ have also been accurately determined by this
-telescope, which fully deserves the glowing eulogium passed upon it by
-the Duke of Argyle in his presidential address at the meeting of the
-British Association at Glasgow, in 1855. “This instrument,” said his
-Grace, “in extending the range of astronomical science as it has done,
-has been the means of throwing certain doubts upon the laws that govern
-the motions of the heavenly bodies, and render it possible that certain
-of the far-distant nebulæ are regulated in their movements by other laws
-than those to which the members of our own system are subjected.”
-
-The clearness with which this telescope exhibits every object within its
-range is so great that the most distant nebulæ are seen with as great
-distinctness as the nearest planet. On directing it towards the moon,
-which is only distant from us about 240,000 miles, the surface of our
-satellite may be explored with a facility almost as great as that with
-which we examine the details of a landscape with an ordinary telescope.
-
-Maedler, a German astronomer, who has measured nearly every mountain and
-valley on the moon’s surface with the greatest exactitude, stated some
-years before Lord Rosse’s telescope was perfected that if a monument as
-large as one of the Pyramids existed on the surface of the moon it could
-have been readily distinguished by the instruments then in use. With
-Lord Rosse’s telescope we can see the surface of our satellite so much
-enlarged that a space 220 feet square could be readily perceived by a
-good observer. This enormous eye, measuring 6 feet in diameter, would
-hardly show us a lunar elephant; but it is certain that if a troop of
-buffaloes, or animals analogous to them, crossed the field of vision,
-they would undoubtedly be perceptible. Masses of troops marching
-backwards and forwards would also be plainly visible, and we may assert
-with something like absolute certainty that there are neither towns nor
-villages in the moon, nor any buildings as large as St. Paul’s of London
-or the colossal railway stations of that metropolis.
-
-This telescope, as we have said before, is the largest hitherto
-constructed, and cost its noble constructor more than 25,000_l._ It must
-also be recollected that it was not a mere scientific toy belonging to
-an amateur philosopher, but a real working instrument in the possession
-of a true man of science, who did work with it that will render his name
-famous while civilization lasts. The present Lord Rosse seems worthy in
-every way of his father’s great name, and has already enriched
-astronomical science with numerous valuable observations.
-
-We shall finish this chapter by a description of the Newtonian telescope
-constructed by M. Léon Foucault. The mirror, instead of being made of
-speculum metal, which is an alloy of tin and copper, is made of glass
-from the famous manufactory of St. Gobain. The first rough grinding
-having been finished, it passed into the workshops of M. Secrétan, the
-optician to the Paris Observatory, to receive its final polish and
-finishing touches from the hand of M. Foucault himself, the most careful
-optical tests being applied to it before the commencement of each
-operation.
-
-The glass mirror having reached the degree of perfection desired, was
-then silvered on its concave surface by being plunged into a bath of
-nitrate of silver, dissolved in water, and mixed with certain
-proportions of gum galbanum, nitrate of ammonia, and oil of cloves. Half
-an hour in this bath was sufficient for the deposition of a film of
-silver of sufficient thickness to bear polishing. When finished, the
-mirror was found to reflect 92 per cent. of the light incident on its
-surface, the loss in the case of achromatic object-glasses and metal
-specula being 20 and 35 per cent. respectively. The substitution of a
-parabolic glass mirror for the ordinary metal speculum offers the triple
-advantage of greater lightness, increased distinctness, and more
-brilliant images. Fig. 48 represents the large silvered glass telescope
-constructed under M. Foucault’s direction for the observatory at
-Marseilles. It measures 32 inches in diameter, and has a focal length of
-a little more than 16 feet, and is put in motion by clockwork of a very
-perfect description, so that when once pointed at a star or planet it
-follows the object, which would otherwise disappear on account of the
-rotation of the earth. The path taken by the rays is precisely the same
-as in Newton’s telescope, the eye-piece being placed at the side of the
-tube, which is provided with a movable platform and staircase for the
-observer.
-
-[Illustration:
-
-  FIG. 48.—Foucault’s Large Telescope.
-]
-
-The optician to whose talent in his art this fine instrument is due, has
-recently executed several small telescopes upon the same model, at such
-a price as to bring them within the reach of amateurs with slender
-purses. The principal part of these telescopes, one of which is
-represented in fig. 49, (see next page), is the mirror, which is about 4
-inches in diameter, and 24 inches’ focal length. The body, which is
-cylindrical, is made of brass, and revolves on two pivots placed
-horizontally at about one-third of its length from the bottom. The
-bearings on which the pivots move consist of two upright standards of
-metal, which are connected at the bottom, and revolve on a pin in the
-middle of the plate of the tripod stand. They are made of such a height
-that the lower portion of the instrument may pass between them, when it
-is necessary to observe objects in the zenith. By the turn of a screw
-the whole of the upper portion of the instrument may be dismounted and
-fixed on a lower standard, so that the observer may work sitting down if
-necessary. The body of the telescope is provided with a finder. One of
-the great advantages of this form of instrument is that it can be used
-for observations on the zenith without giving the observer those
-unpleasant cricks in the neck so inseparable from the use of ordinary
-telescopes in a nearly upright condition. The mirror will bear a power
-of 220 diameters, and shows the mountains of the moon, the phases of
-Mercury and Venus, Saturn and his ring, Jupiter and his satellites, and
-a large number of double stars and nebulæ. It is provided with a set of
-eye-pieces, so that any power almost from 50 to 220 diameters may be
-used at will. The figure on the opposite page will give the amateur a
-good idea of the form and size of this instrument.
-
-
-[Illustration:
-
-  FIG. 49.—Foucault’s Small Telescope.
-]
-
-
-------------------------------------------------------------------------
-
-
-                               PART III.
-
-                             NATURAL MAGIC.
-
-                             --------------
-
-
-
-
-                               CHAPTER I.
-                           THE MAGIC LANTERN.
-
-
-THE illusions of which we have spoken in the first part of this work
-depended principally on the nature of man’s vision, who, we found, was
-the constant and heedless victim of his own powers of sight. We shall
-now examine a series of illusions that are still more extraordinary, but
-which have nothing to do with the deceptions practised on us by our
-visual organs. Instead of being deceived by ourselves, we shall find
-that we are led astray by others whose knowledge of the laws of optics
-is greater than our own, enabling them to construct instruments capable
-of amusing us or imposing on us, according to our ignorance of natural
-laws. Let us hope, however, that the science of optics has now become so
-familiar to most educated people, that no such thing as a real
-imposition can take place, although at the present day there are so many
-exhibitions of the marvellous that ordinary observers have the greatest
-difficulty in accounting for them. In former ages, when the knowledge of
-science was confined to a certain class, the commonest optical facts of
-the present day were taken advantage of to delude the ignorant. The
-deceptions practised by the ancient priests of Egypt, Greece, and Rome
-were undoubtedly many of them of this description. It is a well known
-fact that both plane and concave metallic mirrors were used by the
-ancients, and a passage in Pliny gives an account of certain glass
-mirrors that were made at Sidon. Aulus Gellius, quoting Varro, speaks of
-the reflecting properties of hollow mirrors, and we shall see, as we go
-on, what a number of illusions may be practised by means of a series of
-plane mirrors arranged in a particular way. But we will first devote a
-short time to the curious historical facts connected with the principle
-of the magic lantern which took place long before the modern invention
-of this instrument by Father Kircher.
-
-Brewster says, when treating of this subject, that there can be little
-doubt that the concave mirror was the principal instrument used in
-connexion with the pretended apparitions of the gods and goddesses in
-the ancient temples. In the meagre history of these apparitions that has
-come down to us, we can easily perceive the traces of an optical
-illusion. In the ancient temple of Hercules at Tyre, there existed a
-certain seat made of consecrated stone, out of which the gods rose,
-apparently at the will of the priests. Æsculapius appeared frequently to
-his worshippers in his temple in Tarsus, and the temple of Eugenium was
-famous for the number of gods and goddesses which were constantly
-visiting its sacred precincts. Iamblicus tells us that the priests
-showed the gods to the people in the midst of smoke; and when the great
-magician Marinus terrified his auditory by suddenly showing them the
-statue of Hercules in the midst of a cloud of incense, it was
-undoubtedly a woman who performed the part, dressed up in man’s robes
-for the occasion.
-
-The character of these spectacles in the ancient temples is admirably
-described by Damasius, and there is no difficulty in seeing that optical
-illusions were the means employed to delude the audience. He describes
-the apparition on the wall of a large spot of white, which at first
-appeared at a distance, but gradually came nearer and nearer until at
-last it assumed the form of a divine or supernatural being, of severe
-yet mild aspect and of great personal beauty. This being the
-Alexandrians immediately honoured as Osiris or Adonis.
-
-Amongst more modern examples of this illusion may be mentioned that of
-the Emperor Basil of Macedonia. Inconsolable at the loss of his son,
-this potentate had recourse to the prayers of the Pontiff Theodore
-Lantabaren, who was celebrated for his power of working miracles. The
-conjurer showed the Emperor the image of his dead son magnificently
-attired and mounted on a splendid war-horse. The young man dismounted,
-and, going up to his father, threw himself into his arms and
-disappeared. Salvertius, in speaking of this story, observes
-judiciously, that the deception could only take place through the agency
-of some person who closely resembled the Emperor’s son, and that the
-trick would have been easily discovered when the person embraced the
-Emperor. A better explanation of the affair is, however, afforded by
-supposing that the Emperor saw an aërial image of a person resembling
-his son, and that when he rushed forward to embrace him it disappeared.
-
-The accounts of the operations of the ancient magicians are too meagre
-to give us any idea of the splendour of some of these ancient
-ceremonies. A system of deception such as this, employed as a means of
-government, must have brought into requisition not only the talents of
-all the learned men of the day, but a crowd of accessories calculated to
-astonish and confound the judgment, fascinate the senses, and facilitate
-imposture.
-
-An account of an instance of modern necromancy has been left us by
-Benvenuto Cellini, who played a prominent part in a case of this sort.
-
-He accidentally made the acquaintance of a Sicilian priest, a man of
-great genius and acquirements, and well versed in Greek and Latin
-classical lore. One day the conversation turned on necromancy, and the
-great goldsmith told him that he had the greatest desire to know
-something about this wonderful art, and that he had felt all his life a
-great curiosity to penetrate its mysteries.
-
-The priest replied, that a man ought to have a very resolute and
-fearless character to study this art; but Benvenuto answered he had both
-resolution and courage. The priest went on to say, that if he had the
-heart to try, he would be the means of obtaining the fulfilment of his
-wishes. They consequently agreed upon a plan of necromantic study. One
-evening, Benvenuto invited one of his companions, Vincenzio Romoli, to
-take part in some experiments that were to be made amongst the ruins of
-the Coliseum. They there met the Sicilian priest, who after the manner
-of the ancients began to describe a number of circles in the air in the
-most imposing manner. He had brought with him various gums and perfumes,
-and had made a fire, into which his assistant necromancer was to throw
-them at the proper time. He commenced his conjurations, the ceremony
-continuing about an hour, when there appeared legions of demons, in such
-numbers that the whole of the ruins seemed filled with them. Benvenuto
-was nearly fainting with the perfumes, when the priest roused him by
-telling him to ask for something. He replied, that he wished to be
-transported to the side of his Sicilian mistress; but the demons were
-evidently unpropitious, for nothing came of it. His instructor, however,
-told him that they must repeat their experiments a second time, and that
-Benvenuto must bring with him a child that had never committed sin. The
-next time Benvenuto took with him a boy of twelve years old whom he had
-in his service, and his friends Romoli and Guddi. When they arrived at
-the place of meeting, they found the priest had made the same
-preparations as before. This time, however, he used more powerful
-conjurations, calling on a number of demons by their names, in Hebrew,
-Greek, and Latin; so that the ruin was filled with a still greater mass
-of them than on the other occasion. The fire and perfumes were put under
-the charge of Guddi and Romoli, and he gave Benvenuto a magic picture to
-hold in a certain direction, the boy being placed underneath it. The
-priest told him again to wish to be in the company of his lady love, but
-on his expressing the wish, the magician told him that the demons still
-refused to do his bidding in this way, but that he should visit her once
-more in a month’s time. The poor boy underneath the magic picture was
-seized with a terrible fright, and exclaimed, that he saw millions of
-ferocious spirits and four giants, all endeavouring to break through the
-magic circle the priest had formed. All there were evidently in a most
-abject state of terror, and remained in the place until the church bells
-began to ring for morning prayers, when they returned home, the boy
-declaring that two of the demons preceded them, dancing and gambolling
-before them, and sometimes running along the housetops.
-
-The priest then advised him to try another spiritual _séance_, and
-endeavour to induce the demons to point out sundry pots of buried gold,
-so that they all might become rich, but it does not appear that the
-priest’s advice was followed.
-
-It is impossible to read the foregoing description of what happened,
-without being convinced that the whole affair was an optical illusion,
-and not the mere result of the imagination of those who took part in it.
-The smoke was evidently caused in order to afford a field for the
-exhibition of painted images reflected by concave mirrors, and the
-circle was formed in order that those within it might be within range of
-the images formed on the smoke. The mirrors reflecting the images of the
-demons had undoubtedly already been arranged so that they would fall
-just above the fire, and become visible when the gums began to burn with
-a smoky flame. The perfumes were simply to help to stupify the
-spectators, and aid in working on their imaginations for those
-occurrences which were beyond the reach of optics, for the poor
-unfortunate boy saw things that his companions did not, even to a couple
-of demons dancing through the streets in broad daylight. In fact, it is
-somewhat difficult to draw the line between reality and imagination in
-this case. No doubt the story is considerably exaggerated by Cellini,
-who was a fervid Italian, and prone to believe in wonders, as is
-instanced by his wish to study the black art. The priest, too, whom he
-describes as a man of genius, no doubt had a great influence over the
-famous artist, and made him see a great deal more than was really there.
-
-The introduction of the magic lantern provided the magicians of the
-seventeenth century with a very powerful instrument with which to
-continue their deceptions. The use of the concave mirror, which does not
-appear to have had any accessories worth speaking of, required a
-separate apartment, or at least a hiding-place of some sort that was
-difficult to discover under ordinary circumstances; but the magic
-lantern, inclosing as it did the lamp, the optical apparatus, and the
-figures in a comparatively small spice, was particularly appropriate to
-the wants of the Homes and Davenports of the day, who until then had
-never possessed anything so convenient and portable.
-
-[Illustration:
-
-  FIG. 50.—Section of the Magic Lantern.
-]
-
-The magic lantern shown in figures 50 and 51 consists of a dark box,
-containing a lamp and a concave metallic mirror, constructed in such a
-way that the whole of the rays proceeding from the lamp are reflected
-through the aperture holding the optical portion of the apparatus. In
-front of the box is fixed a double tube C D, one-half of which (D)
-slides in the other. A large plano-convex lens _c_ is fixed at the inner
-extremity of the double tube, and a small one at its outer end. To the
-fixed tube C E is attached a groove _b b_, which serves to hold the
-painted glass. These glasses, or slides as they are generally called,
-are painted with strong transparent colours.
-
-The direct light of the lamp G, as well as that reflected by the mirror
-and passing through the lens _c_, is so concentrated as to project a
-brilliant beam of light through the painted slide, which being in the
-conjugate focus of the large plano-convex lens _d_, the pictures on the
-glass are refracted in a magnified form on the white cloth P Q.
-
-The magic lantern, therefore, consists of a box to hold the lamp, a
-concave mirror, and a convex lens to concentrate the light on the slide,
-and a second convex lens to throw the image on the screen.
-
-
-------------------------------------------------------------------------
-
-[Illustration:
-
-  FIG. 51.—Magic Lantern.
-]
-
-
-------------------------------------------------------------------------
-
-
-
-
-                              CHAPTER II.
-                          THE PHANTASMAGORIA.
-
-
-THE phantasmagoria may be described as a perfected magic lantern, and
-bears the same relation to its prototype that a shilling telescope
-bought in the Lowther Arcade does to one of Dollond’s or Ross’s field
-glasses. The position of the spectators, too, is different, being on the
-other side of the scene which receives the magnified pictures, already
-described when speaking of the magic lantern.
-
-[Illustration:
-
-  FIG. 52.—The Phantasmagoria.
-]
-
-The phantasmagoria lantern is generally mounted on a stand provided with
-castors so that it may be moved about at will. It consists of a box as
-represented in fig. 52, inclosing a lamp with a metallic reflector, the
-bundle of rays being sent through the centre of the tube containing the
-slide and lenses, as before described. The chimney serves to carry off
-the products of combustion generated by the lamp. In fig. 53 we have
-shown the interior of the tube containing the lenses. Between this tube
-and the body of the lantern there is a space within which slide the
-glasses whereon are painted the figures and landscapes that are to be
-thrown on the white screen. The luminous rays given off by the reflector
-in the interior of the lantern pass through a plano-convex lens placed
-with the flat side outwards. In front comes the double convex lens, or
-object-glass, which can be moved backwards and forwards by means of a
-rack and pinion. There is also a movable diaphragm, which is worked with
-a couple of cords, by pulling which the aperture is made larger or
-smaller at will. By moving the lantern backwards and forwards, working
-the rack and pinion and the diaphragm at the same time, the view seen by
-the spectator seems to advance and recede. The pictures are painted on
-glass with transparent colours, the glasses being generally about five
-inches in diameter. To render the illusion perfect it is necessary that
-the spectator should be placed in a partially dark room, being separated
-from the operator by the screen already mentioned. Everything being
-ready, the spectators having but little notion of the distance of the
-screen, a very small picture is shown to them first, the illumination
-being reduced to a minimum by pulling the cords which act on the
-diaphragm. The little picture first seen by them will appear to be
-situated at an enormous distance; but as the lantern is brought almost
-imperceptibly nearer to the screen, the image appears to advance towards
-them in a very surprising manner, at last appearing almost as if it were
-going to fall upon the spectators.
-
-[Illustration:
-
-  FIG. 53.—The Phantascope.
-]
-
-Robertson, an English optician who was settled in Paris some fifty years
-since, was one of the first to exhibit the phantasmagoria with success.
-In order to obtain the best results he used a room some sixty or eighty
-feet long, and twenty-four wide, which he hung entirely with black. Of
-this a strip twenty-five feet long was cut off and devoted to the
-manipulation of the phantasmagoria. This portion of the apartment was
-separated from the spectators by a white calico screen, tightly strained
-from side to side, and at first concealed from view by a black curtain.
-The calico screen, which was about twenty feet square, was well soaked
-in a mixture of starch and fine gum arabic, in order to render it
-semi-transparent. The floor was raised about four or five feet at one
-end in order that the whole of the spectators might have a free and
-uninterrupted view of what was going on.
-
-It is undoubtedly to Robertson that we owe most of the improvements in
-the phantasmagoria. The success of his performances in Paris during the
-first Revolution has never been equalled by any similar exhibition. The
-enthusiasm excited amongst the Parisian public at the time surpassed
-that awakened even by Cagliostro and Mesmer. The spirit which guided
-Robertson in exhibiting these wonders was totally opposed to that which
-animated the two charlatans just mentioned. Robertson, unlike them,
-sought to spread the notion that there was nothing occult or
-supernatural in the marvels he exhibited, but that they resulted simply
-from the application of a few simple laws of optics. We shall presently
-give an account of one these famous _séances_, which were powerful
-enough to distract the attention of the people of that day from the
-stormy events that were going on around them; but we will first allow
-our author to tell the story of his experiments in optics in his own
-words.
-
-“From my very earliest infancy,” he says in his Memoirs, “my lively and
-passionate imagination caused me to be dominated over by the marvellous
-in a very powerful manner. Anything that seemed to go beyond nature in
-any way, excited in me an ardour which then appeared to me capable of
-overcoming all obstacles in order to realize the effects I had
-conceived. Father Kircher, it was said, believed that the magic lantern
-was the invention of the Evil One. All the worse for Father Kircher, who
-was gifted with a great intellect, and many persons were tempted to say
-that he might possibly have some cause for believing in the diabolical
-origin of a simple optical instrument. But as the writer who has thus
-reproached Father Kircher with too much credulity has not cited those
-passages of the work in which this statement may be found, I did not
-think seriously of the matter. Who has not in his younger days believed
-in witches, hobgoblins, and compacts with the devil? I know I did, and
-worse; for I imagined and fully believed that an innocent old woman who
-was a neighbour of ours, really had dealings with Lucifer, as every one
-asserted. I even went so far as to envy her the power of conferring with
-the Evil One, and once shut myself up in my room with an unhappy live
-cock, whose head I cut off in the most barbarous manner, having heard
-that that was the most approved manner of summoning into one’s presence
-the great head of all the demons. I waited for him several hours,
-calling on him to appear, threatening to deny his existence for the
-future if he did not appear, but all to no purpose. The books on magic
-and the black art that I had read had completely turned my head. I
-believed everything that was in them, and I desired ardently to perform
-the wonders they described, even with the aid of the devil. The _Magia
-Naturalis_ of Porta, and the _Recreations_ of Midorge, which treated
-simply of natural phenomena, had no effect upon me, but I was at last
-obliged to fall back on the principles involved in them, in order to
-create the diabolical appearances I had sought after in what I
-considered a truly supernatural manner, until at, last my dwelling
-became a true Pandemonium.
-
-“It is only our grandmothers, it has been said for a long time, who
-believe in magic, witches, and supernatural appearances; but the
-statement is hardly true, seeing how easily the country people fall a
-prey to the first cheat who chooses to invest himself with supernatural
-powers. We have sufficiently ridiculed the superstitions of the
-ancients, and numberless instances may be adduced which are a shame to
-their intelligence, and which gives, so to speak, a denial to the
-stories we have heard of their high state of civilization. But I
-believe, if we were to make a collection of all the stories of ghosts,
-of mysterious appearances, of communications between the living and the
-departed, of the discoveries of hidden treasures, &c., &c., which have
-taken place even since the Revolution, before whose power so many dark
-things have been brought to light, the collection would hardly be less
-bulky than that of the ancient superstitions now happily passed away.”
-
-Robertson then goes on to take great credit to himself for showing the
-world that all the superstitions concerning ghosts, spectral
-appearances, and other illusions of a similar nature, were to be easily
-accomplished, by simply studying natural laws. He appears first to have
-begun his optical experiments with the solar microscope, and we hear of
-his landlord taking an action against him to recover damages for having
-pierced the doors of his rooms with innumerable holes. He studied the
-subject both theoretically and practically for many years, in company
-with his friend Villette, and at last announced a public _séance_ at the
-Pavillon de l’Echiquier at Paris. A multitude of advertisements and
-prospectuses, written in the high-flown style of the time, were issued,
-and distributed throughout the city. The newspapers of the day are full
-of accounts of the extraordinary impression made on the minds of the
-Parisians by Robertson’s wonderful exhibition. The old-fashioned word
-magic lantern was quite abandoned, and the new and high sounding Greek
-appellation, “phantasmagoria,” was heard issuing from every one’s mouth.
-There is an amusing account given of Robertson’s exhibition in one of
-the contemporary journals, written by Poultier, one of the
-representatives of the people. He says: “A decemvir of the republic has
-said that the dead return no more, but go to Robertson’s exhibition and
-you will soon be convinced of the contrary, for you will see the dead
-returning to life in crowds. Robertson calls forth phantoms, and
-commands legions of spectres. In a well-lighted apartment in the
-Pavillon de l’Echiquier I found myself seated a few evenings since, with
-some sixty or seventy people. At seven o’clock a pale thin man entered
-the room where we were sitting, and having extinguished the candles he
-said: ‘Citizens and gentlemen, I am not one of those adventurers and
-impudent swindlers who promise more than they can perform. I have
-assured the public in the _Journal de Paris_ that I can bring the dead
-to life, and I shall do so. Those of the company who desire to see the
-apparitions of those who were dear to them, but who have passed away
-from this life by sickness or otherwise, have only to speak, and I shall
-obey their commands.’ There was a moment’s silence, and a haggard
-looking man, with dishevelled hair and sorrowful eyes, rose in the midst
-of the assemblage and exclaimed, ‘As I have been unable in an official
-journal to re-establish the worship of Marat, I should at least be glad
-to see his shadow.’ Robertson immediately threw upon a brasier
-containing lighted coals, two glasses of blood, a bottle of vitriol, a
-few drops of aquafortis, and two numbers of the _Journal des Hommes
-Libres_, and there instantly appeared in the midst of the smoke caused
-by the burning of these substances, a hideous livid phantom armed with a
-dagger and wearing a red cap of liberty. The man at whose wish the
-phantom had been evoked seemed to recognise Marat, and rushed forward to
-embrace the vision, but the ghost made a frightful grimace and
-disappeared. A young man next asked to see the phantom of a young lady
-whom he had tenderly loved, and whose portrait he showed to the worker
-of all these marvels. Robertson threw on the brasier a few sparrow’s
-feathers, a grain or two of phosphorus, and a dozen butterflies. A
-beautiful woman, with her bosom uncovered and her hair floating about
-her, soon appeared, and smiled on the young man with the most tender
-regard and sorrow. A grave-looking individual sitting close by me
-suddenly exclaimed ‘Heavens! it’s my wife come to life again,’ and he
-rushed from the room, apparently fearing that what he saw was not a
-phantom.
-
-A Swiss asked to see the shade of William Tell. The phantom of the great
-archer was evoked with apparently as much ease as the others. Delille,
-who was present, called for Virgil, whose Georgics he had lately
-translated. The poet appeared, having in his hand a laurel crown, which
-he held out to his French commentator. Many other equally extraordinary
-apparitions were shown at the will of various individuals in the
-audience, and towards the end of the evening Robertson showed his
-judgment, and under very difficult circumstances. A royalist who was
-present asked for the phantom of Louis XVI., the appearance of which
-would no doubt have raised a tumult amongst so many red-hot Republicans,
-had not Robertson replied that before the 18th Fructidor, the day on
-which the French republic declared that royalty was abolished for ever,
-he had had a receipt for bringing dead kings to life again, but that
-same day he lost it, and feared that he should never recover it again.
-The answer was said to have been whispered to Robertson by his friend
-Ponthieu, who saw the difficulty he was in. It was supposed that the
-demand was prompted by an agent of the police, who for some cause had a
-spite against Robertson. In any case the affair made such a noise that
-the next day the exhibition was prohibited by those in authority, and
-seals were placed upon the optician’s boxes and papers. The exhibition
-was, however, afterwards allowed to be continued, and was so successful
-that it had to be transferred to the old Capuchin convent near the Place
-Vendôme.
-
-The whole of Paris rang with eulogiums upon Robertson’s wonderful
-exhibition at the Capuchin Convent. He had purposely chosen the
-abandoned chapel, which was in the middle of a vast cloister crowded
-with tombs and funereal tablets. It was approached by a series of dark
-passages, decorated with weird and mysterious paintings, and the very
-door was covered with hieroglyphics. The chapel itself was hung with
-black, and was feebly illuminated by a single sepulchral lamp. The whole
-assembly involuntarily remained grave and silent, and it was only when
-the first preparations for the exhibition were made, that the audience
-broke into a low murmur. Robertson commenced with an address on sorcery,
-magicians, witches, ghosts, and phantoms, and, having worked the
-spectators up to the proper pitch, he suddenly extinguished the single
-antique lamp already mentioned, plunging the assembly into perfect
-darkness. Then there arose a storm of rain, wind, thunder, and
-lightning. The bells tolled lugubriously as if summoning the dead from
-their tombs beneath the feet of those present; the wind whistled
-mournfully, the rain fell in torrents, the thunder rolled, and the
-lightnings flashed. But suddenly above all this confusion were heard the
-sweet notes of a harmonium, and in the far-off distance the sky was seen
-clearing gradually. A luminous point then made its appearance in the
-midst of the clouds, which gradually became the figure of a man,
-increasing in size every instant, until it seemed to be about to
-precipitate itself on to the spectators. A man in the front row was so
-frightened, that he uttered a scream of terror, when the phantom
-instantly disappeared. A series of spectres then issued suddenly from a
-cave. The shades of great men crowded together round a boat floating on
-a black and sluggish river, which the spectators had no difficulty in
-identifying as the Styx. The shadows gradually disappeared in the
-distance, getting smaller and smaller until they became invisible.
-
-Robertson was extremely careful in all his entertainments to flatter the
-popular ideas of the day. For instance, one of his most famous
-exhibitions consisted in a picture of a tomb, in the middle of which
-Robespierre issued. The figure, as usual, walked towards the spectators;
-but when apparently within a few yards of them, it was struck down by
-lightning. Voltaire, Lavoisier, Rousseau, and other popular favourites
-then appeared on the scene, and disappeared again in the usual manner.
-Robertson generally ended his entertainment with an address something
-like the following:—
-
-    “We have now seen together the wonderful mysteries of the
-    phantasmagoria. I have unveiled to you the secrets of the
-    priests of Memphis. I have shown you every mystery of optical
-    science; you have witnessed scenes that in the ages of credulity
-    would have been considered supernatural. You have, perhaps, many
-    of you, laughed at what I have shown you, and the gentler
-    portion of my audience have possibly been terrified at many of
-    my phantoms; but I can assure you, whoever you may be, powerful
-    or weak, strong or feeble, believers or atheists, that there is
-    but one truly terrible spectacle—the fate which is reserved for
-    us all;”
-
-and at that instant a grisly skeleton was seen standing in the middle of
-the hall (fig. 54).
-
-[Illustration:
-
-  FIG. 54.—Phantasmagoria (ROBERTSON).
-]
-
-Even in those unbelieving days, when scepticism of every sort was riding
-rough-shod over the French people, Robertson had the greatest difficulty
-in disclaiming all approach to the possession of supernatural powers.
-Day after day he received applications from all quarters to reveal the
-secrets of the past, present, and future, to describe events that were
-passing in other countries; and it frequently happened, that after his
-entertainments, he would be asked by several members of his auditory to
-assist them in recovering property that had been lost or stolen from
-them. In the latter kind of cases he generally used to adopt the
-excellent plan of sending his would-be clients to the nearest
-police-office.
-
-
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-
-
-
-
-                              CHAPTER III.
-                        OTHER OPTICAL ILLUSIONS.
-
-
-BY varying the disposition of mirrors, prisms, lenses, and light, an
-infinite number of the most surprising effects may be shown, with a
-comparatively small amount of trouble and expense. We shall, therefore,
-devote this chapter to the explanation of a large number of allusions,
-which have been devised by Robertson and other adepts in the art of
-honest deception.
-
-One of Robertson’s most famous delusions was the “Dance of Demons,” an
-effect he discovered quite accidentally. One evening, while
-experimenting with the phantasmagoria, he suddenly found himself in the
-dark, when two persons, each bearing a light, crossed the room on the
-other side of the screen. A little window which happened to be between
-the lights and the screen, immediately threw its double image on the
-cloth, and the method of multiplying shadows was discovered.
-
-[Illustration:
-
-  FIG. 55.—Wizard Dance.
-]
-
-The figures used in this experiment are cut out of fine cardboard, and
-may be made a foot high or there-abouts. They are placed on a second
-screen in front of the principal one, and by multiplying the lights, as
-shown in fig. 55, you may have as many shadows as you please. The effect
-is much heightened if the figures are cut out so as to show as lights
-when thrown on the screen. A little ingenuity shown in the arrangement
-of the distance and movements of the lights, will produce an endless
-amount of amusing effects. Thus, a small image of the principal figure
-may be produced by carrying the second light to a great distance, and
-the lesser figure may be easily made to jump over the former, by moving
-the candle in a semicircle over the light that is stationary. It is only
-necessary to recollect that whatever movements are made by the lights,
-the shadows of the figures follow their example. With a little ingenuity
-the heads and limbs of the figures may be made moveable; and if one
-assistant attends entirely to the working of the figures, and the rest
-to the lights, an infinite number of changes may be carried out. If
-mounted in a frame, they may be made to throw somersaults, fall down, or
-jump up in the air at will.
-
-A knowledge of optics will often serve to explain with great ease the
-tricks played by conjurers and impostors on princes and other great
-people, for their own vile ends. It is well known that Nostradamus, on
-being consulted by Marie de Médicis on the future destiny of France, was
-shown by him in a mirror events that left no doubt on her mind that she
-would one day share the throne of the Bourbons. These illusions were
-possibly effected in the following manner, and may be readily understood
-by reference to fig. 56.
-
-The throne in the first chamber is reflected in a mirror concealed in
-the canopy overshadowing a second mirror, placed carelessly on a table
-in the room in which the Princess and astrologer are standing. The
-arrangement of the mirrors is such that, on looking into the smaller
-glass, the Princess sees all that is going on in the adjoining chamber.
-The very fact of her consulting Nostradamus on her future fate, shows
-that under certain circumstances, at least, this clever woman was as
-silly as a child. It is not, therefore, to be supposed that she would
-notice that the mirror she was looking into was inclined at such an
-angle that it could not reflect her beautiful face. Nothing could be
-more natural, either, than that this magic looking-glass should be
-placed on a daïs, and shaded by a canopy. Nostradamus, who was a shrewd
-man, could no doubt pretty well see the course that events would take,
-and must consequently have felt quite safe in showing the Princess the
-throne of France occupied by Henry of Navarre. This was not the first
-time that the rulers of the earth were duped by so-called magicians, who
-possessed the knowledge that the angle of reflection was always equal to
-the angle of refraction.
-
-We may also mention, while speaking on this subject, the adventure of
-the Emperor Alexander of Russia, _à propos_ of a singular optical
-experiment at which he was present, which had for its end the changing
-of a man into a wild animal, or _vice versâ_. Certain cynics will
-possibly say that this is by no means difficult, and that it is an event
-that happens every day; but the clever trick at which Alexander was so
-astonished was not moral but purely physical. After having gained much
-money and fame in France, Robertson directed his steps towards Hamburg,
-where the Emperor was at that time stopping. He performed before the
-Czar an experiment that puzzled his Majesty beyond endurance. He showed
-him a man upon whose shoulders he saw successively the head of a calf, a
-lion, a tiger, a bear, and a whole menagerie of other animals. At last,
-the Czar could stand it no longer, and he suddenly rose, put his
-shoulder against the partition, and brought the whole to the ground with
-a loud crash, just at the moment that the confederate was assuming the
-form of a goat. If our readers would like to join the Czar in his
-discovery of the manner in which the trick was performed, they can
-easily do so.
-
-[Illustration:
-
-  FIG. 56.—Nostradamus and Marie de Médicis.
-]
-
-The room in which this trick is to be performed should have a smaller
-one adjoining it, about eight feet square. The magician in the first
-place shows the small apartment to the spectator, who perceives that it
-contains nothing but an empty chair placed against the wall. The
-partition between the two rooms is provided with a small hole, covered
-with glass, exactly opposite the chair, and at about the ordinary height
-of the eyes. On the inner side there are two grooves, in which slide a
-block of wood containing a prism, as shown in fig. 57, which may be
-quickly and easily replaced by a piece of plane glass. On looking
-through this opening, the spectator sees a man sitting in a chair, but
-suddenly, without any apparent cause, the man changes into a goat, a
-sheep or some other animal. The sudden replacing of the prism, which
-takes place without the spectator perceiving it, causes him to see, not
-the floor with the man and chair upon it, but the ceiling, which is
-carpeted exactly in the same way, and is provided with a precisely
-similar chair, upon which is placed a goat or any other animal.
-
-[Illustration:
-
-  FIG. 57.—The Arrangement of the Reversing Prism.
-]
-
-While looking at the goat, the plane glass is substituted for the prism,
-and the man reappears; another movement of the prism, and he changes
-into a sheep, a figure of a sheep having in the meantime replaced that
-of the goat. Of course it is necessary not merely to have the walls,
-floors, and chairs precisely alike, but they must each occupy the same
-relation to each other. If it is desirable only to change the head, it
-is simply necessary to have a lay figure with a moveable head, dressed
-precisely in the same manner as the living operator, in the upper
-portion of the chamber. At the end, by the substitution of the empty
-chair, the individual may be made to disappear entirely.
-
-There may often be seen in the streets of London, a man showing a
-wonderful instrument, consisting of a telescope cut in two, the two
-portions being separated from each other by an interval of three or four
-inches. On looking through the instrument, the spectator of course sees
-the object at which it is pointed; but what is his astonishment to find,
-that when the showman places a brick between the two halves of the
-instrument he sees just as well as before. The showman generally informs
-him that the instrument in question has such powerful lenses, that it
-will not only see through a brick, but even through a policeman’s head
-if it happened to be in the way; and the spectator, having paid his
-penny, goes away perfectly mystified, until, like the young lady who
-believed that all machinery was worked “by a screw, somehow,” he
-comforts himself with the idea that the trick is performed “by a mirror,
-somehow.” The following figure will, however, soon clear up the mystery.
-
-[Illustration:
-
-  FIG. 58.—The Goat Trick.
-]
-
-Let F M, L G be an ordinary telescope tube, to be separated in the
-middle by an interval large enough to insert a brick, the hand, or some
-other opaque object. The whole is fixed on a stand, consisting of a
-square tube with a couple of elbows to it. Between G and L a mirror (A)
-is placed diagonally, which receives the image of the objects to be
-looked at. This mirror sends the image downwards to another placed
-diagonally at C, a third being placed at D, and a fourth at B. The
-horizontal ray, meeting the mirror at A, is consequently bent downwards
-to C, then travels horizontally to D, when it is reflected upwards to B,
-in which it is seen by the eye. Of course a simple tube without any
-lenses at all would serve the same purpose, but the fact of its being a
-telescope serves to distract the attention of the too curious observer.
-
-[Illustration:
-
-  FIG. 59.—How to see through a Brick.
-]
-
-Another illusion of the same kind is often practised at fancy fairs and
-bazaars, when a spectator looking into what he supposes to be an
-ordinary looking-glass, sees his companions instead of himself. The way
-in which this is effected is very simple. A looking-glass is placed
-diagonally across a square box, the apertures in the sides being so
-arranged that the spectator does not perceive that he is looking into a
-glass that is placed at an angle. Of course the exhibitor endeavours to
-show the illusion to two persons at once; and if they are strangers to
-each other, and of the opposite sex, a great deal of fun is made out of
-the trick. A showman at Greenwich made an immense harvest by showing two
-such mirrors, one to all the young girls who wished to see their future
-husbands, and the other to all the young men who wished to see their
-future wives. Of course he had a tolerably good-looking male and female
-confederate to help him. With a couple of mirrors placed back to back in
-a square case, with an opening on each side, the illusion is still more
-perfect, as on looking through any of the holes the box seems to be
-quite empty.
-
-The “Speaking Head” trick is performed on this principle. When the
-curtain is drawn up, the audience perceive an apparently living head
-placed on a small three-legged table, the curtain at the back of the
-stage being quite visible through the legs. By and by the bodiless head,
-which is generally painted in a very fantastic manner, begins to speak,
-answers questions, and ends by singing a song. The trick is performed in
-the following way: The spaces between the legs are filled with a
-looking-glass; consequently, the spectators see the reflection of the
-curtains at the _sides_ of the stage, which are made exactly like those
-at the back, thus giving the table the appearance of standing on three
-slim legs, with nothing between. Behind the looking-glass there is of
-course plenty of space for the body of the man belonging to the magical
-head. The exhibitor naturally takes especial care never to pass in front
-of the table, otherwise the lower part of his body would be reflected in
-mirrors.
-
-[Illustration:
-
-  FIG. 60.—The Polemoscope.
-]
-
-The polemoscope (from two Greek words signifying “war” and “to see”) is
-another instance of double reflection. It was said to have been invented
-by Helvetius, about 1637. Fig. 60 will show the principle of this
-instrument.
-
-The luminous rays coming from a distant object are received upon an
-inclined mirror, which is elevated above the parapet of a fortification,
-and are reflected downwards to a second, which is placed at a
-corresponding angle. If necessary, lenses can be interposed, so as to
-give a magnified view of the distant object that is being examined. By
-means of such an instrument, the movements of the enemy can be followed
-without danger, the apparatus being generally of small size, and not
-attracting notice. Amongst the varieties of this instrument, is one
-whose use is readily seen by inspecting fig. 61, by which it seems to be
-perfectly possible to see with safety all that is going on outside the
-door of the house without being perceived. The line of the mirrors in
-this case is at right angles to that of the polemoscope in fig. 60.
-Amongst the different varieties of polemoscope which have been invented,
-may be mentioned a reflecting opera-glass, which was greatly used by the
-beaux and dandies of the last century. In the tube of this instrument
-was inserted an inclined mirror, which allowed the spectator to point
-his glass in quite a different direction to that of the object he was
-really looking at. In fact, it was constructed somewhat on the same
-principle as the Herschellian or Newtonian telescope, and enabled the
-possessor, while apparently enjoying the play, to observe all that was
-going on in the boxes or pit of the theatre. Years ago, there was a
-little instrument of a similar kind, sold for a penny in the streets of
-London, which consisted of a morsel of looking-glass set at an angle, in
-a pill-box, and which gave the possessor the power of seeing all that
-was going on behind him. Persons who wear dark preservers are often in
-the habit of observing all that is going on behind their backs by the
-reflection seen in the corner of their glasses.
-
-Such are the principal optical recreations founded on the reflecting and
-refracting properties of mirrors and lenses. We shall end this chapter
-by appending to it the description of a few additional optical
-amusements that are quite within the reach of the amateur.
-
-If the reader is in possession of a concave mirror, it may be made the
-means of performing a number of amusing experiments. In front of it is
-placed a plaster head, a skull or any other object, mounted on wheels
-and running along a grooved platform, which is naturally kept perfectly
-concealed from the spectators. The mirror is slightly inclined, so as to
-reflect the image of the object at an angle to the observer’s eye. By
-running the cast backwards and forwards, it will have the appearance of
-advancing and retiring from the spectator in a very imposing manner. A
-dagger may be substituted for the cast, and by being made to work up and
-down on a pivot, will have the appearance of striking at the spectator.
-We have already seen that an experiment of this sort had such an effect
-on Louis XIV. that he drew his sword to defend himself from his
-imaginary aggressor. There is another way of performing this trick, by
-suddenly illuminating the skull or dagger by means of a dark-coloured
-box containing a light, which may be made to throw its reflections on
-the object, by sliding it along a couple of wires. In the case of the
-dagger, however, the hinged arrangement will be found more effective.
-
-[Illustration:
-
-  FIG. 61.—Protection against ill-natured people.
-]
-
-One of Robertson’s tricks was called the “Magic Box,” and he astonished
-a numerous party of visitors who were staying at a country house to
-which he had been invited. One of the gentlemen who was always boasting
-of his freedom from superstitious feelings of any kind, had had several
-arguments with Robertson on the subject of apparitions, and the latter
-thought that he would at any rate surprise his strong-minded friend by
-an easy trick or two. He consequently chose as his confederate a lady to
-whom the gentleman had been paying great attention during the time of
-his visit. Robertson one evening mysteriously delivered a small box to
-him, which he was to place upon his toilet table, and unlock exactly at
-midnight. The gentleman did so, and what was his astonishment to see the
-face of the lady with whose charms he had been so deeply impressed
-suddenly spring out of the box. His look of terror and surprise was
-evidently too much for Robertson’s confederate, who burst into a merry
-peal of laughter, leaving her admirer in a very disconcerted state.
-
-After all we have said on the subject of mirrors, it is not difficult to
-guess how this trick was performed. The box in question was painted
-black on the inside, and contained a concave mirror placed at an angle
-of 45°. The reflection of the lady, who was of course in the next room,
-was carried by means of several plane mirrors placed in boxes
-communicating with each other through the partition of the room, the
-head of the lady only being strongly illuminated, the rest of her figure
-not appearing by being kept quite dark.
-
-The figures reflected from smoke are extremely surprising. To perform
-such experiments a phantasmagoria is necessary. The focus is so adjusted
-that the distant image falls just above a brasier containing lighted
-charcoal. Everything being ready, a few grains of olibanum or other gum
-are thrown on the coals, and the smoke that rises immediately affords a
-screen for the reflection of the images proceeding from the
-phantasmagoria. If the amateur is not the possessor of a magic lantern,
-a properly arranged concave mirror will answer almost the same purpose.
-
-
-------------------------------------------------------------------------
-
-
-
-
-                              CHAPTER IV.
-                       THE PROPERTIES OF MIRRORS.
-
-
-ALMOST every one in his younger days has possessed and broken that
-pretty instrument known as the kaleidoscope. His researches into its
-construction no doubt taught him that it consisted of a cylindrical tube
-in tin or cardboard, with a moveable cap at one end and a small hole at
-the other. In the interior of the tube were found three long glasses,
-blackened on the back, placed at an angle, and kept in position by
-pieces of cork. The moveable cap was provided with two circular pieces
-of glass, one ground and the other transparent, between which were
-placed a number of pieces of coloured glass. On holding the instrument
-up to the light and looking through the eye-hole, a beautifully coloured
-star was seen whose form and hue changed by simply shaking the tube.
-
-The kaleidoscope was invented by Sir David Brewster, and is exceedingly
-simple in principle. We all know that if a luminous object, such as a
-taper, is placed before a mirror, it gives forth rays of light in all
-directions. Amongst these luminous rays, those that fall on the surface
-of the mirror are, of course, reflected in such a manner that the angle
-of reflection is equal to the angle of incidence. If another mirror be
-placed at right angles to the first, and an object be put in the angle,
-the image of it will be multiplied four times. If the angle be
-diminished to 60°, six reflections will be seen, and so on. A
-symmetrical figure is constantly obtained, forming in one case a cross
-composed of four similar portions; in the other a triple star, the
-halves of each ray being similar. It is the symmetry of the figure that
-gives the pleasing effect. In the ordinary kaleidoscope the angle made
-by the reflecting surfaces is thirty degrees, and a star of six rays is
-formed, the halves of each ray being alike. The figures formed in the
-kaleidoscope are simply endless; and if the space between the glasses in
-the moveable cap be filled with bits of opaque as well as transparent
-substances, the varieties of light and shade may be added to those of
-colour. It was at one time the fashion to copy the images formed in the
-kaleidoscope as patterns for room papers, muslins, curtains, shawls, and
-other similar fabrics, but thanks to the spread of artistic taste in
-this country the decorative designer now relies more on his own talent
-than any aid he may receive from optical instruments.
-
-Plane mirrors, as we have seen, reflect objects upright and symmetrical,
-reversing only the sides. Concave mirrors reverse them, and if they are
-not placed exactly in the proper focus, distort them by making one
-portion appear smaller than the other; while convex mirrors reflect them
-in an upright position, but also similarly slightly distorted. But when
-the mirror is not a portion of a sphere, like those whose properties we
-have been considering, the distortion is increased to so great an extent
-as to deform the object so that it is difficult to recognise its nature
-from its reflection. We all know the distortion that our face undergoes
-when reflected from the shining surface of a teapot or spoon, and the
-cylindrical mirrors that hang in the shop windows of many opticians are
-the source of much amusement to the passers by, whose physiognomies are
-shown to them either lengthened to many times their natural size, or
-widened to an extent that is ludicrously hideous, according to the
-position in which the mirror is hung. Such distortions are known to
-opticians as _anamorphoses_, from two Greek words signifying the
-destruction of form; and distorted drawings used to be sold at one time
-which when reflected from the surface of the cylindrical mirror, became
-perfectly symmetrical. Anamorphic drawings may be also made, which when
-looked at in the ordinary manner appear distorted, but when viewed from
-a particular point have their symmetry restored to them. With a little
-knowledge of drawing, it is not difficult to produce these in great
-variety.
-
-Suppose the portrait in fig. 62 to be divided horizontally and
-vertically by equidistant lines comprehended within the square A B C D.
-
-[Illustration:
-
-  Fig. 62.
-]
-
-Upon a second piece of paper draw the figure shown in fig. 63 in the
-following manner. Draw the horizontal line _a b_ equal to A B (fig. 62),
-and divide it into the same number of parts. Through the centre draw a
-perpendicular line to V, and cross it by a line _e d_ parallel to _a b_.
-Lastly, draw V S horizontal to _e d_. The length of the two lines _e_ V
-and S V is quite arbitrary, but the longer you make the former in
-proportion to the latter the greater will be the distortion of the
-drawing. Now draw the lines V 1, V 2, V 3, and V 4, and join S to _a_.
-Wherever S _a_ crosses the divisions 1, 2, _e_, 3, 4, and _b_, draw a
-horizontal line, parallel of course with _a b_. You will thus have a
-trapezium _a b c d_ divided into as many spaces as the square A B C D in
-fig. 62, and it now remains to fill them in with similar portions of the
-figure. Thus, for instance, the nose is in the fourth vertical division,
-starting from the left, and in the third and fourth counting from the
-top; in order, therefore, to make it occupy so lengthened a space it
-must be considerably distorted by the pencil. It will be readily seen
-also that the more numerous the spaces into which the square is divided,
-the easier it will be to draw the distorted picture. It is by this means
-that the _anamorphosis_ shown in fig. 63 has been drawn.
-
-The next thing to do is to find the point of view from which we can see
-the figure in its natural proportions. This will be found to be at a
-distance above the point V equal to the line V S. In order to complete
-the experiment it is simply necessary to place the distorted picture in
-a horizontal position, and fix a piece of cardboard vertically at the
-point V. If a hole be punched in it at a distance from V equal to S, and
-the drawing be looked at through it, the whole of the parts will fall
-into symmetry immediately.
-
-The experiment may be tried first with fig. 63, the hole being made
-rather large, and the eye placed at a distance of from 3 to 4 inches.
-
-It would be difficult, without having recourse to geometrical formulæ,
-to explain how it happens that by placing the eye at a particular point
-the distorted lines of the drawing become symmetrical; but perhaps a
-mechanical demonstration will help to make this difficult subject a
-little plainer.
-
-[Illustration:
-
-  FIG. 63.—Anamorphosis.
-]
-
-Draw in outline any figure upon a piece of cardboard, and make a series
-of pin-holes along the most prominent lines of the drawing, taking care
-that they are pretty close together. Place the perforated card in a
-vertical position on a sheet of paper, so that the rays from a candle or
-lamp may fall, on the flat surface beneath. On looking at the luminous
-figure formed from the drawing, you will find that it is as much
-distorted as the lady’s head in fig. 63, and that the lower you place
-the candle the greater will be the deformity. You may if you please,
-trace the luminous figure on the paper, and the result will appear
-distorted when looked at in the ordinary manner, but symmetrical when
-viewed from the point at which the flame of the candle was placed.
-
-In the foregoing experiments we have spoken of the anamorphic drawings
-as being placed in a horizontal position, but they may be looked at just
-as well vertically, the card with the hole being in this instance
-horizontal. It is also not necessary that the point of sight (V, fig.
-63) should be in the centre of the picture; it may be placed at one side
-or the other, care being taken to draw all the divisional lines so that
-they meet at this particular spot. A few experiments with a candle and a
-perforated figure will soon show the student how to accomplish this.
-
-Anamorphoses by reflection may be prepared, if this principle is carried
-out, which appear a mass of confused lines until they are reflected in a
-cylindrical mirror. Formerly opticians were accustomed to construct
-anamorphoses which became symmetrical pictures when viewed in a conical
-mirror; but the fashion for such toys appears to have gone out. Such
-drawings were extremely difficult to make, and the mirrors, having to be
-ground and polished with great care, were very expensive.
-
-Some experimentalists have carried the subject so far that, by looking
-at the drawing of an object in particular positions, it changed into
-quite a different subject. In the cloister of an abbey that once existed
-in Paris, there were two anamorphoses of this kind. They were the work
-of a certain Father Niceron, who has left behind him a treatise in Latin
-on optical wonders, entitled _Thaumaturgus Opticus_, which contains a
-long essay on anamorphoses. One of these pictures represented St. John
-the Evangelist writing his Gospel; the other Mary Magdalene. When looked
-at in the ordinary manner, they appeared to be landscapes; but when the
-observer placed himself in a particular position, they changed into the
-figures we have mentioned.
-
-
-------------------------------------------------------------------------
-
-
-
-
-                               CHAPTER V.
-                            CHINESE SHADOWS.
-
-
-WHILE upon the subject of optical wonders, we should hardly be forgiven
-if we did not give a description of the amusement known as Chinese
-shadows, or Fantocini. In the winter time it is difficult to pass
-through any of the large thoroughfares of London after nightfall,
-without seeing a crowd admiring the popular fantocini farces of the
-“Broken Bridge,” or “Billy Button;” and although these dramatic
-exhibitions are not always free from vulgarity, they are received with
-vociferous applause by at least the younger portion of the audience.
-
-The apparatus for the exhibition of the fantocini is generally very
-simple. The screen on which they are shown is generally made of calico
-rendered semi-transparent with copal varnish, and the figures are cut
-out of cardboard. Frames containing landscapes and scenes of different
-kinds are also provided, which are cut out in the same material. The
-_dramatis personæ_ are generally made with moveable limbs, which they
-throw about in the most unanatomical manner, and the showman is often
-endowed with ventriloquial talents of no mean order. This amusement is
-to be found in all parts of the world, from the Strand and Tottenham
-Court Road London, to the streets of Algiers and Java. A graphic writer
-in the _Magasin Pittoresque_ gives a pleasant description of the
-fantocini, as exhibited at the Arabs’ theatre in the Mohammedan quarter
-of the city of Algiers. It was on the occasion of the feast of the
-Bairam, which immediately follows the termination of the Ramadan, or
-Mohammedan Lent. The theatre, which was the only one frequented by the
-Arab population, consisted simply of a long vaulted hall, without seats,
-boxes, or galleries; but the audience, who had already been there some
-time, did not seem to regard the omission as of any consequence, but had
-seated themselves on the ground with great coolness, chatting in
-whispers, and waiting patiently until the director should consider the
-place full enough to begin the performance. Half an hour elapsed, and
-the spectators still chatted on quite unconcernedly; an hour, and yet
-there was no hissing or stamping of feet from the grave and patient
-spectators. At last they reached the maximum, and a boy came forward and
-blew out the few lamps with which the theatre was lighted, leaving them
-to smoulder away with a perfume that was certainly not Oriental in its
-character. First came the legend of the Seven Sleepers; then
-Scheherazade relating her bewitching stories to the Sultan. These were
-followed by Aladdin and the Wonderful Lamp, a story that is as popular
-in Algiers as it is in London or Paris; the whole culminating in a kind
-of burlesque, in which a great deal of gross fun was mixed up with a
-number of rebellious allusions. The devil, for instance, who is of
-course one of the members of the troupe, is portrayed as a French
-soldier, bearing a cross on his breast like an ancient Crusader. After
-him came Carhageuse, who is the buffoon of the Eastern stage, and who
-makes violent but unsuccessful love to a charming young Jewess. There
-was a poor barber who was raised to the dignity of grand vizier, his
-successor’s head being cut off by the yataghan of the Oriental Jack
-Ketch, to the great delight of the people. Then a wretched Jew receives
-the bastinado, amidst vociferous applause, which increases still higher
-when the ears of an unhappy Giaour are cut off and thrown to the dogs.
-Throughout the piece, it is of course the Mussulman who always triumphs,
-like the French guards at the _Cirque Impériale_, or the British
-grenadiers at old Astley’s. The performance concluded with a grand naval
-battle between the Moorish and Spanish fleets. The drum as usual served
-for cannon, there was a great deal of smoke and confusion, and the
-Christian fleet gradually sank under the continuous fire of the
-Mussulmans amidst the plaudits and bravos of the crowd.
-
-[Illustration:
-
-  FIG. 64.—Effect of cut paper-work.
-]
-
-In Java, the subjects of the fantocini are generally taken from the
-native mythology. The screen on which the shadows are exhibited is ten
-or twelve feet long, and five feet high, and the figures are cut of
-thick leather, their limbs being moved by thin pieces of nearly
-transparent horn.
-
-In fig. 64 we see another kind of Chinese shadows, in which the lights
-of the figure are cut out. These pictures are perfectly unrecognisable
-as being even the basest imitation of any known form; but when their
-shadows are thrown on the wall, the cut-out portions show us lights,
-whilst those that have been left form the shadows. On the Boulevard des
-Capucines, at Paris, there used to be a man who managed to pick up a
-good living by selling these candle shadows. Of course he used to carry
-on his trade of an evening, and with a strong lamp he would throw the
-shadows of his figures on the white walls of the houses, or the blind of
-a shop window, or even on the pavement. With a little care and ingenuity
-a number of these amusing cards may be easily designed. In showing them,
-care must be taken to choose the best distances between the light and
-the paper, and between this latter and the wall. If the card be placed
-too close to the wall, the resulting shadows will be too dark, and the
-outlines too sharp; if, on the contrary, the light is placed too far
-off, the outlines become confused, and the proper effect is lost.
-
-Shadows have been applied before now to the propagation of seditious
-ideas. “In 1817,” says an esteemed French author, “one winter’s night we
-were all sitting round the table listening to my father, who was reading
-aloud an interesting book of the period, when a friend of our family,
-who had been formerly an officer of the Empire, entered the room. He was
-a serious, upright, soldierly man, and wore his coat buttoned up to his
-chin. He had hardly replied to our salutations, when he drew a chair to
-the table, and made a sign with his hands and eyes that plainly
-indicated silence and discretion. There was something in the expression
-of his countenance that seemed to show that he had something mysterious
-in store for us, and we fully expected to hear some extraordinary news,
-or to see him bring out a Bonapartist pamphlet of more than usual
-importance. Our surprise was consequently great when we saw him slowly
-unscrew the top of his cane, which was turned out of boxwood, and
-presented nothing very remarkable either in form or material. He,
-however, took up a copybook which was lying on the table, placed it at a
-certain distance from the lamp, and then laid upon it the little piece
-of turned boxwood. At first we noticed nothing at all extraordinary, and
-he smiled at our want of intelligence, until at last my youngest brother
-cried out suddenly, ‘Look! there’s the head of Napoleon!’ and truly
-enough, we found, on looking more attentively at the shadows of the
-turned knob of the cane, that their profile was that of the great exile,
-most correctly and clearly portrayed. The old captain’s face lighted up
-at the sight, and the tears came into his eyes. ‘We shall see him
-again,’ he murmured in a low voice, and he hummed the burden of a
-Bonapartist song then in vogue. During the rest of the evening he was
-very lively, and proved to us most conclusively, that before six months
-the _Grande Armée_ would be revenged for their defeat at Waterloo. Some
-weeks after, there was hardly a soldier in the town that did not possess
-a stick or a tobacco-pipe stopper, turned in this fashion, but one day a
-panic seized everybody, and the canes and pipe stoppers were all burnt.”
-
-[Illustration:
-
-  Seal.                    Cane.
-  FIG. 65.—Seditious Toys.
-]
-
-Fig. 65 represents historic heads cut in this way. During the
-Shakespeare Tercentenary excitement, a London turner made quite a little
-fortune by making heads of the great poet on the same principle.
-
-
-------------------------------------------------------------------------
-
-
-
-
-                              CHAPTER VI.
-                  POLYORAMA—DISSOLVING VIEWS—DIORAMA.
-
-
-THE description of the polyorama naturally follows that of the
-phantasmagoria, being a practical application of precisely the same
-principles. In the case of the polyorama, however, two or even more
-lanterns of the best construction, are used. There are therefore two
-sets of lenses identical in every particular, placed side by side, in
-the same line, the foci of both being adjusted for the same spot, so
-that the images refracted from each may superpose each other without
-difficulty. In each instrument there are the same pictures, but they
-differ in certain particulars, as we shall see presently.
-
-In the phantascopes shown in figs. 52 and 54 there are two sets of
-lenses; the first carries a glass bearing the image of a skeleton in a
-winding sheet, while on the glass belonging to the second a naked
-skeleton is portrayed. If, therefore, at a given instant the first
-lantern is shut off, the spectators see the winding sheet torn, as it
-were, suddenly from the spectre before them. The first lantern being
-turned on once more, the skeleton is instantly reclothed in its hideous
-garb.
-
-It is of course not necessary always to choose such horrible subjects
-for representation, as it is possible to produce changes of a much more
-agreeable nature. For instance, a volcano may be depicted during its
-tranquillity, with the sun shining on its verdant sides, and surmounted
-with a gently rising wreath of smoke. Then it may be shown at night,
-with its crater vomiting flames and red-hot stones, while streams of
-lava are flowing beneath. By proper mechanism, one lantern may be
-gradually shut and the other as gradually opened, producing an effect
-that appears perfectly natural, from the gentle change which takes
-place. Daylight, twilight, and moonlight effects may be easily made to
-succeed each other in their proper order, and the most opposite scenes
-may be made to change each other by proper appliances. Those who have
-seen the dissolving views at the Polytechnic, know what effects are
-produced by this very simple means. A virgin forest changes to a crowded
-church, which in turn dissolves into a scene on the Alps.
-
-The diorama, properly so called, invented by the illustrious Daguerre,
-differs completely in principle from the apparatus we have just been
-describing. As its etymology indicates, the pictures shown are seen
-_through_. As in the case of the polyorama, there are two different
-effects painted upon the cloth, which are brought out by a double system
-of illumination.
-
-[Illustration:
-
-  FIG. 66.—Diorama.
-]
-
-Fig. 66 will show the way in which these changes are managed. The large
-picture, which is hanging vertically, is painted both in front and
-behind. The front is illuminated by reflection from a semi-transparent
-screen placed over it, which receives the light of the floor above. The
-back is lighted from the windows behind, which are provided with blinds
-to regulate the amount of light. The effects produced by the diorama
-were truly marvellous, and Daguerre had a special talent for this kind
-of painting. His famous _Midnight Mass_, which was exhibited at the
-Regent’s Park, was one of the most renowned of his works. The scene
-first represented a dark, empty church, feebly lighted by a small altar
-lamp, but gradually the lights appeared here and there, worshippers
-congregated in front of the altar, filling the nave and aisles. In Paris
-the same scene was exhibited, representing the interior of the Church of
-St. Germain l’Auxerrois with such perfect reality, that a countryman
-actually threw a halfpenny against the painted canvas, to see whether he
-were really in a church or not.
-
-The next scene represented the destruction of the village of Goldau,
-near Lucerne, by a landslip. First there appeared a smiling fertile
-valley, its sides crowned with verdure; a storm gradually rose, the rain
-fell, the wind blew, the lightnings flashed, and the thunder rolled in
-the distance. Darkness at last closed in, and when the sun once more
-rose over the valley, nothing was to be seen but a mass of fallen rocks.
-
-
-------------------------------------------------------------------------
-
-
-
-
-                              CHAPTER VII.
-                            THE STEREOSCOPE.
-
-
-HAVING devoted so much space in the preceding chapters to optical
-amusements of a purely recreative character, it is only right that we
-should now say a few words on certain instruments of a less frivolous
-character than those we have lately been considering, and which deserve
-at our hands the most serious attention. We shall, therefore, in the
-present chapter, speak of an ingenious instrument which serves to show
-in relief the images of objects depicted on a flat surface. We have
-already seen, that although we have two eyes, provided with lenses and
-screens by means of which the images of things around us are formed, we
-only perceive a single object; and the student has no doubt long since
-wondered why nature has bestowed two eyes upon us, when only one would
-have apparently served the same purpose. This question was for a long
-time a complete puzzle to philosophers, and it was not until Professor
-Wheatstone made his experiments on binocular vision in 1838, that the
-matter received a satisfactory explanation. He showed that each eye
-receives a different impression of any object upon the retina, and that
-it is in consequence of the union of these slightly dissimilar images
-that the sensation of relief is experienced. A one-eyed man or a Cyclops
-would only partially perceive relief in the objects presented to his
-view, in consequence of a single image being sent to his brain. He
-would, no doubt, after examining the things he saw with his hands, know
-they were solid, and generally see them so; but if a new object were
-presented to his view he would have some difficulty in knowing whether
-it had a flat surface or not.
-
-[Illustration:
-
-  FIG. 67.
-]
-
-The principle of binocular vision may be explained as follows: If a
-playing die, such as is represented in fig. 67, be held out at arm’s
-length in the position indicated in the figure, and looked at first with
-the left eye and then with the right, we shall find that in the first
-case we see a little of the three dots on the left-hand side, and in the
-second we lose sight of the three dots and see a little of the single
-one on the right-hand side. The images seen by each eye are, therefore,
-slightly dissimilar, and it stands to reason that, if by any means we
-can combine two slightly dissimilar flat pictures of a solid object, we
-shall see it in relief. This was proved practically by Professor
-Wheatstone, who constructed an instrument capable of effecting the
-desired union, and which has since been called the stereoscope, from two
-Greek words signifying ‘to see solid.’ The instrument remained for a
-long time fallow, so to speak, from the difficulty of drawing two
-pictures that should be identical in size and details, although
-dissimilar in the arrangement of their perspective. It was, therefore,
-not until photography enabled us to do this with the greatest ease and
-exactitude that the stereoscope became common. The instrument first
-devised by Professor Wheatstone, was what is termed a reflecting
-stereoscope, and was expensive to make and cumbrous to use. It was
-modified by Sir David Brewster, by the substitution of prisms for
-reflectors, and was thus made cheaper and more portable. The refracting
-form of stereoscope is so familiar to most people, that it really needs
-no description. It will only be necessary to mention that the prisms
-used in the eye-pieces are made by cutting a double convex lens in two,
-and reversing the halves. They are so placed that the centre of each
-prism is just in the centre of each eye; but as the eyes of different
-people vary in distance, an arrangement is generally added so that the
-eye-pieces may slide from side to side. Being cut from lenses, the
-prisms have a magnifying power; consequently other means are provided
-for sliding them up and down to suit the length of focus in different
-eyes.
-
-[Illustration:
-
-  FIG. 68.—Stereoscope.
-]
-
-In fig. 69 we can follow the path of the rays proceeding from each
-picture, and reach the eyes apparently from a spot exactly between the
-two.
-
-In the reflecting stereoscope two mirrors are joined together at right
-angles to each other, the two pictures being placed at each side, at a
-distance corresponding to their size. The reflecting instrument,
-although not so portable, is in some sort superior to the other,
-inasmuch as pictures of any size can be seen by it, whilst in the
-smaller instrument the size of the photograph is limited by the distance
-at which the eyes are placed.
-
-[Illustration:
-
-  FIG. 69.—The Principle of the Refracting Stereoscope.
-]
-
-It should be mentioned, that no optical instrument of any kind is
-absolutely necessary to obtain a stereoscopic effect from two suitable
-drawings or photographs, as it is quite possible by a little management
-of the eyes to cause the two images to combine with each other.
-Referring again to fig. 67, it will be perceived that the two figures of
-the dice are about an inch and a half from each other. Holding the book
-at about ten inches from the eye, they are viewed by squinting strongly
-until the _right_ eye looks at the _left_ die, and the _left_ eye at the
-right. This may be also done by converging the eyes on a point beyond
-the centre of the figure, which may be easily done by looking at a point
-midway between the two. In both cases the images at first appear
-doubled, and we see four dice, but a little practice will soon enable
-you to cause the two inside images to coalesce, and so give the effect
-of relief. It is true that even then three images are seen, but the eye
-soon grows accustomed to neglect them altogether. This habit is a very
-pleasant acquirement for the London _flâneur_, who can thus see in
-perfection the numberless stereoscopic views now shown in our
-shop-windows without the intervention of an instrument of any kind.
-
-The method of photographing subjects for the stereoscope is very simple,
-and consists in taking two views of the object to be depicted, from two
-different points. According to the distance of these points from each
-other, so will the resulting pictures appear in greater or less relief.
-This is readily seen in some stereoscopic portraits which have been
-taken at a large angle, and consequently show such increased relief as
-to produce distortion. Theoretically, the interval of the two points of
-view ought to be two inches and a half, that being the average distance
-between the two eyes; but in practice it is better to increase it in the
-case of portraits or other near objects to about twelve inches, and in
-that of views to even several feet. Brewster’s original rule for taking
-stereoscopic photographs, was to place the cameras one foot apart for
-every twenty-five feet of distance. The beautiful stereoscopic pictures
-of the moon photographed by Mr. Warren de la Rue were taken at more than
-1,000 miles’ distance, in order to obtain the necessary relief. The
-principle of the stereoscope has received many useful applications in
-the way of book illustrations, art teaching, and anatomical
-demonstration, and has thus gained a position among philosophical
-instruments that it did not at first possess.
-
-A combination of the principles of the phenakistiscope (fig. 4) and
-stereoscope, has resulted in the invention of an instrument called the
-stereotrope. A number of binocular photographs of some object in
-motion—a steam-engine, for instance—are taken when the moving parts are
-in different positions, and mounted on two revolving discs, the images
-being combined by means of a pair of semi-lenses, as in the ordinary
-refracting stereoscope.
-
-We cannot leave this subject without describing the pseudoscope, also
-the invention of Professor Wheatstone. If a stereoscopic pair of
-photographs of some solid body—a ball, for instance—are mounted the
-reverse way, that is to say, if the picture intended to be looked at by
-the right eye is placed on the left, the relief of the object will be
-reversed, and the ball will appear as a hollow hemisphere. If,
-therefore, we can by means of lenses or prisms cause the image of any
-natural object, as seen by the right eye, to be conveyed to the left,
-and _vice versâ_, we shall see the relief reversed. A conical cap will
-appear in relief as a cone, a globe will look like a hollow sphere, and
-the human face will take the semblance of the inside of a mask. The same
-deception may be effected by looking at a seal through a short-focused
-lens, so that the image shall seem reversed. In this case, the light
-coming apparently from the wrong side, and shining on the parts in
-relief, gives them the appearance of being hollow. An intaglio will, of
-course, appear in relief when so looked at. Photographs of gems and
-bas-reliefs will also present a pseudoscopic appearance, if looked at in
-a light coming from the opposite side to that in which they were taken.
-The same appearance may be seen sometimes in wall papers having patterns
-painted in strong relief.
-
-
-------------------------------------------------------------------------
-
-
-
-
-                             CHAPTER VIII.
-                 THE CAMERA OBSCURA AND CAMERA LUCIDA.
-
-
-THE construction of the camera obscura is founded on the fact that the
-rays of light, when collected into a point either by being passed
-through a small hole or a converging lens, form an image of the objects
-from which they proceed at the point of meeting. This may be readily
-tried by piercing the shutter of a room with a small hole, and holding a
-piece of paper within a short distance of it. It will be noticed that
-the smaller the hole the more distant will be the image formed. The
-first person who observed this fact was John Baptist Porta, an Italian
-philosopher who lived in the latter part of the seventeenth century. He
-noticed that when a screen was placed opposite a small hole in the
-shutter of his room, the objects outside were depicted on it in a
-reversed position with moderate distinctness; but that when a biconvex
-lens was placed over the hole, the picture was rendered much more
-distinct. This was the first attempt at the formation of the camera
-obscura, an instrument that has since bestowed such incalculable
-benefits on humanity.
-
-[Illustration:
-
-  FIG. 70.—The Camera Obscura.
-]
-
-The shape of the images so formed is independent of the shape of the
-opening, which, as long as it is sufficiently small, may be square,
-oval, or triangular. This may be easily seen when the sun shines through
-the intervals between the leaves of a shady avenue or bower of trees.
-The image of the sun as a circular patch of light is seen scattered over
-the surface of the ground, although the accidental intervals formed by
-the leaves above were of a thousand different shapes. These images at
-the time of an eclipse of the sun are very surprising, taking, as they
-do, the form of a crescent, more or less large according to the
-magnitude of the eclipse.
-
-This property possessed by the rays of light, of depicting on a screen
-the forms and colours of the objects from which they proceed when passed
-through a small aperture or a lens, is taken advantage of in most places
-famous for their natural scenery. The apparatus employed for this
-purpose is comparatively simple, consisting merely of a dark wooden hut,
-with a whitened table in the centre, and a mirror and lens in the apex
-of the roof. In fig. 70 we have a section of a camera obscura of this
-kind. The mirror and lens at the top of the apparatus are made to
-revolve, so as to bring every part of the landscape into view in turn. A
-camera obscura in a position commanding a view of moving objects, such
-as ships sailing to and fro, or the busy streets of a populous town, is
-an unending source of amusement, and may be easily and cheaply
-constructed.
-
-The camera obscura has been much utilized for taking hasty but exact
-sketches of various places. For this purpose it is made very light, and
-mounted on three legs carrying at their junction a flat table, whereon
-is placed the paper to receive the drawing. The tripod is covered with a
-black curtain, which, falling over the artist, effectually excludes all
-the rays of light except those which pass through the lens and are
-reflected downwards by the mirror. In the better kind of apparatus the
-mirror is replaced by a prism, which throws a clearer image than a
-mirror upon the screen.
-
-It is on these properties of the camera obscura that the art of
-photography was founded. Everybody who saw the beautiful images formed
-by this instrument was struck with the idea that by some means or other
-they could be fixed on paper. After numberless attempts the
-long-wished-for goal was at length arrived at; and now optics, aided by
-chemistry, is enabled to depict for us natural objects of every kind,
-from the distorted limb of the hospital patient to the beautiful forms
-of the queens and empresses of the world—from the tiniest animalcule to
-the great sun itself, who is compelled by the might of science to paint
-his own portrait for us with all his faults and imperfections.
-
-The lenses used for photographic purposes have only reached their
-present state of perfection after ceaseless labours of the philosophers
-and opticians of all countries. At first only a single lens was used,
-but it was found that the rays which exercised a chemical action did not
-meet in the same point as the rays of light, for it must be remembered
-that it is not the light we see that acts upon the substances used in
-photography, but another influence, known as actinism. It was also found
-that a single lens would not give a flat picture when the whole of its
-aperture was used, the edges of the image being always blurred and
-indistinct. This latter defect was found to be partially obviated by
-decreasing the opening, but this remedy shut off the light and prolonged
-the process. Gradually these two defects were removed, and now every
-photographer, no matter how humble, is possessed of a lens capable of
-taking a clear picture, every detail of which is perfectly distinct and
-faithful.
-
-The camera lucida bears a great analogy to the camera obscura in the
-purpose for which it is used, though not in the principle on which it is
-constructed. It is employed, like the preceding instrument, for
-obtaining faithful copies of a landscape, a building, or even of another
-drawing. It was invented by Dr. Wollaston, in 1804, and consists of a
-little four-sided prism, of which fig. 71 is a section.
-
-[Illustration:
-
-  FIG. 71.—Section of Camera Lucida.
-]
-
-The angle at A is a right angle; the angle B measures 67½°, the angle C
-135°, and the angle D is, of course, equal to B. It is mounted on a
-sliding foot, so that it may be raised or lowered at will, or turned in
-a horizontal direction. The path of the rays in this case is easy to
-follow, the object to be copied being placed at L, and the eye at I. On
-looking downwards the image of the object to be drawn is seen on the
-paper; and if the eye is placed so that the edge of the prism will just
-cut the pupil in two, the paper and pencil will be seen at the same
-time. It will be seen from the diagram, that the rays proceeding from L
-strike on the surface A B at right angles, and, being then reflected
-from C B, pass upwards again to point E. The direction of the rays is in
-reality a little more complicated than this. In the case of distant
-objects it is impossible to see both the object and the pencil at the
-same time; a lens is sometimes introduced at I to modify this defect.
-The original instrument has also been modified by the introduction of a
-triangular prism, in conjunction with plates of coloured glass, but the
-difficulty of rendering the image and the paper of the same strength is
-very great. The instrument is also hard to use, from the additional
-difficulty of always keeping the head in the same position, for the
-least movement from left or right is sufficient to throw the whole
-drawing out.
-
-A simple camera lucida may be made out of a small piece of
-looking-glass, mounted at an angle of 45°, or half-way between the
-horizontal and the perpendicular. If this be turned towards the drawing
-or view to be copied, and the left eye applied to the mirror, the image
-of the object will be seen on the paper below, and the pencil may be
-guided with the right. The proper use of this simple little instrument
-depends in a great measure upon the focus of each eye being the same.
-The light falling on the paper, too, requires very careful adjusting,
-otherwise the brighter object will eclipse the other. It is a good plan,
-too, to whiten the pencil or pen used, so that it may not so easily be
-lost when drawing the brighter parts of the object. We have seen
-excellent drawings made from plants by means of a little instrument of
-this kind, which simply consisted of a piece of looking-glass inserted
-in a cork stuck in a glass bottle.
-
-
-------------------------------------------------------------------------
-
-
-
-
-                              CHAPTER IX.
-                           THE SPECTROSCOPE.
-
-
-WE now come to speak of an instrument which may fairly rank, after the
-telescope and microscope, as one of the most wonderful discoveries of
-modern optical science. By its means we have not only discovered four
-new elementary bodies, which are found in certain minerals in
-inconceivably small quantities, but we have also determined the chemical
-composition of some of the remotest stars and nebulæ.
-
-In 1701 Newton discovered that if an ordinary ray of white light was
-admitted through a small hole into a dark chamber, and thence passed
-through a triangular prism, it became decomposed into a coloured band,
-known as the solar spectrum. As we have already explained that this
-decomposition is caused by the different coloured rays that make up
-white light being bent unequally by the action of the prism, we trust
-the following explanations will be readily understood. In 1802 Dr.
-Wollaston, an English philosopher, discovered that by using a narrow
-slit, instead of a round hole, the resulting spectrum was no longer
-continuous, but was divided at intervals by dark lines extending across
-it in a direction parallel to the edges of the prism. These lines
-attracted considerable attention at the time, but it was not until 1815,
-that Fraunhofer, an optician of Munich, investigated them with accuracy.
-He mapped and counted no less than six hundred of them, identifying
-eight of the most conspicuous by the first eight letters of the
-alphabet. Their positions are as follows:—
-
-    A.  Beginning of red.
-    B.  Middle of red.
-    C.  Beginning of orange.
-    D.  Middle of yellow.
-    E.  Middle of green.
-    F.  Beginning of blue.
-    G.  Middle of indigo.
-    H.  Middle of violet.
-
-The designations of these lines have been retained to the present day,
-and they have been named after the Munich philosopher, being known as
-Fraunhofer’s lines. They are to be seen in all parts of the spectrum,
-and increase in number and fineness according as the width of the slit
-through which the light passes is diminished. It may be asked, how it
-happens that they increase in proportion to the narrowness of the
-aperture admitting the light? A little consideration will soon show the
-reason of this.
-
-When a beam of light is passed through a hole of, let us say, the eighth
-of an inch in diameter and decomposed by a prism, the spectrum so
-produced is imperfect, inasmuch as an infinite number of spectra are
-thus superposed, and for this reason, that the rays of light entering on
-the right side of the aperture will give a spectrum falling in a
-different place to that formed by the rays entering on the left. In
-order, therefore, to diminish the confusion caused by the superposition
-of a number of spectra, the aperture ought to be reduced to a narrow
-slit. When the thin slice of light passing through the slit is
-decomposed by the prism, we find that not only is the purity of the
-colours greatly increased, but the lines in question make their
-appearance more or less in all parts of the coloured band.
-
-These lines are very unequally distributed, some being crowded together
-in masses, while others are extremely faint, and are separated by large
-intervals. Their position is well marked and determined, no matter from
-what source we obtain our beam of sunlight. Whether the spectrum be
-produced from the sun itself, or from the reflected light proceeding
-from the moon or planets, they are still found in the same place; only
-that in the latter case they are not so numerous, on account of the
-light being much fainter. For many years the cause of these lines
-remained a complete mystery, and it was not until Bunsen and Kirchhoff
-undertook their investigation that a satisfactory explanation of their
-origin was arrived at. In order to explain this, we must consider
-briefly the properties of the spectra of flames, and other luminous
-bodies.
-
-If, instead of the light of the sun, we examine prismatically the light
-given off by an incandescent body, such as a white-hot piece of
-platinum, we shall find that the lines seen in the solar spectrum are
-absent, and that we have a continuous band of coloured light quite
-uninterrupted by dark spaces or bands. The same absence of lines is seen
-in the spectra of the electric light and the flame of an ordinary
-candle, the light in each of these cases being produced by particles of
-carbon in a state of vivid incandescence. But if we examine the flame of
-incandescent gases, we shall find a spectrum of an entirely new kind.
-Thus if we examine an ordinary gaslight through a slit with a prism, we
-shall obtain a continuous spectrum, in consequence of the luminous
-portion of the flame consisting of solid carbon in a state of
-incandescence; but if we turn down the flame, so as to lessen the amount
-of carbon to be burned, we shall find the whole of that body is
-converted into feebly luminous gas, giving off a faint reddish blue
-light. If we now again examine it in the same manner, we shall find that
-the spectrum produced consists of black spaces, here and there crossed
-by a few faint coloured lines or bands. The reason of this is obvious:
-in the faint flame caused by the carbon and hydrogen in a state of
-luminous vapour, which only have a few of the colours of the spectrum,
-which, when passed through the prism, fall into their proper places. All
-substances with which we are acquainted are capable of being converted
-into luminous vapour by means of heat, and when thus burnt produce
-flames of more or less faint luminosity, generally characteristically
-coloured. A piece of soda inserted in the wick of a spirit-lamp gives a
-yellow tinge to the flame; a morsel of saltpetre (nitrate of potash) or
-nitrate of strontia will give a purple and crimson tint respectively.
-These hues are caused by the metals sodium, potassium, and strontium
-contained in these salts being converted into luminous vapour. On
-analyzing these coloured flames with a prism, as before, we should find
-in the case of the soda a single broad yellow line, situated just in the
-middle of the yellow portion of the spectrum, the rest of the space
-where the spectrum should be being perfectly dark. The reason of this is
-pretty simple. Sodium burns with a pure yellow flame, consequently when
-passed through a prism it cannot split into any other colours, but takes
-its place in the position belonging to yellow of that particular hue.
-Were it a little more orange or green in tint, it would take its place
-nearer to the red or violet end of the spectrum. The light from
-saltpetre, which contains potassium may next be examined. It will be
-found to tinge the flame with the spirit-lamp of a beautiful purple. We
-can almost guess what will happen when this flame is submitted to the
-action of the prism. We shall find that the purple light emitted will
-split into red and violet, which will immediately arrange themselves in
-their proper positions according to their hues. If in like manner we
-substitute nitrate of strontia for saltpetre, we shall get a splendid
-crimson flame which is decomposed by the prism into red, orange, or
-blue.
-
-On submitting the compounds of the other elements to the same tests, we
-shall find that each of them, when converted into luminous gas, is
-capable of producing coloured lines of various kinds when the light of
-their flames is passed through a prism. If, therefore, we had a number
-of salts of whose composition we were ignorant, all we need do is to
-burn them in a spirit-lamp, and by the number and position in the lines
-of their spectra we should be able to tell immediately of what they were
-composed.
-
-The spectra of nearly all the elements capable of being connected with
-luminous gas have been determined with great accuracy. Perhaps the
-number and position of the lines of a few spectra will be interesting to
-the student.
-
-_Sodium._—This is the metallic base of soda salts, and gives a double
-bright yellow line in the middle of the yellow.
-
-_Potassium._—The base of the various salts of potash. It gives one line
-in the extreme red, one in the middle of the red, one in the violet, and
-a peculiar glow in the centre of the spectrum.
-
-_Strontium._—The base of the strontia salts, of which the nitrate is
-used as the principal ingredient in the red fire of the theatres. It
-gives a group of lines in the red and orange, and a beautiful blue one
-in the middle of the blue.
-
-_Barium._—The base of the baryta salts, one of which is used in making
-green fire. It gives several strong lines in the green, and a few in the
-red, orange, and yellow.
-
-After the position of the spectral lines of most of the elements had
-been discovered, Messrs. Bunsen and Kirchhoff were one day examining the
-saline deposit of a spring which issues from the earth near Durkheim, in
-the Palatinate, and were surprised to find that a blue line belonging to
-no known metal made its appearance in addition to the potassium, sodium,
-and other lines produced by the saline ingredients of the water. These
-philosophers immediately concluded that the unknown line was caused by
-an unknown metal, and they at once set to work to obtain a larger
-quantity of the saline residue from the spring. They evaporated down no
-less than forty tons of water, and succeeded in isolating the new
-substance, which turned out to be a metal resembling potassium. While
-examining the residue more carefully, a new, dark _red_ line, beyond
-that belonging to potassium, was discovered, pointing to the existence
-of a second new element, which was also afterwards obtained in the pure
-state. These two new metals, which closely resemble potassium in their
-properties, were named in accordance with the lines given by them when
-converted into luminous gas. The first was called cæsium, from _cœsius_,
-Lat. light blue; and the other, rubidium, from _rubidus_, Lat. dark red.
-Since the publication of MM. Bunsen and Kirchhoff’s experiments, these
-two elements have been found in comparatively large quantities in
-various minerals, and these properties have been closely studied.
-
-Spectrum analysis has yielded us two more new metals since first these
-philosophers applied the prism to the determination of the chemical
-composition of various bodies. Mr. W. Crookes, F.R.S., an English
-chemist of eminence, while examining the flame of a deposit obtained
-during the manufacture of sulphuric acid from a certain sulphur mineral
-found in the Hartz mountains, perceived a brilliant green line with
-which he was previously unacquainted, which quickly flashed into view,
-and then disappeared. After numerous experiments on various other
-minerals (for the deposit he had first experimented upon only yielded
-him a few grains of the new body), Mr. Crookes succeeded in discovering
-a comparatively large quantity of it in a sulphur mineral found in
-Belgium. The new element was found to be a heavy metal, closely
-resembling lead in its properties. It was named by the discoverer,
-thallium, from the Greek word _thallos_, a green twig, from the
-brilliancy of the single green line that indicates its presence. In like
-manner, Messrs. Reich and Richter have discovered a fourth new metal,
-which has been named _indium_, from its principal lines being found in
-the centre of the _indigo_ of the spectrum.
-
-The delicacy of spectrum analysis may be imagined from the fact that a
-quantity of sodium amounting to less than the _two-millionth_ of a grain
-can be detected by its means. Indeed, it has taught us that sodium in
-one form or other exists almost everywhere. This mode of analysis is
-only serviceable to indicate the composition of any salt or other
-substance, the quantities of the different elements found by its use
-having no influence on the appearances brought out by the prism. Thus, a
-substance which has only been contaminated with sodium from being
-handled by warm fingers, will show the yellow bands as strongly as if it
-contained a large proportion of that metal.
-
-For ordinary experiments in spectrum analysis the apparatus used is very
-simple. It consists of a tube with a fine slit at one end, and a convex
-lens at the other, for concentrating the light from the coloured flame
-upon the centre of the prism. After the light passes through the prism,
-it is examined by a small telescope of low magnifying power. The lamp
-used may be either a spirit-lamp or a colourless gas flame into which
-the substance to be examined is introduced upon a platinum wire.
-
-We now come to another very important discovery, made by means of our
-prism and narrow slit—the determination of the composition of the
-photosphere or mass of luminous vapour surrounding the body of the sun.
-
-A simple experiment will show how this brilliant discovery was arrived
-at. The light of a candle or other flame containing incandescent _solid_
-matter is passed through the spectroscope, and is found to decompose
-into a continuous spectrum, uninterrupted by dark lines. Between the
-light and the slit a spirit-lamp is placed, but no difference in the
-appearance of the spectrum is perceived. Introduce, however, the
-smallest portion of a soda salt into the non-luminous flame of the
-second-lamp, and a broad black line is immediately seen, crossing the
-middle of the yellow portion of the band of colour. Remove the sodium
-flame and the band disappears; but do the same with the lamp producing
-the spectrum, and the spectrum of course disappears, and the dark band
-caused by the sodium flame is changed to the yellow line produced by
-that metal. The same experiments may be tried with potassium, strontium,
-and other metals; and we shall always find that when a coloured flame is
-introduced between an incandescent solid and its continuous spectrum, it
-produces a series of black lines corresponding to the substances by
-which it is coloured. Thallium, in like manner, would give a black band
-in the middle of the green, and indium a similar one in the indigo.
-(Fig. 6, Frontispiece).
-
-The exact position of the black band in the middle of the yellow is
-shown in the coloured figure of the spectrum so beautifully printed in
-the frontispiece of this book, and it has been found to correspond
-exactly with the dark line D of the solar spectrum. The inference from
-this fact is obvious. The incandescent portion of the sun gives off
-light corresponding in its properties to that emitted by the solid
-matter contained in the candle flame, but the photosphere containing the
-vapour of sodium cuts off that portion corresponding to the sodium line.
-Accurate measurements prove that numberless other lines occurring in the
-solar spectrum are due to the vapours of other well known metals
-existing on the earth. Amongst these may be mentioned potassium, calcium
-(the base of lime), iron, nickel, chromium, and several others. This
-discovery with regard to the sun has resulted in the spectral
-examination of a large number of the fixed stars and nebulæ. For
-centuries the fixed stars refused to answer all questions put to them by
-mortals. The telescope showed them merely as bright points. Their nature
-and origin remained a beautiful mystery, until Dr. Miller, Mr. Huggins,
-Father Secchi, and a few other philosophers interrogated them in a
-manner that could not fail to draw forth an answer. They brought their
-light within range of their prisms, and forthwith they declared
-themselves to be suns like our own. It is true that before this they
-were looked on by most astronomers as bodies analogous to our own sun,
-but it was only reasoning from analogy, after all; but we are now able
-to assert with all the certainty that is compatible with human
-fallibility that many of these heavenly bodies are possessed of an
-incandescent centre, surrounded by a photosphere or envelope of gaseous
-matter in a luminous condition. It would be impossible to give a list of
-all the stars that have been examined up to the present time; the
-composition of the photospheres of a few must therefore suffice. It is
-singular that the elements hitherto discovered in the stars are those
-which are more or less abundant on the earth. Amongst them we may name
-hydrogen, nitrogen, sodium, magnesium, barium, iron, antimony, bismuth,
-tellurium, and mercury. The bright star in the constellation of Orion
-known as Betelgeux is one of the most singular in composition, the lines
-of its spectrum indicating the absence of hydrogen. If, as Messrs.
-Huggins and Miller suggest, the worlds revolving round this star are
-also deficient in this element, they would be without water, like our
-moon.
-
-Upon a very clear night it may be noticed that the stars are not all of
-the same colour, but that many of them appear to be of a ruddy or
-yellowish tint. The cause of this is plainly seen when they are
-submitted to spectral analysis. Thus, Sirius, which is a brilliant white
-star, shows but three dark lines, while one of the stars in the
-constellation of Hercules shows several groups of bands in the red,
-blue, and green portions of its spectrum, fully accounting for its
-orange tint.
-
-The double star _β_ Cygni is a very beautiful example of the
-distribution of colour between two members of a stellar group. One star
-shows a strong spectrum with the blue and violet portions almost totally
-blotted out, while its companion is similarly circumstanced with respect
-to the yellow and orange portions of its spectrum. The colour of one is
-consequently orange, while the other is of a delicate blue. If these
-stars are the principal members of a system, the alternation of blue and
-orange days must be indeed a singular phenomenon to those who inhabit
-their planets.
-
-In some of the stars lines have been discovered which do not possess any
-equivalent amongst those produced by terrestrial matter; they
-consequently contain elements of which we know nothing; at the same
-time, however, it has been found that terrestrial elements exist in some
-of the remote nebulæ, which are so distant that their light takes many
-thousands of years to reach our earth.
-
-Spectrum analysis has decided the grand question of the physical
-composition of the nebulæ. Those bodies were supposed, with some reason,
-to be aggregations of stars, like our Milky Way, which only required
-telescopes of sufficient power to resolve them. That they partly consist
-of gaseous matter in a luminous condition is evidenced by their showing
-a series of bright lines in the spectroscope, exactly like those
-produced by terrestrial gases. Their light is therefore not emitted by a
-solid or liquid incandescent body, but by a glowing gas. The lines
-mentioned by Messrs. Huggins and Miller showed that the nebula in the
-sword-handle of Orion consists of hydrogen and nitrogen in a state of
-luminous incandescence. Not the slightest trace of a continuous spectrum
-can be detected in the light emanating from this body; consequently,
-according to present hypotheses, it contains no solid matter at all. A
-number of other nebulæ have given similar results.
-
-There are numerous star clusters which, unlike the true nebulæ, give
-continuous spectra when their light is submitted to the action of the
-prism. Of these may be specially mentioned the great clusters in
-Andromeda and Hercules, which give continuous spectra, interrupted by
-dark bands on the red and orange. The light thrown by these experiments
-upon the nebular hypotheses of Sir William Herschel, who considered that
-true nebulæ consisted of the primordial gaseous matter out of which suns
-and stars have been elaborated, is very great, and will be appreciated
-even by those whose knowledge of astronomy is small.
-
-Spectral analysis has also been the means of our witnessing a celestial
-conflagration, and understanding the cause of this marvellous event. It
-is well known to most people that from time to time stars have suddenly
-burst upon us, and have almost as suddenly disappeared. The theories
-advanced to account for these singular celestial visitors, have been
-more numerous than satisfactory. In May 1866, a star of the second
-magnitude suddenly burst forth in the Northern Crown, and was almost
-immediately noticed by Mr. Huggins who brought every power of prism and
-telescope to bear upon this extraordinary celestial phenomenon. He found
-the spectrum of the star to consist of two distinct spectra, one being
-formed by four bright lines, the other analogous to the spectra of the
-sun and stars. Consequently two kinds of light were given off by this
-star; one forming a series of bright lines indicative of luminous gas,
-the other consisting of a continuous spectrum, crossed by dark lines,
-showing the existence of a solid body in a state of incandescence,
-surrounded by a photosphere of luminous vapours. Two of the bright lines
-undoubtedly showed the presence of hydrogen in a state of illumination,
-the great brightness of the lines indicating that the burning gas was
-hotter than the photosphere. These facts taken in conjunction with the
-suddenness of the outburst in the star, and its immediate decline in
-brightness from the second down to the eighth magnitude in twelve days,
-suggest the startling speculation that the star had become suddenly
-wrapped in the flames of burning hydrogen, consequent possibly on some
-violent convulsion in the interior of the star having set free enormous
-quantities of this gas. As the free hydrogen became exhausted, the
-spectrum showing the bright lines gradually waned until the star
-decreased in brilliancy. It must not be forgotten that the event seen by
-Mr. Huggins occurred many years ago, and that the light emitted by this
-marvellous celestial convulsion has been travelling to us ever since.
-
-Comets and meteors have been submitted to the test of spectral analysis.
-The former erratic visitors have been but few and small since stellar
-spectrum analysis has been perfected. In January 1866, Mr. Huggins
-brought his apparatus to bear upon a small comet, which gave a somewhat
-unexpected result. When the object was viewed in the spectroscope, two
-spectra were distinguishable—a very faint continuous spectrum of the
-tail, showing that it reflected solar light, and a bright space towards
-the centre of the spectrum, indicating that the nucleous was
-self-luminous and gaseous.
-
-Mr. Alexander Herschel—the nephew and the grandson of Sir John and Sir
-William Herschel—has recently succeeded in obtaining indications of the
-composition of the meteors that people the heavens in the months of
-August and November. The principal result of his observations appears to
-be, that sodium in a state of luminous vapour is present in the trains
-left behind these singular bodies.
-
-Lightning has also been similarly examined, and lines showing that
-hydrogen and nitrogen were rendered luminous during the electrical
-discharge, were seen with great distinctness. In fact, the applications
-of the prism to scientific discovery are almost endless, and in
-describing them it is difficult to tell where to draw the line.
-
-Before quitting this subject, it will be as well to say a few words on
-the fluorescent rays of the spectrum, to which allusion has already been
-made towards the end of Chapter IV., Part II. It was there said that the
-chemical power of the spectrum extends to some distance beyond the
-extreme violet, a fact that may be readily proved by exposing a piece of
-photographic paper to the action of the dark portion of the spectrum.
-Professor Stokes found that there were means of rendering these rays
-visible to the eye by altering their rate of vibration. This he found
-was possible by passing them through the solutions of certain
-substances, such as sulphate of quinine, horse-chestnut bark, &c. We
-have already said, that light vibrating at the rate of from 458 to 727
-billion times a second, was capable of exciting luminous sensations upon
-the optic nerve. The latter is the rate of vibration of the extreme
-violet ray, and it has been found that the eyes of many persons are not
-sufficiently sensitive to be influenced by it; it is, therefore, just
-probable that there are animals whose eyes are so much more sensitive
-than ours, that they can see rays that exist far beyond those seen by
-us. Now, as difference of colour is produced by difference in the rate
-of vibration, it follows that those whose eyes are sensitive enough to
-perceive the extreme violet rays, see tints of violet that are
-inappreciable by others.
-
-The power of sulphate of quinine in reducing the luminous vibrations is
-easily seen by passing a tube filled with the solution successively
-through each of the colours of the spectrum formed by a quartz prism;
-the ordinary colours will pass through the liquid as if it were simply
-water, but on arriving near the violet extremity a gleam of pale blue
-light will shoot across the tube, and continue to increase. As it is
-moved onwards the light will gradually die away, until a point is
-reached nearly equal in length to the whole of the visible spectrum,
-when it will disappear altogether. It is somewhat singular that no
-substance has yet been found that will increase the refrangibility of
-the dark rays beyond the red end of the spectrum. There are many
-artificial flames which produce this dark light (if we may use such a
-paradoxical expression) in greater quantity than the sun, whose light is
-no doubt greatly deteriorated in this respect during its passage through
-the atmosphere. The substance of which the prism is made also greatly
-influences the length of the invisible portion of the spectrum. By using
-a quartz prism and lenses of the same material Professor Stokes, found
-that the spectrum of the electric light could be traced for a distance
-equal to six times that of the visible portion.
-
-The action of certain substances in rendering the invisible rays of
-light perceptible may be easily shown by any one possessing a
-horse-chestnut tree. A weak decoction of the inner portion of the bark
-having been made and filtered through blotting-paper, or at any rate
-allowed to settle, the room is made quite dark and a piece of common
-brimstone is ignited. The pale blue light given off is comparatively
-feeble, but it is very rich in the ultra-violet rays; consequently, when
-the infusion of horse-chestnut bark is poured into a tall jar of water,
-beautiful waves of phosphorescent light are seen flashing backwards and
-forwards as the two liquids mingle. The tincture of stramonium is also
-possessed of this property, and characters traced on paper with it,
-although nearly invisible by ordinary daylight, appear distinctly when
-examined by the light of burning sulphur.
-
-
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-
-
-
-
-                               CHAPTER X.
-                      SPECTRES—THE GHOST ILLUSION.
-
-
-WE close our account of the wonders of optics by a description of the
-ghost illusion, which has been exhibited with such great success by M.
-Robin, the well-known French conjurer, Mr. Pepper, the enterprising
-manager of the Royal Polytechnic Institution, and several others. Before
-doing so, however, we will say a few words on those unpleasant
-visitations known as spectres, to which some people are liable, either
-through an over-worked brain or some organic disease.
-
-The peculiar appearances known as spectres in optics are certain
-illusions of vision in which an object is apparently presented to the
-view which does not really exist. In such cases either the brain, the
-retina, or the optic nerve are unnaturally excited, and made sensitive
-to an appearance that, physically speaking, does not exist. There is
-such a close connexion between the senses and the mind, that we
-continually, and without knowing it, transfer to the physical world that
-which belongs to the domain of thought. A picture which has struck us
-during the day will reappear to us at night during sleep, with every
-detail perfect, or possibly under a form modified by the capricious
-wanderings of our thoughts. A sudden fright may sometimes be the cause
-of optical illusions which will pursue us unceasingly. Fear, despair,
-passion, ambition, and other violent mental phases, are capable of
-evoking images closely connected with the state of our brain,
-appearances that we often take for realities, and whose truths we have
-to test by our faculty of reasoning, before we can set them down as
-positive illusions. “In the most insignificant phenomena,” says Sir
-David Brewster, “we find that the retina is so powerfully influenced by
-exterior impressions as to retain the images of visible objects for a
-long time after they have passed out of sight; besides, this portion of
-the eye is so strongly influenced by local impressions of which we know
-neither the nature nor the origin, that we see the shapeless forms of
-coloured light moving about in the dark. In fact we have, in the cases
-of Newton and many others, examples of the ease with which the
-imagination revivifies the images of luminous objects for months or even
-years, after these impressions took place. After the occurrence of such
-phenomena, the mind can readily comprehend how thin is the division that
-separates reality from those spectral illusions which during a
-particular state of health have afflicted the most intelligent men, not
-merely those belonging to the community at large, but also the most
-learned philosophers.”
-
-Spectres may properly be divided into two classes, those which may be
-termed subjective, which result from some unnatural action of our minds
-or bodies, and which properly belong to the science of physiology, and
-those which may be called objective, which are caused by some peculiar
-illusion acting on us from without. We shall pass lightly over the
-first, illustrating them by a single example, while we shall pay more
-serious attention to those belonging to the second class.
-
-Sir Walter Scott, in his _Letters on Demonology and Witchcraft_,
-mentions a remarkable instance of the first order of spectres. A doctor
-of eminence was called in to attend a gentleman who occupied a high
-place in a particular department connected with the administration of
-justice. Until the time that the physician’s services became necessary,
-he had shown strong common sense and extraordinary firmness and
-integrity in every case in which he had been called upon to arbitrate.
-But after a certain epoch his temper became saddened, although his mind
-preserved its habitual strength and calmness. At the same time, the
-feebleness of his pulse, the loss of appetite, and impaired digestion
-seemed to point out to his medical adviser the existence of some serious
-source of disturbance. At first the sick man seemed inclined to keep the
-cause of the change in his health a profound secret; but his melancholy
-bearing, confused answers, and the badly disguised constraint with which
-he sharply replied to the interrogations of the doctor, caused the
-latter to seek for information as to the cause of the disorder in other
-directions. He made minute inquiries of the various members of his
-unhappy patient’s family, but he could obtain no explanation of the
-mystery. Every one was lost in conjecture as to the reason of the
-alarming condition of the patient, which did not appear to be justified
-by any loss of fortune or beloved friends. His age rendered the idea of
-an unsuccessful love affair improbable, and his known integrity
-precluded the possibility of remorse. The doctor accordingly was
-compelled to return once more to the straight road, and he used the most
-serious arguments with his patient to induce him to conquer his
-obstinacy. At last the doctor’s efforts took effect; the patient allowed
-himself to be convinced, and manifested his desire to open his mind
-frankly to the doctor. They were accordingly left alone, all the doors
-were securely fastened, and the patient made the following singular
-avowal.
-
-“You cannot be more firmly convinced, my dear friend, than I am myself,
-that I am on the eve of death, crushed by the fatal malady which has
-dried up the sources of my life. You remember, without doubt, the
-disease of which the Duke of Olivarez died in Spain?”
-
-“From the idea,” replied the doctor, “that he was pursued by an
-apparition in whose existence he did not believe, and he died from the
-continual presence of this imaginary vision weighing down his strength,
-and breaking his heart.”
-
-“Well, my dear doctor,” the patient went on, “I am in the same
-condition, and the presence of the vision that persecutes me is so
-painful and frightful, that my reason is totally helpless in controlling
-the effects of my imagination, and I feel that I am dying from the
-effects of an imaginary illness. My visions began two or three years
-since. At first I found myself embarrassed from time to time by the
-presence of a great cat, which appeared and disappeared I knew not how.
-But at last the truth flashed across my mind, and I was compelled to
-look upon the creature, not as an ordinary domestic animal, but as a
-vision which had its origin in some derangement of the organs of sight
-or in my imagination. I have no antipathy to cats, in fact I am rather
-fond of them, so I endured the presence of my imaginary companion so
-well that at last I treated the whole affair with indifference. But at
-the end of several months the cat disappeared, and was replaced by a
-spectre of greater importance, and whose exterior was, to say the least
-of it, very imposing. It was neither more nor less than one of the high
-officials of the House of Lords, in the full dress belonging to his
-dignity.
-
-“This personage, who was in court dress, with a bag-wig on his head, and
-a sword by his side, his coat splendidly embroidered and his _chapeau
-bras_ under his arm, glided along by my side like a shadow. Whether I
-was in my own house or elsewhere, he mounted the stairs before me, as if
-to announce my coming. Sometimes he seemed to mix with the company,
-although it was evident that no one remarked his presence, and I was the
-sole witness of the chimerical honours that this imaginary individual
-seemed to render to me. This phantasy of my brain did not make a very
-strong impression on me, although it made me conceive doubts as to the
-state of my health, and the effects it would produce upon my reason.
-
-“This second phase of my malady, like the first, also came to an end.
-Some months after, the usher of the Upper House ceased showing himself,
-and he was replaced by an apparition that was at once wearing to the
-mind and terrible to the sight. It was a skeleton. Whether I was alone
-or in company this frightful image of death never quitted me; it dogged
-my footsteps and followed me everywhere, and seemed to be a shadow
-inseparable from myself. It was in vain that I repeated to myself a
-hundred times over that the vision was not real, and was only an
-illusion of my senses. The reasoning of philosophy and my religious
-principles, strong though they are, are powerless to triumph over the
-influence that besets me, and I feel that I shall die a victim to this
-cruel evil.”
-
-“It seems then,” interrupted the doctor, “that this skeleton is always
-before your eyes?”
-
-“It is my evil fate to see it continually before me.”
-
-“In which case it is at this moment visible to your eyes?”
-
-“It is at present.”
-
-“And in what part of the room do you imagine that you see it now?” asked
-the doctor.
-
-“At the foot of my bed,” replied the patient: “when the curtains are
-half open I can see it place itself in the empty space between them.”
-
-“You say that you are convinced that it is only an illusion,” replied
-the doctor; “have you the firmness to convince yourself of it
-positively? Have you the necessary courage to get up and go and place
-yourself in the position which appears to be occupied by the spectre, in
-order to demonstrate to yourself positively that it is only a vision?”
-
-The unfortunate man sighed and shook his head.
-
-“Well,” went on the doctor, “let us try another plan.”
-
-He quitted the chair on which he was sitting, at the head of his
-patient’s bed, and placing himself between the half opened curtains, in
-the place where the patient had pointed out the skeleton, he asked if
-the apparition was still visible.
-
-“Not the whole of it,” answered the patient, “because you are standing
-between him and me; but I see his skull looking at me over your
-shoulder.”
-
-In spite of his philosophy, the learned physician could not help
-starting to hear that the spectre was immediately behind him. He had
-recourse to other questions, and tried endless remedies, but without
-success. The prostration of the patient, however, increased, and he died
-in the same distress of mind in which he had passed the last months of
-his life. This example is a sad proof of the power of the imagination
-over the life of the body even when the terrors endured are powerless in
-destroying the judgment of the unfortunate sufferer. We will say more;
-men who have the strongest nerves are not free from similar illusions.
-
-The second kind of spectres, in which the science of optics plays so
-important a part, is the result of the imagination being deceived by art
-with the assistance of science.
-
-These spectres are displayed in the ghost trick which has been practised
-at various Parisian theatres for a number of years, with very great
-success, more especially at the _Théâtres du Châtelet_ and _Dejazet_.
-The Adelphi, in London, also employed Mr. Pepper to heighten the effect
-of the excellent acting of Mr. Toole and Mrs. Alfred Mellon, in the
-dramatic version of Dickens’ “Haunted Man,” by the introduction of
-various spectral effects. And the same trick was also called into
-requisition with some success in several of the minor theatres in New
-York and other cities of the United States. At the Polytechnic, in
-London, very remarkable effects were produced, and few who ever saw them
-will forget the surprise they felt at seeing the first representation of
-an imponderable ghost endowed with motion, and even speech. Amongst the
-most successful productions in this way was the entertainment of M.
-Robin, one of the cleverest of the many successors of the great Robert
-Houdin, the prince of prestidigitators. M. Robin claims to be the
-inventor of the ghost illusion, and to have shown it frequently since
-1847. Whether this be so or not it is not our business to decide, but we
-can testify that his exhibition in the Boulevard du Temple drew all
-Paris to see it. Evening after evening he not only “called spirits from
-the vasty deep,” but “made them come.” He pierced them with swords, he
-fired pistols through them, and he made them appear and disappear at his
-slightest wish. He showed the Zouave at Inkermann, lying dead amongst a
-heap of slain, who at the familiar sound of the drum, rose, pale and
-grave, and showed the bleeding wounds from which he died. Amongst other
-scenes shown by M. Robin was one of a spectre appearing to an armed man,
-who after trying in vain to shut out the vision from his sight fires a
-pistol at the intruder. Fig. 72 shows the scene as seen by the audience,
-and fig. 73, the method by which the illusion is worked. The theatre is
-shown in section. On the left, at the end, are seen the spectators; on
-the right is the stage upon which the scene is represented. Beneath the
-stage is an actor clothed in white to personate a ghost, whose image is
-reflected by the glass above.
-
-[Illustration:
-
-  FIG. 72.—The Spectre. An optical illusion.
-]
-
-[Illustration:
-
-  FIG. 73.—How to produce Spectres.
-]
-
-This glass is placed at an angle, and fills up the whole of the front of
-the stage, the edges being carefully concealed by curtains. The glass of
-course must be of a very large size, and should be of the very best
-quality, so that it cannot be seen by the audience. The actor must take
-care to place himself in such a position as to counteract the effect
-produced by the glass being placed at an angle. At first the cavalier is
-seen sitting at a table. After soliloquizing for a time in a very
-remorseful manner touching several murders that he has committed, the
-ghost of one of his victims gradually appears. This is effected by
-gently turning the electric light upon the concealed actor. The murderer
-and victim parley for a short time, when the former, being unable to
-withstand the reproaches of the ghost any longer, fires a pistol at him
-point-blank. The ball of course takes no effect, so the villain draws a
-sword, but before it has left its scabbard the spirit of the victim has
-vanished with a mocking laugh, or, in other words, the electric light is
-suddenly turned off. The management of the light is exceedingly
-difficult under these circumstances; the theatre, the stage, and the
-portion beneath ought to be lighted in a very careful manner, for if
-either is too bright or too dark it mars the whole effect. It must be
-remembered, too, that the person performing the part of the spectre and
-the real actor above cannot see each other, consequently all their
-action has to be carried on by guess-work. The actor below has to walk
-along an inclined plane, keeping himself exactly at right angles to it.
-Again, the movements of the latter are obliged to be reversed; for the
-cavalier already mentioned drew his sword with his left hand in order
-that the reflected figure should appear to use the right.
-
-When well arranged, the ghost trick leaves far behind all the efforts of
-a similar nature that were obtained by the ancients in the way of
-magical illusions. It is also incontestably true, contrary to what some
-people have supposed, that they were unable to perform this illusion in
-the way we have described, for they were ignorant of the method of
-manufacturing and polishing glass plates of sufficient size and
-clearness for the purpose.
-
-The production of living but impalpable spectres is thus a completely
-modern achievement, as we have already proved, and which has taken its
-place amongst the applications of science to stage art, to the total
-exclusion of all effects depending for their production on the
-old-fashioned phantasmagoria and magic lantern.
-
-
-
-                                THE END.
-
-
-
-
-------------------------------------------------------------------------
-
-
-
-
- ● Transcriber’s Notes:
-    ○ Missing or obscured punctuation was silently corrected.
-    ○ Typographical errors were silently corrected.
-    ○ Inconsistent spelling and hyphenation were made consistent only
-      when a predominant form was found in this book.
-    ○ Text that was in italics is enclosed by underscores (_italics_).
-    ○ The use of a caret (^) before a letter, or letters, shows that the
-      following letter or letters was intended to be a superscript, as
-      in S^t Bartholomew or 10^{th} Century.
-
-
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-
-<div style='text-align:center; font-size:1.2em; font-weight:bold'>The Project Gutenberg eBook of The Wonders of Optics, by Fulgence Marion</div>
-
-<div style='display:block; margin:1em 0'>
-This eBook is for the use of anyone anywhere in the United States and
-most other parts of the world at no cost and with almost no restrictions
-whatsoever. You may copy it, give it away or re-use it under the terms
-of the Project Gutenberg License included with this eBook or online
-at <a href="https://www.gutenberg.org">www.gutenberg.org</a>. If you
-are not located in the United States, you will have to check the laws of the
-country where you are located before using this eBook.
-</div>
-
-<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Title: The Wonders of Optics</div>
-
-<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Author: Fulgence Marion</div>
-
-<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Translator: Charles W.  Quin</div>
-
-<div style='display:block; margin:1em 0'>Release Date: April 19, 2021 [eBook #65114]</div>
-
-<div style='display:block; margin:1em 0'>Language: English</div>
-
-<div style='display:block; margin:1em 0'>Character set encoding: UTF-8</div>
-
-<div style='display:block; margin-left:2em; text-indent:-2em'>Produced by: deaurider, Barry Abrahamsen, and the Online Distributed Proofreading Team at https://www.pgdp.net (This file was produced from images generously made available by The Internet Archive)</div>
-
-<div style='margin-top:2em; margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK THE WONDERS OF OPTICS ***</div>
-
-<div  class='figcenter id001'>
-<img src='images/cover.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='small'>The cover image was created by the transcriber and is placed in the public domain.</span></p>
-</div>
-</div>
-<div class='pbb'>
- <hr class='pb c000' />
-</div>
-<div>
-  <h1 class='c001'>THE WONDERS OF OPTICS.</h1>
-</div>
-<div class='pbb'>
- <hr class='pb c002' />
-</div>
-<div id='i004a'  class='figcenter id002'>
-<span class='pageno' id='Page_I'>I</span>
-<img src='images/i004a.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p>Spectrum showing the absorptive power of Sodium vapour (Fig. 6).</p>
-</div>
-</div>
-
-<div id='i004b'  class='figcenter id003'>
-<img src='images/i004b.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p>Solar Spectrum (Fig. 5).</p>
-</div>
-</div>
-
-<div id='i004c'  class='figcenter id004'>
-<img src='images/i004c.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p>Action of a prism on a ray of light (Fig. 7).<br />Eng.<sup>d</sup> by A. Robin N.Y.</p>
-</div>
-</div>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='pageno' id='Page_II'>II</span>THE</div>
-    <div class='c000'><span class='c004'>WONDERS OF OPTICS.</span></div>
-    <div class='c005'>BY</div>
-    <div class='c000'><span class='c006'>F. MARION.</span></div>
-    <div class='c007'>TRANSLATED FROM THE FRENCH, AND EDITED BY</div>
-    <div class='c000'><span class='c006'>CHARLES W. QUIN, F.C.S.</span></div>
-    <div class='c008'><i>ILLUSTRATED WITH SEVENTY ENGRAVINGS ON WOOD, AND A COLOURED FRONTISPIECE.</i></div>
-  </div>
-</div>
-
-<hr class='c009' />
-
-<div class='nf-center-c0'>
-<div class='nf-center c007'>
-    <div><span class='c010'>NEW YORK:</span></div>
-    <div><span class='c011'>CHARLES SCRIBNER’S SONS,</span></div>
-    <div><span class='xsmall'>SUCCESSORS TO</span></div>
-    <div><span class='small'>SCRIBNER, ARMSTRONG, &amp; CO.</span></div>
-  </div>
-</div>
-
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div class='chapter'>
-  <span class='pageno' id='Page_ix'>ix</span>
-  <h2 class='c012'>PREFACE.</h2>
-</div>
-<hr class='c013' />
-<p class='c014'><span class='sc'>The</span> present work needs but little introduction to
-the English public. The author, M. F. Marion, who
-holds a high official scientific position in Paris, is well
-known, especially in Europe, as a popular writer on
-the “Wonders of Optics,” and kindred subjects. As a
-rule, the original text has been strictly adhered to by
-the Translator, but in a few instances certain anecdotes
-of a local character have been altered so as to be more
-generally applicable, or condensed to make room for
-the chapter on the Spectroscope, which is entirely
-original.</p>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div class='chapter'>
-  <span class='pageno' id='Page_xi'>xi</span>
-  <h2 class='c012'>CONTENTS.</h2>
-</div>
-<hr class='c013' />
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='c006'>PART I.</span></div>
-    <div class='c000'><span class='c006'>THE PHENOMENA OF VISION.</span></div>
-    <div class='c003'><span class='c015'>CHAPTER I.</span></div>
-  </div>
-</div>
-
-<table class='table0' summary=''>
-<colgroup>
-<col width='86%' />
-<col width='13%' />
-</colgroup>
-  <tr>
-    <td class='c016'>&nbsp;</td>
-    <td class='c017'><span class='small'>PAGE</span></td>
-  </tr>
-  <tr>
-    <td class='c016'>THE EYE</td>
-    <td class='c017'><a href='#Page_15'>15</a></td>
-  </tr>
-</table>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='c010'>CHAPTER II.</span></div>
-  </div>
-</div>
-
-<table class='table1' summary=''>
-<colgroup>
-<col width='86%' />
-<col width='13%' />
-</colgroup>
-  <tr>
-    <td class='c016'>THE STRUCTURE OF THE EYE</td>
-    <td class='c017'><a href='#Page_22'>22</a></td>
-  </tr>
-</table>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='c010'>CHAPTER III.</span></div>
-  </div>
-</div>
-
-<table class='table0' summary=''>
-<colgroup>
-<col width='86%' />
-<col width='13%' />
-</colgroup>
-  <tr>
-    <td class='c016'>THE ERRORS OF THE EYE</td>
-    <td class='c017'><a href='#Page_30'>30</a></td>
-  </tr>
-</table>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='c010'>CHAPTER IV.</span></div>
-  </div>
-</div>
-
-<table class='table0' summary=''>
-<colgroup>
-<col width='86%' />
-<col width='13%' />
-</colgroup>
-  <tr>
-    <td class='c016'>OPTICAL ILLUSIONS</td>
-    <td class='c017'><a href='#Page_36'>36</a></td>
-  </tr>
-</table>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='c010'>CHAPTER V.</span></div>
-  </div>
-</div>
-
-<table class='table0' summary=''>
-<colgroup>
-<col width='86%' />
-<col width='13%' />
-</colgroup>
-  <tr>
-    <td class='c016'>THE APPRECIATION OF COLOUR</td>
-    <td class='c017'><a href='#Page_44'>44</a></td>
-  </tr>
-</table>
-<div><span class='pageno' id='Page_xii'>xii</span></div>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='c010'>CHAPTER VI.</span></div>
-  </div>
-</div>
-
-<table class='table0' summary=''>
-<colgroup>
-<col width='86%' />
-<col width='13%' />
-</colgroup>
-  <tr>
-    <td class='c016'>ILLUSIONS CAUSED BY LIGHT ITSELF</td>
-    <td class='c017'><a href='#Page_53'>53</a></td>
-  </tr>
-</table>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='c010'>CHAPTER VII.</span></div>
-  </div>
-</div>
-
-<table class='table0' summary=''>
-<colgroup>
-<col width='86%' />
-<col width='13%' />
-</colgroup>
-  <tr>
-    <td class='c016'>THE INFLUENCE OF THE IMAGINATION</td>
-    <td class='c017'><a href='#Page_60'>60</a></td>
-  </tr>
-</table>
-<hr class='c013' />
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='c010'>PART II.</span></div>
-    <div class='c000'><span class='c006'>THE LAWS OF LIGHT.</span></div>
-    <div class='c003'><span class='c010'>CHAPTER I.</span></div>
-  </div>
-</div>
-
-<table class='table0' summary=''>
-<colgroup>
-<col width='86%' />
-<col width='13%' />
-</colgroup>
-  <tr>
-    <td class='c016'>WHAT IS LIGHT?</td>
-    <td class='c017'><a href='#Page_73'>73</a></td>
-  </tr>
-</table>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='c010'>CHAPTER II.</span></div>
-  </div>
-</div>
-
-<table class='table0' summary=''>
-<colgroup>
-<col width='86%' />
-<col width='13%' />
-</colgroup>
-  <tr>
-    <td class='c016'>THE SOLAR SPECTRUM</td>
-    <td class='c017'><a href='#Page_84'>84</a></td>
-  </tr>
-</table>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='c010'>CHAPTER III.</span></div>
-  </div>
-</div>
-
-<table class='table0' summary=''>
-<colgroup>
-<col width='86%' />
-<col width='13%' />
-</colgroup>
-  <tr>
-    <td class='c016'>OTHER CAUSES OF COLOUR</td>
-    <td class='c017'><a href='#Page_94'>94</a></td>
-  </tr>
-</table>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='c010'>CHAPTER IV.</span></div>
-  </div>
-</div>
-
-<table class='table0' summary=''>
-<colgroup>
-<col width='86%' />
-<col width='13%' />
-</colgroup>
-  <tr>
-    <td class='c016'>LUMINOUS, CALORIFIC, CHEMICAL, AND MAGNETIC PROPERTIES OF THE SPECTRUM</td>
-    <td class='c017'><a href='#Page_100'>100</a></td>
-  </tr>
-</table>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='c010'>CHAPTER V.</span></div>
-  </div>
-</div>
-
-<table class='table0' summary=''>
-<colgroup>
-<col width='86%' />
-<col width='13%' />
-</colgroup>
-  <tr>
-    <td class='c016'>THE LAWS OF REFLECTION.—MIRRORS</td>
-    <td class='c017'><a href='#Page_106'>106</a></td>
-  </tr>
-</table>
-<div><span class='pageno' id='Page_xiii'>xiii</span></div>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='c010'>CHAPTER VI.</span></div>
-  </div>
-</div>
-
-<table class='table0' summary=''>
-<colgroup>
-<col width='86%' />
-<col width='13%' />
-</colgroup>
-  <tr>
-    <td class='c016'>METALLIC BURNING MIRRORS</td>
-    <td class='c017'><a href='#Page_117'>117</a></td>
-  </tr>
-</table>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='c010'>CHAPTER VII.</span></div>
-  </div>
-</div>
-
-<table class='table0' summary=''>
-<colgroup>
-<col width='86%' />
-<col width='13%' />
-</colgroup>
-  <tr>
-    <td class='c016'>LENSES</td>
-    <td class='c017'><a href='#Page_127'>127</a></td>
-  </tr>
-</table>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='c010'>CHAPTER VIII.</span></div>
-  </div>
-</div>
-
-<table class='table0' summary=''>
-<colgroup>
-<col width='86%' />
-<col width='13%' />
-</colgroup>
-  <tr>
-    <td class='c016'>OPTICAL INSTRUMENTS.—THE SIMPLE AND COMPOUND MICROSCOPE. THE SOLAR AND PHOTO-ELECTRIC MICROSCOPE</td>
-    <td class='c017'><a href='#Page_141'>141</a></td>
-  </tr>
-</table>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='c010'>CHAPTER IX.</span></div>
-  </div>
-</div>
-
-<table class='table0' summary=''>
-<colgroup>
-<col width='86%' />
-<col width='13%' />
-</colgroup>
-  <tr>
-    <td class='c016'>THE TELESCOPES OF GALILEO, GREGORY, NEWTON, HERSCHEL, LORD ROSSE, AND FOUCAULT</td>
-    <td class='c017'><a href='#Page_150'>150</a></td>
-  </tr>
-</table>
-<hr class='c013' />
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='c010'>PART III.</span></div>
-    <div class='c000'><span class='c006'>NATURAL MAGIC.</span></div>
-    <div class='c003'><span class='c010'>CHAPTER I.</span></div>
-  </div>
-</div>
-
-<table class='table0' summary=''>
-<colgroup>
-<col width='86%' />
-<col width='13%' />
-</colgroup>
-  <tr>
-    <td class='c016'>THE MAGIC LANTERN</td>
-    <td class='c017'><a href='#Page_173'>173</a></td>
-  </tr>
-</table>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='c010'>CHAPTER II.</span></div>
-  </div>
-</div>
-
-<table class='table0' summary=''>
-<colgroup>
-<col width='86%' />
-<col width='13%' />
-</colgroup>
-  <tr>
-    <td class='c016'>THE PHANTASMAGORIA</td>
-    <td class='c017'><a href='#Page_183'>183</a></td>
-  </tr>
-</table>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='pageno' id='Page_xiv'>xiv</span><span class='c010'>CHAPTER III.</span></div>
-  </div>
-</div>
-
-<table class='table0' summary=''>
-<colgroup>
-<col width='86%' />
-<col width='13%' />
-</colgroup>
-  <tr>
-    <td class='c016'>OTHER OPTICAL ILLUSIONS</td>
-    <td class='c017'><a href='#Page_196'>196</a></td>
-  </tr>
-</table>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='c010'>CHAPTER IV.</span></div>
-  </div>
-</div>
-
-<table class='table0' summary=''>
-<colgroup>
-<col width='86%' />
-<col width='13%' />
-</colgroup>
-  <tr>
-    <td class='c016'>THE PROPERTIES OF MIRRORS</td>
-    <td class='c017'><a href='#Page_216'>216</a></td>
-  </tr>
-</table>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='c010'>CHAPTER V.</span></div>
-  </div>
-</div>
-
-<table class='table0' summary=''>
-<colgroup>
-<col width='86%' />
-<col width='13%' />
-</colgroup>
-  <tr>
-    <td class='c016'>CHINESE SHADOWS</td>
-    <td class='c017'><a href='#Page_223'>223</a></td>
-  </tr>
-</table>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='c010'>CHAPTER VI.</span></div>
-  </div>
-</div>
-
-<table class='table0' summary=''>
-<colgroup>
-<col width='86%' />
-<col width='13%' />
-</colgroup>
-  <tr>
-    <td class='c016'>POLYORAMA—DISSOLVING VIEWS—DIORAMA</td>
-    <td class='c017'><a href='#Page_231'>231</a></td>
-  </tr>
-</table>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='c010'>CHAPTER VII.</span></div>
-  </div>
-</div>
-
-<table class='table0' summary=''>
-<colgroup>
-<col width='86%' />
-<col width='13%' />
-</colgroup>
-  <tr>
-    <td class='c016'>THE STEREOSCOPE</td>
-    <td class='c017'><a href='#Page_236'>236</a></td>
-  </tr>
-</table>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='c010'>CHAPTER VIII.</span></div>
-  </div>
-</div>
-
-<table class='table0' summary=''>
-<colgroup>
-<col width='86%' />
-<col width='13%' />
-</colgroup>
-  <tr>
-    <td class='c016'>THE CAMERA OBSCURA AND CAMERA LUCIDA</td>
-    <td class='c017'><a href='#Page_242'>242</a></td>
-  </tr>
-</table>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='c010'>CHAPTER IX.</span></div>
-  </div>
-</div>
-
-<table class='table0' summary=''>
-<colgroup>
-<col width='86%' />
-<col width='13%' />
-</colgroup>
-  <tr>
-    <td class='c016'>THE SPECTROSCOPE</td>
-    <td class='c017'><a href='#Page_249'>249</a></td>
-  </tr>
-</table>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='c010'>CHAPTER X.</span></div>
-  </div>
-</div>
-
-<table class='table0' summary=''>
-<colgroup>
-<col width='86%' />
-<col width='13%' />
-</colgroup>
-  <tr>
-    <td class='c016'>SPECTRES—THE GHOST ILLUSION</td>
-    <td class='c017'><a href='#Page_264'>264</a></td>
-  </tr>
-</table>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div class='chapter'>
-  <span class='pageno' id='Page_xv'>xv</span>
-  <h2 class='c012'>LIST OF ILLUSTRATIONS.</h2>
-</div>
-<hr class='c013' />
-<table class='table2' summary=''>
-<colgroup>
-<col width='8%' />
-<col width='80%' />
-<col width='12%' />
-</colgroup>
-  <tr>
-    <td class='c018'><span class='small'>FIG.</span></td>
-    <td class='c016'>&nbsp;</td>
-    <td class='c017'><span class='small'>PAGE</span></td>
-  </tr>
-  <tr>
-    <td class='c018'>1.</td>
-    <td class='c016'>Section of the Eye</td>
-    <td class='c017'><a href='#i024'>24</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>2.</td>
-    <td class='c016'>A Camera Obscura</td>
-    <td class='c017'><a href='#i027'>27</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>3.</td>
-    <td class='c016'>The Phenakistiscope</td>
-    <td class='c017'><a href='#i054'>54</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>4.</td>
-    <td class='c016'>Disc of the Phenakistiscope</td>
-    <td class='c017'><a href='#i055'>55</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>5.</td>
-    <td class='c016'>Solar Spectrum</td>
-    <td class='c017'><a href='#i004b'><i>Frontispiece</i></a></td>
-  </tr>
-  <tr>
-    <td class='c018'>6.</td>
-    <td class='c016'>Absorption of Light by Sodium Vapour</td>
-    <td class='c017'><a href='#i004a'><i>ib.</i></a></td>
-  </tr>
-  <tr>
-    <td class='c018'>7.</td>
-    <td class='c016'>Action of a Prism on the Solar Rays</td>
-    <td class='c017'><a href='#i004c'><i>ib.</i></a></td>
-  </tr>
-  <tr>
-    <td class='c018'>8.</td>
-    <td class='c016'>The Recomposition of Light</td>
-    <td class='c017'><a href='#i086'>86</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>9.</td>
-    <td class='c016'>Recomposition of Light by means of a Concave Mirror</td>
-    <td class='c017'><a href='#i087'>87</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>10.</td>
-    <td class='c016'>Recomposition of Light by means of a number of Mirrors</td>
-    <td class='c017'><a href='#i088'>88</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>11.</td>
-    <td class='c016'>Newton’s Disc</td>
-    <td class='c017'><a href='#i089'>89</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>12.</td>
-    <td class='c016'>Newton’s Rings</td>
-    <td class='c017'><a href='#i095'>95</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>13.</td>
-    <td class='c016'>Reflection from Plane Surfaces</td>
-    <td class='c017'><a href='#i107'>107</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>14.</td>
-    <td class='c016'>Refraction</td>
-    <td class='c017'><a href='#i108'>108</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>15.</td>
-    <td class='c016'>Experimental Proof of Refraction</td>
-    <td class='c017'><a href='#i108b'><i>ib.</i></a></td>
-  </tr>
-  <tr>
-    <td class='c018'>16.</td>
-    <td class='c016'>The Effects of Plane Mirrors</td>
-    <td class='c017'><a href='#i109'>109</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>17.</td>
-    <td class='c016'>Reflection from the Surface of Water</td>
-    <td class='c017'><a href='#i110'>110</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>18.</td>
-    <td class='c016'>Concave Mirror</td>
-    <td class='c017'><a href='#i111'>111</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>19.</td>
-    <td class='c016'>Conjugate Foci</td>
-    <td class='c017'><a href='#i113'>113</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>20.</td>
-    <td class='c016'>Virtual Focus</td>
-    <td class='c017'><a href='#i114'>114</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>21.</td>
-    <td class='c016'>Concave Mirror</td>
-    <td class='c017'><a href='#i114b'><i>ib.</i></a></td>
-  </tr>
-  <tr>
-    <td class='c018'>22.</td>
-    <td class='c016'>Magnifying Effect of Concave Mirrors</td>
-    <td class='c017'><a href='#i115'>115</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>23.</td>
-    <td class='c016'>The Reversal of real Images</td>
-    <td class='c017'><a href='#i115b'><i>ib.</i></a></td>
-  </tr>
-  <tr>
-    <td class='c018'>24.</td>
-    <td class='c016'>Diminishing Power of Convex Mirrors</td>
-    <td class='c017'><a href='#i116'>116</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>25.</td>
-    <td class='c016'>Burning Mirror</td>
-    <td class='c017'><a href='#i124'>124</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>26.</td>
-    <td class='c016'>Double Convex Lens</td>
-    <td class='c017'><a href='#i127'>127</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>27.</td>
-    <td class='c016'>Forms of Lenses</td>
-    <td class='c017'><a href='#i128'>128</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>28.</td>
-    <td class='c016'>Path of a Ray through a Convex Lens</td>
-    <td class='c017'><a href='#i129'>129</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>29.</td>
-    <td class='c016'>Path of Divergent Rays through a Convex Lens</td>
-    <td class='c017'><a href='#i129b'><i>ib.</i></a></td>
-  </tr>
-  <tr>
-    <td class='c018'>30.</td>
-    <td class='c016'>Conjugate Foci</td>
-    <td class='c017'><a href='#i130'>130</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>31.</td>
-    <td class='c016'>Images formed by Convex Lenses</td>
-    <td class='c017'><a href='#i131'>131</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>32.</td>
-    <td class='c016'>Magnifying Property of Convex Lenses</td>
-    <td class='c017'><a href='#i132'>132</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>33.</td>
-    <td class='c016'>Diminishing Effect of Concave Lenses</td>
-    <td class='c017'><a href='#i132b'><i>ib.</i></a></td>
-  </tr>
-  <tr>
-    <td class='c018'><span class='pageno' id='Page_xvi'>xvi</span>34.</td>
-    <td class='c016'>Cannon of the Palais Royal</td>
-    <td class='c017'><a href='#i134'>134</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>35.</td>
-    <td class='c016'>Fresnel’s Lighthouse Apparatus</td>
-    <td class='c017'><a href='#i136'>136</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>36.</td>
-    <td class='c016'>Lantern of a First-Class Lighthouse</td>
-    <td class='c017'><a href='#i140'>140</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>37.</td>
-    <td class='c016'>The Compound Microscope</td>
-    <td class='c017'><a href='#i143'>143</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>38.</td>
-    <td class='c016'>The Theory of the Compound Microscope</td>
-    <td class='c017'><a href='#i144'>144</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>39.</td>
-    <td class='c016'>Photo-Electric Microscope</td>
-    <td class='c017'><a href='#i147'>147</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>40.</td>
-    <td class='c016'>Solar Microscope</td>
-    <td class='c017'><a href='#i148'>148</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>41.</td>
-    <td class='c016'>The Galilean Telescope</td>
-    <td class='c017'><a href='#i155'>155</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>42.</td>
-    <td class='c016'>The Astronomical Telescope</td>
-    <td class='c017'><a href='#i156'>156</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>43.</td>
-    <td class='c016'>Section of an Astronomical Telescope</td>
-    <td class='c017'><a href='#i157'>157</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>44.</td>
-    <td class='c016'>Section of the Gregorian Telescope</td>
-    <td class='c017'><a href='#i160'>160</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>45.</td>
-    <td class='c016'>Gregorian Telescope</td>
-    <td class='c017'><a href='#i161'>161</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>46.</td>
-    <td class='c016'>Section of a Newtonian Telescope</td>
-    <td class='c017'><a href='#i162'>162</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>47.</td>
-    <td class='c016'>Herschellian Telescope</td>
-    <td class='c017'><a href='#i164'>164</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>48.</td>
-    <td class='c016'>Foucault’s Large Telescope</td>
-    <td class='c017'><a href='#i169'>169</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>49.</td>
-    <td class='c016'>Foucault’s Small Telescope</td>
-    <td class='c017'><a href='#i171'>171</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>50.</td>
-    <td class='c016'>Section of the Magic Lantern</td>
-    <td class='c017'><a href='#i179'>179</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>51.</td>
-    <td class='c016'>Magic Lantern</td>
-    <td class='c017'><a href='#i182'>182</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>52.</td>
-    <td class='c016'>The Phantasmagoria</td>
-    <td class='c017'><a href='#i184'>184</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>53.</td>
-    <td class='c016'>The Phantascope</td>
-    <td class='c017'><a href='#i185'>185</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>54.</td>
-    <td class='c016'>Phantasmagoria (<span class='sc'>Robertson</span>)</td>
-    <td class='c017'><a href='#i194'>194</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>55.</td>
-    <td class='c016'>Wizard Dance</td>
-    <td class='c017'><a href='#i198'>198</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>56.</td>
-    <td class='c016'>Nostradamus and Marie de Médicis</td>
-    <td class='c017'><a href='#i201'>201</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>57.</td>
-    <td class='c016'>The Arrangement of the Reversing Prism</td>
-    <td class='c017'><a href='#i203'>203</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>58.</td>
-    <td class='c016'>The Goat Trick</td>
-    <td class='c017'><a href='#i205'>205</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>59.</td>
-    <td class='c016'>How to see through a Brick</td>
-    <td class='c017'><a href='#i207'>207</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>60.</td>
-    <td class='c016'>The Polemoscope</td>
-    <td class='c017'><a href='#i210'>210</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>61.</td>
-    <td class='c016'>Protection against ill-natured People</td>
-    <td class='c017'><a href='#i213'>213</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>62.</td>
-    <td class='c016'>&nbsp;</td>
-    <td class='c017'><a href='#i218'>218</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>63.</td>
-    <td class='c016'>Anamorphosis</td>
-    <td class='c017'><a href='#i220'>220</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>64.</td>
-    <td class='c016'>Effect of Cut Paper-work</td>
-    <td class='c017'><a href='#i225'>225</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>65.</td>
-    <td class='c016'>Seditious Toys</td>
-    <td class='c017'><a href='#i229'>229</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>66.</td>
-    <td class='c016'>Diorama</td>
-    <td class='c017'><a href='#i234'>234</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>67.</td>
-    <td class='c016'>&nbsp;</td>
-    <td class='c017'><a href='#i237'>237</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>68.</td>
-    <td class='c016'>Stereoscope</td>
-    <td class='c017'><a href='#i238'>238</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>69.</td>
-    <td class='c016'>The Principle of the Refracting Stereoscope</td>
-    <td class='c017'><a href='#i239'>239</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>70.</td>
-    <td class='c016'>The Camera Obscura</td>
-    <td class='c017'><a href='#i243'>243</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>71.</td>
-    <td class='c016'>Section of Camera Lucida</td>
-    <td class='c017'><a href='#i247'>247</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>72.</td>
-    <td class='c016'>The Spectre—an Optical Illusion</td>
-    <td class='c017'><a href='#i272'>269</a></td>
-  </tr>
-  <tr>
-    <td class='c018'>73.</td>
-    <td class='c016'>How to produce Spectres</td>
-    <td class='c017'><a href='#i273'>271</a></td>
-  </tr>
-</table>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='pageno' id='Page_15'>15</span><span class='c019'>THE WONDERS OF OPTICS.</span></div>
-  </div>
-</div>
-
-<hr class='c013' />
-
-<div class='nf-center-c0'>
-  <div class='nf-center'>
-    <div><span class='c006'>PART I.</span></div>
-    <div class='c000'><span class='c015'>THE PHENOMENA OF VISION.</span></div>
-  </div>
-</div>
-
-<hr class='c009' />
-<div class='chapter'>
-  <h2 id='ch1-01' class='c012'>CHAPTER I.<br /><span class='c020'>THE EYE.</span></h2>
-</div>
-<p class='c014'><span class='sc'>The</span> Eye is at once the most wonderful and the most
-useful of all our organs of sense. It is especially by
-means of the eye that we gain a knowledge of the exterior
-world. Our other senses are far more limited in
-their action: thus the sense of touch only extends to
-objects within our reach; the sense of taste is only a
-delicate and exquisite modification of the sense of touch;
-the sense of smell can only be exercised on substances
-that are close to us; and the use of our ears is limited
-by the distance at which the loudest sound ceases to
-impress them. But the eye has the privilege of extending
-its dominion, whether for mere enjoyment or for
-serious instruction, far beyond the limits of this little
-world. Not only is it the origin of all our ideas upon
-<span class='pageno' id='Page_16'>16</span>every object that comes within its ken; not only does
-it reveal to us our own position and that of our surroundings;
-but, thanks to the discoveries of modern
-science, it is able to admire, on the one hand, a world
-of infinite minuteness that remained unknown to us for
-centuries, and, on the other, the immeasurable immensity
-of the starry universe.</p>
-
-<p class='c021'>Admirable as the eye undoubtedly is through the
-possession of the power of vision, it is also capable of
-enchanting us by its own particular beauties. Not to
-speak of its internal mechanism, which we shall consider
-very fully by and by, let us for a moment examine
-its outward appearance. Have you never, dear reader,
-been enchanted with a pair of soft and gentle eyes, or
-with a couple of black orbs veiled with long dark lashes,
-or with those wondrous eyes that rival the heavens in
-colour and depth, shedding on you rays of light whose
-mute eloquence was irresistible? If it be true that
-man’s face is the canvas upon which the affections and
-desires of his mind are depicted as soon as they are
-formed, the eyes are unquestionably the central point
-of the picture, and it is in them, as in a looking-glass,
-that every sentiment that passes across our brain is reflected.</p>
-
-<p class='c021'>When the mind is undisturbed, says Buffon, all the
-parts of the face are in a state of repose; their proportion,
-unity, and general appearance indicate the
-pleasing harmony of our thoughts and the perfect calmness
-of our mind; but when we are agitated, the human
-face becomes a living picture, in which the passions
-that disturb us are depicted with equal force and
-delicacy, a picture in which every emotion is expressed
-by a stroke, every action by a letter, so to speak; in
-which the quickness of the impression outstrips the will,
-and reveals by the most sympathetic signs the image of
-our secret trouble.</p>
-
-<p class='c021'><span class='pageno' id='Page_17'>17</span>It is more especially in the eyes, adds this great
-naturalist, that these signs are manifested and recognised.
-The eye is connected with the mind more than
-any other organ: it seems almost to be in contact with
-it and to participate in all its movements; it expresses
-in obedience to it the strongest passions and the most
-tumultuous emotions, as well as the gentlest thoughts
-and most delicate sentiments, and reproduces them in
-all their force and purity just as they have sprung into
-existence; it transmits them with exquisite rapidity
-even to the minds of others, where they once more become
-impressed with all their original fire, movement,
-and reality. The eye both receives and reflects the
-light of thought and the warmth of sentiment, and is
-at once the sense of the mind and the tongue of the
-intellect. Persons who are short-sighted, or who squint,
-have much less of this external intelligence that dwells
-in the eye. It is only the stronger passions that can
-bring the other features of the face into play, that are
-depicted on their physiognomy; and the effects of fine
-thought and delicate feeling are rendered apparent with
-much greater difficulty.</p>
-
-<p class='c021'>The elegant author of <i>L’Histoire Naturelle</i> rightly
-thinks that we are so accustomed only to see things
-from the outside, that we are hardly aware how much
-this exterior view of everything influences the judgment
-of even the gravest and most thoughtful of us. Thus
-we are apt to set down a man as unintellectual whose
-physiognomy does not particularly strike us; and we
-allow his clothes, and even the manner in which he wears
-his hair, to influence our judgment of him. Hence, our
-author goes on to say, not wholly without some show
-of reason, that a man of sense ought to look upon his
-clothes as part of himself, because they really are so in
-the eyes of others, and play an important part in the
-general idea that is formed of him who wears them.</p>
-
-<p class='c021'><span class='pageno' id='Page_18'>18</span>The vivacity or languor of the movement of the eyes
-forms one of the chief characteristics of facial expression,
-and their colour helps to render this characteristic more
-striking. The different colours seen in the eye are dark
-hazel, or black, as it is generally called, light hazel, blue,
-greenish grey, dark grey, and light grey. The velvety
-substance which gives the colour to the iris is arranged
-in little ramifications and specks, the former being directed
-towards the centre of the eye, the latter filling up
-the gaps between the threads. Sometimes they are both
-arranged in so regular a manner that instances have
-been known in which the irises of different eyes have
-appeared to be so much alike that they seemed to have
-been copied from the same design. These little threads
-and specks are held together by a very fine network.</p>
-
-<p class='c021'>The commonest colours seen in the eye are hazel and
-blue, and it mostly happens that both these colours are
-found in the same individual, giving rise to that peculiar
-greenish-grey hue that is far from being uncommon.
-Buffon thinks that blue and black eyes are the most
-beautiful, but this of course is a matter of taste. It is
-true that the vivacity and fire which play so important
-a part in giving character to the eye, are more perceptible
-in dark eyes than in those whose tints are lighter;
-black eyes, therefore, have greater force of expression,
-while in blue eyes there is more softness and delicacy.
-In the former we see a brilliant fire, which sparkles
-uniformly on account of the iris, which is of the same
-colour throughout, giving in all parts the same reflection;
-but a great difference may be perceived in the intensity
-of the light reflected from blue eyes, from the fact of the
-various tints of colour producing different reflections.
-There are some eyes that are remarkable for being almost
-destitute of colour, and appear to be constituted in an
-abnormal manner. The iris is tinted with shades of
-blue and grey of so light a hue that it appears quite
-<span class='pageno' id='Page_19'>19</span>white in some places. The shades of hazel in such eyes
-are so light that they are hardly distinguishable from
-grey and white, in spite even of the contrast of colour.</p>
-
-<p class='c021'>For our part, we think that the beauty of the eye
-consists not so much in its colour, or even in its harmony
-with the rest of the face, but in its expression.</p>
-
-<p class='c021'>There are also numerous instances of green eyes.
-This colour is, of course, much less frequent than blue,
-grey, or hazel. It often happens, too, that the two
-eyes vary in colour in the same individual. This defect
-is not confined to the human species, being shared by
-the horse and the cat. In most other animals the colour
-of the two eyes is always similar. The colour of
-the eye in most animals is either hazel or grey. Aristotle
-imagined that grey eyes were stronger than blue, that
-those persons whose eyes are prominent cannot see so
-far as others, and that brown eyes are less valuable in
-the dark than those of another tint; but modern investigations
-have failed to bear out the ancient philosopher’s
-ideas with regard to the human eye.</p>
-
-<p class='c021'>Although the eye appears to move about in every
-direction, it has in reality only one movement, that of
-rotation round its centre, by means of which the eyeball
-rises or falls, or passes from side to side at will.
-In man the eyes are parallel with each other in relation
-to their axes; he can consequently direct them at pleasure
-upon the same object: but in most animals this
-parallelism is wanting. In some cases the eyes of animals
-are set almost back to back, rendering it impossible
-for them to see the same object with both eyes at
-once.</p>
-
-<p class='c021'>Buffon makes the remark, that after the eyes, the
-eyebrows contribute more strongly than any other part
-of the face towards giving character to the physiognomy,
-being, inasmuch as they differ in their nature from the
-other features, more apparent by contrast, and hence
-<span class='pageno' id='Page_20'>20</span>strike us more than any other portion of the countenance.
-They are, in fact, a shadow in the picture,
-bringing its colour and drawing into strong relief. The
-eyelashes also contribute their effect; when they are
-long and thick, they overshadow the eye, making its
-glance appear softer and more beautiful. The ape is
-the only other animal besides man that possesses two
-eyelashes, the rest having them only on the upper eyelid.
-Even in man they are more abundant in the upper
-eyelid than in the lower. The eyebrows have but two
-movements, upward and downward, governed by the
-muscles of the forehead. In the action of frowning we
-not only lower them, but move them slightly towards
-each other. The eyelids serve to protect the eyeball,
-and keep the cornea from becoming dry. The upper
-eyelid has the power of raising and lowering itself, the
-lower one being almost destitute of movement. Although
-the motion of the eyelids is an effort of will, there are
-times when it is impossible to keep them open, as for
-instance when we are overpowered by sleep, or when
-the eyes are suddenly subjected to the effects of strong
-light. The eyelid is a most admirable arrangement for
-the protection of the eye, and it is almost impossible to
-admire this provision of nature too much, even when
-we confine ourselves to an outward examination of it.
-It is not merely the outward mechanism and motion of
-the eyelids, nor the colour of the eyes, that constitutes
-their beauty; we have already said that the leading
-characteristic of the eye was <i>expression</i>. It is this expression
-which causes the eye to appear to speak, to fire
-up suddenly, to sparkle with flashes of light, to languish
-or conceal itself underneath its lashes, to raise itself
-with inspiration, or to pierce the abyss of thought, just
-according to the particular sentiment governing the
-mind at the moment. Hence it is expression that constitutes
-the true beauty of the eye: every one knows
-<span class='pageno' id='Page_21'>21</span>instances of eyes which, while at rest, would never be
-noticed by anybody, but which, when once animated by
-intense eloquence, lend to the voice of their possessor
-an unexpected power, which moves and transports the
-listener to an extent infinitely beyond that resulting
-from the simple spoken words.</p>
-
-<p class='c021'>Enough, however, has been said upon the external
-aspect of the human eye; we will, therefore, at once
-endeavour to penetrate the circle in which are contained
-the wonders that this little book is intended to
-describe. The object of these lines is not so much to
-describe the beauty of man’s glances, nor the value of
-his senses, but rather to make known those illusions to
-which the most sagacious of all his senses is apt to fall
-a prey. But before entering the temple it was but
-right to have bestowed a little admiration upon the
-façade. By the way, as we are about to describe many
-illusory wonders, do not let us commence by deceiving
-ourselves with regard to our first marvel—the eye itself.
-A great philosopher calls the eyes the windows of the
-soul, and, although meant as a poetical image, the saying
-is not far from the truth; for the optic nerve by
-which we see external objects, is an extension of the
-nerves of the brain, whose functions and actions are
-an unfathomable mystery.</p>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div class='chapter'>
-  <span class='pageno' id='Page_22'>22</span>
-  <h2 id='ch1-02' class='c012'>CHAPTER II.<br /><span class='c020'>THE STRUCTURE OF THE EYE.</span></h2>
-</div>
-<p class='c014'><span class='sc'>Of</span> all the senses, says an ardent admirer of nature,
-the sight is certainly that which furnishes the mind with
-the quickest and most widely-extended perceptions. It
-is the source of the richest treasures of the imagination,
-and of our ideas of the beauty, order, and unity of the
-world around us. How unhappy are those whom a
-hard fate has deprived of the sense of sight from their
-birth! Alas! the finest day and the darkest night
-differ in nothing as far as they are concerned; the light
-of heaven never brings joy into their hearts. The
-enamelled beauties of a bed of flowers, the varied plumage
-of the peacock, the glories of the rainbow are
-alike unknown to them. They cannot contemplate from
-the mountain height the beauties of the valley beneath;
-the fields golden with the harvest, the meadows smiling
-with verdure, and watered by winding rivers, and the
-habitations of man dotted about here and there over
-the surface of this magnificent picture. To them is
-unknown the sight of the mighty ocean; and the
-innumerable legions of the cloud army of Heaven are
-to them as if they did not exist. The impenetrable
-obscurity which surrounds them allows them neither the
-contemplation of what is grandest in man’s outward
-aspect, nor even the admiration of those qualities which
-they themselves would hold most dear.</p>
-
-<p class='c021'><span class='pageno' id='Page_23'>23</span>A strong sentiment of pity should, therefore, animate
-the breast of every right-thinking man, when he
-considers the unhappy condition of those who are born
-blind.</p>
-
-<p class='c021'>The eye infinitely surpasses in its complexity and
-beauty of structure all the other organs of sense, and
-is most unquestionably the most marvellous object that
-the human mind is capable of examining and understanding.
-Let us first examine the external parts of
-this wonderful organ. With what a singular system of
-entrenchments and defences do we find the eye provided!
-It is itself placed in the head at a certain
-depth, and surrounded on all sides by solid bone, so that
-it is only with the greatest difficulty that it is hurt by
-accident from without. The eyebrows also play their
-part as protection to the eye, and prevent the perspiration
-from entering and irritating the organ. The eyelids
-too are always ready to rush to the rescue, whether
-to protect the eye from outward attacks, or to shade it
-from too strong a light during sleep. The eyelashes
-not only add to the beauty of the eye, but they shade
-it from the too brilliant light of the sun, and act as
-advanced guards to prevent the entrance of dust or any
-other foreign body with which the eyes might be injured.</p>
-
-<p class='c021'>But its internal structure is still more admirable.
-The globe of the eye is almost spherical and measures
-nearly one inch in diameter. <a href='#i024'>Fig. 1</a> is a view of the
-eyeball, showing the details of its structure; the various
-membranes surrounding it have been cut away in order
-that it may be better examined. If we commence our
-examination by the exterior portion of the front, we
-shall first find immediately beneath the eyelashes a perfectly
-transparent membrane (<span class='fss'>C</span>), called the <i>cornea</i>. It
-is a prolongation of the hard opaque external coating
-of the eye, called the <i>sclerotic membrane</i>, and marked
-S in the figure. The cornea is sufficiently hard in its
-<span class='pageno' id='Page_24'>24</span>nature to present a strong resistance to any violence
-from without.</p>
-
-<p class='c021'>Immediately beneath the cornea and in contact with
-it is the <i>aqueous humour</i>, a thin transparent liquid occupying
-a small portion of the front of the eye.</p>
-
-<p class='c021'>Next comes the <i>iris</i>, a circular disc perforated with a
-round hole in the middle, and coloured with various
-shades of blue, brown, and grey.</p>
-
-<div id='i024'  class='figcenter id005'>
-<img src='images/i024.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p>Fig. 1.—Section of the Eye.</p>
-</div>
-</div>
-
-<p class='c021'>The opening in the centre, which appears like a black
-spot when the eye is examined, is not really an object,
-but simply an aperture, capable of changing its size
-according to the quantity of light striking the eye.
-This change of size in the opening, or pupil, as it is
-popularly called, is effected by the contraction or expansion
-of the iris, which thus possesses the peculiar
-property of exactly proportioning the amount of light
-that enters the eye, so that there is never too much or
-too little. It is through the pupil that the rays of
-light proceeding from the various objects around us pass
-into the interior of the eye, and form an image upon
-the retina, as will be afterwards explained.</p>
-
-<p class='c021'>Immediately behind the pupil is <span class='fss'>O</span>, a bi-convex lens
-to transmit the rays of light to the retina. It is generally
-called the <i>crystalline lens</i>.</p>
-
-<p class='c021'><span class='pageno' id='Page_25'>25</span>From the crystalline lens to the back of the eyeball,
-is a space more or less globular in form, containing
-a gelatinous diaphanous mass somewhat resembling
-white of egg in appearance, and called the <i>vitreous
-humour</i>.</p>
-
-<p class='c021'>Behind the vitreous humour, and immediately opposite
-the pupil and lens, is the most delicate and important
-of all the membranes of the eye, the <i>retina</i>,
-which serves as a screen whereon are received the
-images of the objects around us. This membrane is an
-expansion of the optic nerve <span class='fss'>N</span> leading from the brain,
-and lines the whole of the interior of the eye. The
-eye is also enveloped in a second membrane (<span class='fss'>C</span>), called
-the <i>choroid</i>, which is impregnated with a black pigment.
-Round this is wrapped a third membrane, the <i>sclerotic</i>
-(<span class='fss'>S</span>), which unites with the cornea in front of the eyeball.</p>
-
-<p class='c021'>The crystalline lens through which all the rays pass
-before they reach the retina, possesses the marvellous
-power of being able to modify its curvature in such a
-manner as to adapt itself to the distance of the object
-seen, and thus throw a distinct image on the retina.
-When we come to talk of the properties of lenses, we
-shall see that the focus of a lens differs for objects at
-different distances; if, therefore, the eye were not provided
-with some such means for altering the focus of the
-crystalline lens, we should only see objects distinctly at
-one particular point. The crystalline lens consists of
-infinite numbers of extremely thin transparent little
-plates, each of which is in itself composed of fine fibres
-so united together as to be capable of a small degree of
-compression or extension. Hence the power of the
-lens to alter its form according to circumstances. It is
-calculated that the human eye contains over five millions
-of the laminæ above referred to. With such wonders
-is the world of nature replete,—wonders that we
-daily and hourly pass by without examination.</p>
-
-<p class='c021'><span class='pageno' id='Page_26'>26</span>It is by means of this ingenious and inimitable structure
-of the eye that external objects pass from the domain
-of the material world into that of the mind, and
-become accessible to every faculty of our brain. Of its
-own accord, and without apparently any effort of our
-own will, does this marvellous mechanism adapt itself
-to all the variations of distance and intensity of light,
-a power possessed by no instrument as yet constructed
-by the hand of man—being capable, as it is, of distinguishing
-instantaneously between the distance of the
-remotest nebulæ and that of the letters forming this
-page. This wonderful organ, writes Brewster, may be
-considered as being the sentinel that guards the passage
-between the world of matter and that of mind, and as
-the medium through which they interchange all their
-communications. The optic nerve perceives the objects
-written on the retina by the hand of nature, and conveys
-them to the brain in all their integrity of form and
-colour.</p>
-
-<p class='c021'>The path of the rays of light and the formation of
-images upon the retina are shown in the preceding
-figure. At first sight it will be perceived that the objects
-thereon depicted are in a reversed position, that is
-to say, when we look at a view similar to that shown in
-<a href='#i027'>fig. 2</a>, we should find, if we had any means of observing
-the positions of objects reflected on our retina, that the
-flock of sheep coming up the road were at the top of the
-eye, while the trees, the roof of the house, and the chimney
-were in the contrary position. Similar reversed
-images may be seen in dark rooms, by holding a screen
-before any little crack or pinhole in the door or shutter
-of the room. In <a href='#i027'>fig. 2</a> the keyhole of the door is represented
-as playing the part of a lens. The author, in
-common with almost every other boy, observed this fact
-at a very early age, and the idea immediately struck
-him that it would be only necessary to fix these images
-<span class='pageno' id='Page_27'>27</span>to procure exact representations of natural scenery; but
-in making inquiries into the subject, he found that his
-juvenile observations had been made a little too late,
-photography having already gained the end he intended
-striving for.</p>
-
-<div id='i027'  class='figcenter id006'>
-<img src='images/i027.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 2.</span>—A Camera Obscura.</p>
-</div>
-</div>
-
-<p class='c021'>Seeing that the images of all objects appear on our
-retina upside down, the student is naturally disposed to
-ask how it happens that we do not see them in that
-position. Physiologists and natural philosophers have
-advanced numerous theories on the subject. Some,
-with Buffon, admit at once that it is by habit and education
-of the eye that we see objects unreversed.
-Others, like the great physiologist Müller, imagine that
-as we see everything upside down, and not a single object
-only, we have no points of comparison, and practically
-ignore the reversal. The truth, however, appears
-to be that it is the brain, and not the eye, that possesses
-the power of determining the real position of what we
-see. That the eye alone has no power of determining
-the positions of objects by itself, may be easily proved
-by showing a person an astronomical object, such as the
-moon through a telescope. Unless the observer has
-<span class='pageno' id='Page_28'>28</span>been already familiarized with the appearance of our
-satellite, he will not know whether the image he sees is
-reversed or not. It is the brain, therefore, and the
-brain only, that has the power of determining the position
-of objects around us, without taking into consideration
-the reversed picture of them that is depicted on our
-retina. The student who takes an interest in the structure
-of this important organ, would do well to procure
-a sheep’s or bullock’s eye from the butchers, and dissect
-it carefully with a sharp penknife and pair of scissors.
-The image formed on the retina may be easily seen by
-cutting away the sclerotic and choroid coatings at the
-back of the eye.</p>
-
-<p class='c021'>The ordinary distance of distinct vision for small
-objects, such as the letters of a book, is from ten to
-twelve inches. But possibly there do not exist two
-pairs of eyes in the world whose foci are the same.
-Even in the same individual it frequently happens that
-the focal length of the eyes differs considerably. In
-some persons the focus of the eye is so reduced that
-they are obliged to bring the object they are examining
-within six, and even four inches of their eyes, before
-they can see it. This defect is known ordinarily as
-<i>short sight</i>, and results from the too great convexity of
-the cornea and crystalline lens. It is corrected by
-wearing spectacles with concave glasses. Others again,
-on the contrary, place the book or object they are looking
-at, at a greater distance from the eye than that
-named. Such people are called long-sighted, and the
-defect results from the too great flatness of the cornea
-and the crystalline lens. The fault is of course corrected
-by the use of spectacles containing convex
-lenses.</p>
-
-<p class='c021'>Long-sightedness is generally the result of old age,
-and it may be taken as a fact that the older we grow
-the flatter becomes the crystalline lens. Hence short-sighted
-<span class='pageno' id='Page_29'>29</span>people have been known to recover their sight
-perfectly as they advance in years through the natural
-process of the flattening of the crystalline lens. These
-matters, however, will be more fully treated of when
-we begin to speak of the properties of lenses of different
-forms and curvatures.</p>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div class='chapter'>
-  <span class='pageno' id='Page_30'>30</span>
-  <h2 id='ch1-03' class='c012'>CHAPTER III.<br /><span class='c020'>THE ERRORS OF THE EYE.</span></h2>
-</div>
-<p class='c014'><span class='sc'>It</span> is with our own organization that we shall commence
-our task of exposing the illusions that we shall
-meet with during our optical experiments,—in fact with
-that wonderful and important organ of our body that
-we are apt to look upon as sure and infallible, but
-which we shall find is deceiving us constantly, and
-hourly proving the fallacy of the popular saying, that
-“every one must believe his own eyes.” In ancient
-times there existed a school of sceptics who doubted
-everything beginning with Pyrrho, the great theorist,
-and ending with the follower of his school who doubted
-the existence of muscular force even after he had received
-a sound box on the ear from an opponent of his
-system of philosophy. If any of our readers were to
-become followers of Pyrrho, they might easily do so
-when considering the numberless illusions we shall
-describe to them, if they did not remember that if our
-senses are subject to error, we have a brain to set them
-right: our mind, if logical and well regulated, soon
-discovers errors of observation, and speedily places our
-judgment on the most solid basis. We shall find endless
-instances of this throughout our little book. If
-we are dazzled with illusions from time to time we shall
-as often recover ourselves; and no matter how beautiful
-or interesting these deceptions may appear, we shall
-speedily be able to convince ourselves that they are
-<span class='pageno' id='Page_31'>31</span>unreal. In this chapter we shall only speak of those
-errors of the eye of which we have actually lost all
-cognizance, so effectually has our judgment succeeded
-in counteracting their influence.</p>
-
-<p class='c021'>We all know that the first thing a child does with its
-eyes, even when it is only five or six weeks old, is to
-turn them towards the most brilliant object within its
-reach. Instinctively and without being aware of it,
-the child’s eye seems to seek the light. The whole of
-nature, from the lowest plant to the baby in the cradle,
-appears more or less endowed with this instinct of turning
-towards the light.</p>
-
-<p class='c021'>From the time that children begin to distinguish objects,
-their eyes are liable to be affected by two causes
-of error. Before being able to judge of the position of
-things surrounding them, they see everything upside
-down; they consequently acquire a false impression of
-the position of objects. The next cause of error that is
-likely to mislead them is the fact of their seeing everything
-double, a separate image of everything being
-formed on each eye; and it can only be by the experience
-gained through the sense of touch that they can
-acquire the knowledge necessary to rectify these errors,
-and see those objects single which appear to them
-double. This error of sight, as well as the first one, is
-set right so easily in the end, that although in reality
-we see everything double and upside down, we imagine
-that we see them single, and in their proper positions,
-a state of things brought about entirely through another
-sense exercising its power over our judgment; and
-it is hardly too much to say that, if that sense were deprived
-of the power of feeling, our eyes would deceive
-us, not only as to the number, but the position of the
-objects within our view.</p>
-
-<p class='c021'>It is very easy to convince ourselves that we really
-see objects double, although we imagine them to be only
-<span class='pageno' id='Page_32'>32</span>single. We have only to look at the same object first
-with the right eye, and we shall see it directly against
-some portion of the wall of the room in which we are
-sitting; then looking at it with the left eye, we shall
-see that it covers a different part of the wall. This experiment
-is easily tried, and is very convincing. Thus
-we see that an image is formed on both eyes, and we
-consequently see the object, whatever it may be, repeated
-twice. By degrees, however, the eyes gain the
-power of converging their axes on objects at different
-distances, so that they fall on similar portions of each
-retina, and so convey a single impression to the brain.
-Thus, for instance, if we look at a pencil held up at
-arm’s length, and then, without changing the position
-of the eyeball, look at some distant object, we shall see
-it double. Let us, however, converge the eyes upon it,
-and the two images unite. Reverse the experiment by
-now looking at the pencil without converging the eyes
-upon it, and we shall see that object double in its turn.
-The same thing happens if we push aside one of the
-eyes with the finger while looking at any object.
-During severe illness it often happens that the patient
-from extreme weakness loses the power of convergence,
-and consequently sees every thing double, and we continually
-see children’s faces wearing a most distressing
-appearance through having temporarily lost the power
-of moving the muscles of the eye. It is a common expression
-to use in speaking of drunken people, that they
-see double, but the saying, unlike many others, is no
-metaphor; when a man gets drunk he loses his power
-over the muscles of his eye, just as he does over those
-that sustain his body, and the instinctive closing of one
-eyelid, in order that he may see objects single, is an
-effort of his weakened judgment to set things right once
-more.</p>
-
-<p class='c021'>While on this subject we may mention the experiment
-<span class='pageno' id='Page_33'>33</span>made by the famous English surgeon Cheselden upon a
-boy who was born blind, and upon whom he operated
-successfully.</p>
-
-<p class='c021'>This boy, who was thirteen years old at the time that
-Cheselden restored to him the sense of sight, was not
-born absolutely blind, his affliction having been caused
-by a cataract or film spread over the eyeball, which allowed
-him to distinguish night from day, or black from
-white or scarlet when placed in a very good light,
-although he was unable to perceive the form of things
-around him. At first Cheselden operated on a single
-eye, perfectly restoring its power; but so little idea of
-distance did the new sense convey to the boy’s mind
-that for a long time he imagined that everything touched
-his eyeball, just as those he felt touched his skin, and
-it was only by the sense of touch that he could persuade
-himself of the fallacy of his supposition. At first he
-had no perception of form whatever, and could only
-recognize objects he had already been familiar with
-after he had felt them all over. He was a long time,
-for instance, before he could distinguish between the
-dog and the cat without touching them, and was greatly
-surprised to find that the persons and things he had
-liked best when blind were not always the pleasantest
-to his newly acquired sense. His ideas of size, too,
-were all at fault, and he could not, for a long time, be
-made to understand how his father’s picture could be
-got into the back of his mother’s watch; even after he
-had possessed his sight for a comparatively long time,
-he could still only recognise people he had known during
-his blindness by touching their faces. Whenever he
-saw a new object he looked at it attentively for some
-time, in order, as it were, to learn its form by heart;
-but his memory was at first so overtaxed that he continually
-forgot his visual impressions, and mistook one
-thing for another. He was more than two months before
-<span class='pageno' id='Page_34'>34</span>he could appreciate form as depicted in a painting
-or drawing, having hitherto learned to consider pictures
-as flat objects. When, however, he began to understand
-the power of light and shade in producing the representations
-of solid objects, he was often extremely surprised
-to find the surface on which they were depicted
-quite flat when he touched it. The same thing frequently
-happens to ourselves, when looking at the photographs
-of bas-reliefs for instance. If these objects be well
-photographed, with the proper arrangement of light
-and shade, the illusion is so complete that the finger
-involuntarily touches the paper to feel if the surface is
-not really raised. In the Bourse at Paris there are
-some figures painted to represent bas-reliefs in so wonderful
-a manner, that numberless bets have been made,
-lost and won, over them. When feeling such representations
-of solid objects, the boy would often ask
-those around him which of his senses was deceiving him,
-his sight or his touch.</p>
-
-<p class='c021'>At first he saw everything of an enormous size, but
-as he saw things larger than those around him, he found
-the latter diminish. He also imagined that there was
-nothing beyond the room he was in, and could not be
-brought to comprehend how the house could be larger.
-When the sight of the second eye was restored to him a
-year afterwards, he at first saw every object of an enormous
-size, just as in the case of the first eye; but as
-he had now the perfectly educated organ to help him as
-well as his sense of touch, he soon began to see things
-under their natural appearances.</p>
-
-<p class='c021'>While he was in ignorance of what sight really meant,
-he was not particularly anxious to undergo the operation,
-saying that he did not think it possible to derive
-more pleasure from things that he liked than he did
-while he was blind. But now that his sight was restored
-he found every fresh object a new pleasure. When
-<span class='pageno' id='Page_35'>35</span>first he was shown the landscape from the top of a high
-hill, he was so delighted that he exclaimed that he had
-found another sense. When his second eye was operated
-upon, he saw things apparently twice as large with both
-eyes as with the one already restored to him. Even at
-first he seemed to have no difficulty in converging the
-eyes on any object.</p>
-
-<p class='c021'>These extracts from the history of Cheselden’s patient
-show us how utterly incapable the eye must be of
-rightly understanding the number, position, size, and
-form of objects without frequently correcting our impressions
-by the aid of the sense of touch.</p>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div class='chapter'>
-  <span class='pageno' id='Page_36'>36</span>
-  <h2 id='ch1-04' class='c012'>CHAPTER IV.<br /><span class='c020'>OPTICAL ILLUSIONS.</span></h2>
-</div>
-<p class='c014'><span class='sc'>Besides</span> the errors of sight already spoken of, there
-are other illusions, which are either common to all persons
-or confined to certain individuals, the knowledge
-of which will serve as a fitting prelude to the descriptions
-of those which are artificial.</p>
-
-<p class='c021'>The following defect, for instance, is one which is
-little known, but notwithstanding our ignorance of its
-existence it is nevertheless true that we all suffer from
-it. There is in every one’s eye a blind spot, totally incapable
-of experiencing the effects of the rays of light
-when they impinge upon it. For objects situated opposite
-to this particular spot we are as completely blind as
-if we had no eyes at all. To convince yourself of the
-truth of this assertion it is only necessary to try the
-following simple experiment.</p>
-
-<p class='c021'>Place upon a piece of white paper two small wafers,
-or two blots of ink about an inch and a half apart.
-Take the sheet in your right hand, and hold it up parallel
-to the lines of the eyes; shut the <i>left</i> eye, and fix
-the <i>right</i> eye on the centre of the <i>left</i> wafer or ink-spot.
-Move the sheet of paper steadily towards the eye, until
-it is about two inches and a half or three inches’ distance
-from it, and you will find that in a certain position
-the <i>other</i> wafer or ink-spot will disappear, although it
-is evidently still in the field of view. Having discovered
-<span class='pageno' id='Page_37'>37</span>this point which differs for different eyes, you will find
-that if you diminish or increase the distance of the
-paper you will once more see the missing object. The
-same thing happens if you move the eye from the centre
-of the wafer. The same experiment may be repeated
-with the left eye with a precisely similar result.</p>
-
-<p class='c021'>It has been found by experiment that this particular
-blind space exists exactly over the base of the optic
-nerve, at the spot where it joins the eye. (<a href='#i024'>Fig. 1</a>).
-Thus we see that the nerve which actually conveys the
-impression of sight to the brain is in itself incapable of
-being excited by light. In such cases as these Nature
-seems to laugh at us, and escapes from our grasp just
-as we are most confident in our power of wresting her
-secrets from her; indeed we may compare her to a wise
-and good-natured mother, who, though always amiable
-and willing to instruct those about her, sometimes
-smiles when her children fancy they are as learned as
-she is.</p>
-
-<p class='c021'>If we do not perceive the constant recurrence of the
-phenomenon just mentioned, it is because when both
-eyes are open the object whose image falls on the blind
-spot in one eye is seen by the other, the insensible
-portions of each eye being on opposite sides. Not only
-this: the spot being always situated on the outer and
-indistinct portion of the image reflected on the retina,
-we do not take notice of it; for as every one has no
-doubt observed, it is only the small portion of the
-object we are looking at exactly opposite the centre of
-the eye that is perfectly distinct and clear, all the rest
-being confused in its details, although quite visible.</p>
-
-<p class='c021'>Again, we may account for our not noticing it by the
-fact of our seeing clearly only those things which
-specially attract our attention—a fact first noticed by
-Mariotte. We see only what we wish to see with our
-physical eyes, as well as those of our mind. If our
-<span class='pageno' id='Page_38'>38</span>attention is attracted by a particular portion of a landscape,
-we see only that, and nothing else. If it is
-fixed on some subject that we are contemplating inwardly,
-we see nothing at all, although our eyes may
-not only be wide open, but absolutely fixed on some
-particular object. For instance, suppose a sportsman
-is out in the fields preceded by his dogs, Bran and
-Ponto. If he follows the movements of Bran with
-attention, he becomes the only object animate or inanimate,
-that depicts itself on his retina. Ponto may
-jump and caper in vain: he is lost to his master’s eye
-as much as if he were not there at all; his mind is
-entirely fixed on the beauty of Bran’s coat, on the fit
-of his collar, or fifty other things, and he sees nothing
-else. But let the sportsman begin to think of the
-number of birds he shot yesterday, or how he will find
-time to get up to the grouse in Scotland, or of that
-fine stag he missed when he was last amongst the
-heather, and dogs, cover, and landscape will fade from
-his sight as effectually as if he had been struck with
-blindness. Let him, however, strike his foot against a
-stump, or let the dogs suddenly begin to point, and he
-instantly receives back his sight, which but a few
-moments before he had lost to all intents and purposes.</p>
-
-<p class='c021'>The phenomena of <i>ocular spectra</i> and <i>complementary
-colours</i> experienced by every one forms a curious
-chapter in the history of those illusions which take
-their origin in the eye itself. Every one has noticed
-that after looking fixedly at a bright light or a striking
-colour for a few moments, the eye preserves an impression
-of the object for a certain time. A very light
-window looked at intently for several seconds will leave
-the impression of its cross-bars on the retina for several
-minutes, the colour of the image changing at every
-movement of the eye. The same effect may be observed
-<span class='pageno' id='Page_39'>39</span>when looking at the setting sun, or a flaring gas light.
-If the light at which we look is coloured, we shall see
-the complementary colour in the impression left on the
-retina. Sir David Brewster was one of the first to
-notice and experiment upon these very interesting
-facts.</p>
-
-<p class='c021'>If we cut out any simple figure, a small cross for instance,
-in scarlet paper, place it upon a white background
-and look at it fixedly for a minute or two, we
-shall find that its tint will gradually become duller.
-If we now suddenly look at a piece of white paper,
-we shall see the cross depicted upon it in green, which
-is the complementary colour to red. It should be explained,
-that the complementary of any colour is that
-which is necessary to make white light. Thus, blue,
-yellow, and red (as we shall find out when we come to
-speak of the prismatic spectrum), mixed in certain proportions,
-form white light; consequently the complementary
-of orange, which is composed of red and yellow,
-will be blue; of green, which is yellow and blue,
-red; of purple, which is blue and red, yellow, and <i>vice
-versâ</i>. The complementary of black is white, and of
-white, black as a rule; but if the white object be very
-brilliant, the black spectrum will speedily become coloured.
-The impression left by the setting sun is of this
-character. At first, while the eye is open, the image
-is black, then brownish red, with a light blue border;
-but if the eye be shut suddenly, it becomes green, with
-a red border, the brilliancy of colour being apparently
-in proportion to the strength of the impression. These
-spectra may be perceived for a long time, if the eye is
-gently rubbed with the finger now and then. Some
-eyes are more impressionable in this respect than
-others, and Beyle gives an instance of an individual
-who saw the spectrum of the sun for years, whenever
-he looked at a bright object. A modern instance of
-<span class='pageno' id='Page_40'>40</span>this occurred lately to an amateur astronomer who was
-looking at an eclipse of the sun. He unfortunately
-used a glass that was not sufficiently smoked, and the
-image of the sun’s disc, with the black space caused by
-the intervening moon, remained on his retina for months
-after. This gentleman’s case afforded an instance of
-the necessity of attention in order to see any object,
-for after the first few days he only became sensible of
-his unfortunate mishap when his attention was called to
-it by some accidental circumstance. These facts were
-so inexplicable to Locke, that he consulted Newton on
-the subject, and was surprised to learn that the great
-philosopher himself had suffered for several months
-from a sun-spectrum in the eye.</p>
-
-<p class='c021'>Without affirming that optical illusions are the cause
-of all the supposed supernatural appearances of which
-we have heard so much, there is no doubt that in many
-instances the eye plays an important part in deluding
-the brain. The following example, also cited by Beyle,
-will show this clearly. A horseman dressed in black,
-and riding a white horse, was trotting along a portion
-of the road, which through a sudden break in the
-clouds was brilliantly illuminated by the rays of the
-sun. The black figure of the man was projected
-against a white cloud, and the horse appeared doubly
-brilliant from being seen against the dark-coloured road.
-A person who was greatly interested in the arrival of
-the horseman was watching them with great attention,
-when suddenly the horse and his rider disappeared behind
-a wood. An instant after the observer was terrified
-at seeing a <i>white</i> cavalier on a <i>black</i> horse projected
-on a white cloud at which he was accidentally
-looking. It may be readily imagined that such an occurrence,
-followed up by a succession of unusual events,—such
-as illness, death, or any other series of misfortunes,—might
-even in the present day add a chapter to
-the history of the marvellous.</p>
-
-<p class='c021'><span class='pageno' id='Page_41'>41</span>To the illusions to which, like the preceding, we are
-all subject, may be added those resulting from some abnormal
-conformation, or some disease of the eye, in
-those who labour under them. An example of this occurs
-in the case of double or triple vision, many remarkable
-instances of which are mentioned by Müller,
-the celebrated physiologist.</p>
-
-<p class='c021'>Although, as before explained, the image of an object
-is depicted at the same time on both our eyes, still we
-only see one impression, in consequence of the two
-images being carried to the brain from corresponding
-portions of the retina. If this relation be disturbed by
-any cause, or if the eyes are not converged exactly
-upon the same point, a double image is the result.
-The first of these facts may be proved by looking at
-the moon, for instance, with the left eye shut; on
-suddenly opening it, two images will be seen for an
-instant. The second is instantly proved by pushing
-either of the eyes aside with the finger, when looking
-at any object.</p>
-
-<p class='c021'>It is necessary, however, to distinguish between these
-effects and true double vision, as well as a certain defect
-which exists in the eyes of many people, consisting in the
-apparent multiplication of distant objects by the same
-eye. In these cases, there is a superposition of images
-upon the retina, each having its proper bounds. With
-the majority of individuals afflicted in this way, it
-only happens when they look at a very distant object,
-the moon or stars for instance. There are many, however,
-who suffer from it in the case of everything they
-look at, whether far or near. Stephenson, who was affected
-with it, made it the subject of many interesting experiments.
-When he looked at a clear mark on a white
-ground, and gradually walked away from it, not only
-did the image become indistinct, but it seemed to unfold
-itself into several, independently of many others much
-<span class='pageno' id='Page_42'>42</span>more indistinct, more especially two situated on each
-side, whose distance increased the farther he walked
-away. As these latter images became more and more
-separated, they also became more confused. The image
-seen by the right eye was a little higher than that
-seen by the left. Griffin states, that after having used
-the telescope for any length of time, the eye that he
-kept shut always saw objects triple and double for
-some hours afterwards. These phenomena are possibly
-connected in some way with the disposition of the
-plates and fibres of which the crystalline lens of the
-eye is composed.</p>
-
-<p class='c021'>Semi-vision, or <i>hemiopia</i> as it is called, is much more
-rare and more difficult to explain than the phenomena
-of double vision; and consists in the power of being
-able to see only the right or left half of the object
-looked at, the separation being vertical when the eyes
-of the observer are in the same horizontal line. Thus,
-in looking at the word <span class='sc'>Newton</span>, the person so afflicted
-would only see either the letters <span class='sc'>New</span> or <span class='fss'>TON</span> according
-to which half of the eyes were defective.</p>
-
-<p class='c021'>Wollaston was afflicted with hemiopia on two different
-occasions; the first time after violent exercise,
-during two or three hours, when he could see distinctly
-only the left-hand halves of the objects he was looking
-at. Both eyes were similarly affected, and the phenomenon
-only lasted about a quarter of an hour. Twenty
-years afterwards he suffered again from the same
-accident, but on this occasion in the contrary manner;
-that is to say, he only saw the right halves of the
-objects he was looking at—to use his own words, he
-could only see the right half of every friend he met.
-At certain distances from the eye, one of two persons
-would become invisible, and by simply changing
-his own position or that of the persons he was near,
-he could make one or other of them, or indeed both,
-<span class='pageno' id='Page_43'>43</span>disappear at will. It must be acknowledged that
-similar tricks of Dame Nature, due to an unconscious
-insensibility of the eye, are most singular, and at first
-sight appear to have a supernatural origin.</p>
-
-<p class='c021'>Bartholin mentions the case of a hysterical woman
-who was afflicted with hemiopia horizontally, and saw
-all natural objects cut in two, the lower halves being
-invisible. In this instance it was only the left eye that
-was defective.</p>
-
-<p class='c021'>Another interesting example of optical illusion is the
-luminous sensation produced internally when the eye,
-or the neighbouring parts, are struck or stimulated by
-friction or electricity. These appearances are experienced
-even by those who have lost their sight. Müller
-states that a case was submitted to a legal tribunal to
-decide whether the luminous sensations which are perceptible
-when we rub our eyes are really light. The
-matter in dispute was whether a man who was attacked
-by robbers in the dark, could see and recognise them by
-means of the light produced in his eyes by a violent blow
-on the head; but he does not tell us how the question
-was decided. With regard to internal causes, Humboldt
-tells us that a man whose eye had been extirpated,
-was sensible of luminous appearances whenever he was
-galvanized. Lincke states that a man whose eye had
-been removed by a surgical operation, saw next day all
-kinds of luminous phenomena, which tormented him cruelly
-with the idea that after all his eye had been saved.
-When he shut the perfect eye, he fancied he saw with
-the missing eye circles of fire, persons dancing, and similar
-appearances for several days. These facts are
-analogous to those told of persons who have had their
-legs and arms amputated, but who, notwithstanding,
-apparently feel pain in their lost limbs.</p>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div class='chapter'>
-  <span class='pageno' id='Page_44'>44</span>
-  <h2 id='ch1-05' class='c012'>CHAPTER V.<br /><span class='c020'>THE APPRECIATION OF COLOUR.</span></h2>
-</div>
-<p class='c014'><span class='sc'>Most</span> people understand each other sufficiently to
-agree in their ideas about various colours. Thus every
-one agrees in saying that poppies are red, that the sky
-is blue, and the leaves green; but if any one were to
-assert that the sky was red, that the leaves were blue,
-and poppies green, who could possibly contradict him?</p>
-
-<p class='c021'>This statement may appear a paradox, and an absurdity
-to many of our readers, but it is really a problem
-that has engaged the attention of many of our greatest
-philosophers. Who can prove that what I see as yellow
-may not appear blue to you, or that what you see
-red is not green to me? You would possibly explain
-the doubt by saying that because we both agree in calling
-a buttercup yellow, that we see the same colour. I
-call a buttercup yellow, because I have learnt since my
-childhood to give this name to the particular sensation
-I experience when I look at one of these flowers; but
-that is no proof that the sensation I feel is similar to that
-felt by everybody else, and it is not merely possible, but
-probable, that our personal sensations of colour are essentially
-different, although the arbitrary words we use
-to designate them are the same.</p>
-
-<p class='c021'>It may be remarked in parenthesis, that colour is not
-an entity, but is simply the effect of certain properties
-of surface or interior structure possessed by every substance
-<span class='pageno' id='Page_45'>45</span>with which we are acquainted. The old saying,
-that “all cats are black in the dark,” is really a profound
-philosophical truth, which is not only true of cats
-but of the reddest rose that ever grew in a garden, the
-bluest violet that ever was plucked, the prettiest girl
-that ever was kissed under the mistletoe. It is a sad
-thing to think of, that when we put the candle out, and
-step into bed, we become blacker than the blackest negro
-that was ever emancipated. But without light
-there can be no colour, for there is no material, so to
-speak, from which to manufacture it. White light, as
-we have said before, is made up of red, blue, and yellow,
-and it is by the absorption of one or all of these
-that all tints are formed. The surface of a poppy leaf
-has the power of absorbing all the blue and a little of
-the yellow, reflecting the whole of the red and the remainder
-of the yellow, the mixture of the two forming
-scarlet. The surface of a marigold acts differently; all
-the blue is absorbed, as in the case of the poppy, and a
-good deal of the red with it, leaving just a little to
-brighten up the yellow which is reflected with it. Some
-substances, white marble for instance, have no power
-of splitting the light into colours, absorbing some and
-reflecting others, but reflect the whole of it in its integrity.
-Others again, like black velvet, absorb nearly
-the whole, just reflecting sufficient to enable us to see
-its surface.</p>
-
-<p class='c021'>We began this chapter by speculating on the probability
-of our seeing different colours to our neighbours,
-and we shall now proceed to show that our speculations
-in that direction are not so absurd as they appear to be
-at first sight.</p>
-
-<p class='c021'>The phenomenon of colour-blindness, or the insensibility
-of the eye to certain colours, has been for many
-years past a puzzle both to the physiologist and the
-philosopher. Perhaps the most remarkable case of the
-<span class='pageno' id='Page_46'>46</span>sort is that mentioned first by Huddart, and quoted by
-Sir David Brewster, of a shoemaker named Harris,
-living at Maryport, in Cumberland, who was utterly
-incapable of distinguishing any colour at all, and saw
-everything white, grey or black. The first time that
-Harris noticed this defect, was when he was about four
-years old; having found the stocking of a playmate in
-the street, he returned it to him at his cottage, and
-noticed that every one said it was a red stocking, but he
-could not understand why they should call this particular
-stocking red, as it seemed to him to be like every
-other. This circumstance remained in his mind, and a
-few more similar observations confirmed his suspicions
-that he had some defect of sight that prevented him
-from seeing as others did. He also observed that other
-children pretended to distinguish cherries from their
-leaves by what they called their colour, whilst he could
-see no difference between them, except those of shape
-and size. He also noticed that by means of the difference
-of colour, others could distinguish cherries on a tree
-at a much greater distance than he could; whilst he, on
-the contrary, could see other things at greater distances
-than his companions. Harris had two brothers, whose
-eyes were similarly defective; one of these, that Huddart
-examined, mistook green for yellow constantly, and
-orange for light green.</p>
-
-<p class='c021'>In the <i>Philosophical Transactions</i> Scott describes a
-similar defect in his own powers of vision. He states
-that he was unable to distinguish green, and that the
-colours known as crimson and pale blue presented no
-difference of hue. He further confesses his inability to
-see any difference between bright green and bright red,
-although he could distinguish between red and yellow,
-dark blue, and almost every shade of blue, except sky-blue.
-He goes on to relate how he married his daughter
-to a worthy young man of his acquaintance, and that
-<span class='pageno' id='Page_47'>47</span>the day before the wedding the bridegroom came to his
-house in a full suit of black, as he thought. He was
-greatly displeased to see him appear in mourning on
-such an occasion, and took an opportunity to remonstrate
-with him on the subject. But what was his surprise to
-hear his daughter exclaim, in loud tones of counter remonstrance,
-that she had rarely seen her lover in a coat
-of such a pretty colour, and that her father’s eyes must
-deceive him on this as on many other occasions. Scott’s
-father, his maternal uncle, one of his sisters, and two of
-his sons had the same defect of sight. Dr. Mitchell
-mentions the case of a naval officer who for his ordinary
-uniform chose a blue coat and waistcoat and red trousers,
-fully believing that they were all of the same colour. A
-tailor of Plymouth, also mentioned by Dr. Mitchell,
-mended a black silk waistcoat with a piece of crimson,
-and another put a red cloth collar to a blue coat. Several
-celebrated men have suffered from colour-blindness.
-Amongst them may be mentioned Dugald Stewart, the
-great philosophical writer; John Dalton, the originator
-of the atomic theory; and Troughton, the philosophical
-instrument maker. Dugald Stewart first discovered the
-defect on hearing a member of his family admire the
-contrast of colour between the leaves and fruit of a Siberian
-crab-tree, while he could see no difference between
-them, except in point of form and size. John Dalton
-could not distinguish blue from crimson, and he could
-only see two colours, blue and yellow, in the prismatic
-spectrum. Troughton could see no difference between
-dark crimson, bright orange, and yellow—in fact, he
-could only distinguish blue from yellow.</p>
-
-<p class='c021'>In an article on this subject, published in the <i>Magasin
-Pittoresque</i> for 1846, a Swiss physician gives some
-interesting examples, which are worth repeating. In
-the solar spectrum obtained by passing a ray of light
-through a triangular prism, and which is composed of
-<span class='pageno' id='Page_48'>48</span>the following colours,—red, orange, yellow, green, blue,
-indigo, and violet,—Dalton could only see yellow, blue,
-and violet. Rose-colour by day appeared to him a pale
-blue, but at night it seemed to take an orange hue. By
-day crimson seemed to be dirty blue, and red cloth dark
-blue. Dr. Whewell having asked him one day to describe
-the colour of the doctor’s scarlet gown, Dalton pointed
-to the trees around them, and declared he could distinguish
-no difference in their colour; and one day having
-dropped a stick of red sealing-wax in the grass, he had
-the greatest difficulty in finding it again. Since Dalton’s
-time over five hundred distinctly marked instances
-of this imperfection have been noticed, and Professor
-Prévost, of Geneva, has named it <i>Daltonism</i>, an extremely
-unphilosophical piece of pathological nomenclature,
-which has unfortunately received the sanction
-of too many great physiologists to be abolished. Blindness
-might just as well be called <i>Homerism</i> or <i>Miltonism</i>.</p>
-
-<p class='c021'>Colour-blindness is much more frequent than is generally
-supposed, for those who are afflicted with it are
-mostly ignorant of the defect, and frequently practise
-trades or professions in which perfect sensibility to the
-different hues of colour is quite indispensable. An instance
-of this occurred some time since in the case of
-an engine-driver, who allowed his engine to run into a
-luggage train, through not noticing the red danger signal.
-At his examination it was proved that he was
-colour-blind, and could not distinguish red from green.
-Partial colour-blindness is, no doubt, the cause of the
-frequent disputes that we hear about the tints of certain
-objects; to say nothing of the glaring instances of
-bad taste in the arrangement of colour that are now-a-days
-so common. Out of forty boys at a school at Berlin
-who were examined by Leebech, he found five who
-were quite confused in their notions of colour, and
-<span class='pageno' id='Page_49'>49</span>could not distinguish between ordinary shades of the
-same hue. This affliction is in many cases hereditary,
-descending from father to son. It is singular that instances
-of colour-blindness are much more common
-amongst men than amongst women, for out of over five
-hundred cases there were only four in which females
-were the sufferers. It seems also that persons with
-grey eyes are more frequently colour-blind than those
-whose eyes are blue or brown. To the list of great
-men who were colour-blind, we must not forget to add
-the celebrated Italian historian, Sismondi.</p>
-
-<p class='c021'>Physiologists consider that there are two kinds of
-colour-blindness,—one where only two colours are seen,
-the other where more than two are perceptible. Daubeny
-Turberville, an oculist of Salisbury, mentions a
-case of the former, in which a young girl, like the Maryport
-shoemaker mentioned by Brewster, could only distinguish
-between black and white, everything between
-the two being of different shades of grey. This girl, singularly
-enough, could see to read in twilight a quarter
-of an hour after her companions. This sharpness of sight
-appears to be not at all uncommon amongst those who
-are colour-blind. Spurzheim mentions the ease of a
-whole family who were afflicted in the same way as
-Turberville’s patient. All the male members of Troughton’s
-family were equally incapable of distinguishing any
-colours but blue and yellow.</p>
-
-<p class='c021'>The cases of colour-blindness where more than two
-colours are distinguishable, are much more common.
-Goethe, the great German poet, who dabbled a great
-deal in optics, knew two young men who, although they
-possessed powerful sight, and could distinguish between
-white, black, grey, yellow, and orange, were at a loss
-when the shades between dark red and rose-colour were
-in question. A piece of dried carmine appeared bright
-red to them, and a faint carmine hue on a white shell,
-<span class='pageno' id='Page_50'>50</span>and a rose-leaf, light blue; the leaves of trees and grass
-appeared yellow, and they confounded rose-colour, blue,
-and violet together. Goethe supposed them to be incapable
-of perceiving blue and its several hues, and called
-their defect by a high sounding Greek name, <i>akyanoblepsy</i>,
-or blue-blindness. Péclet mentions two other
-persons, also brothers, who likewise were incapable of
-distinguishing between blue, violet, and rose-colour.
-Like Professor Whewell, they confounded the dull scarlet
-of the trousers of the French infantry with the
-leaves of the trees. Yellow appeared to them more brilliant
-than any other colour. Doctor Sommer and his
-brother could not distinguish between red and its derivatives
-and other colours; they could only distinguish
-between yellow, blue, white, and black. Doctor Nicholl
-mentions a child that could only see red, yellow,
-and blue, in the spectrum. It could distinguish green,
-but called it brown when it was dark, and pink when it
-was pale. The same physiologist knew a man who
-called red green, and brown dark green. A young lady
-who was an amateur artist, could not perceive a piece of
-scarlet cloth hanging on a hedge that was close to her,
-although others could see it plainly half a mile off. One
-day she gathered, as a great curiosity, a lichen which
-she supposed to be of a bright scarlet hue, but which
-was in reality of a beautiful green. Another time she
-could see no difference between carmine and prussian
-blue. A gardener living at Clydesdale, who began life
-as a weaver, was compelled to give up his first trade because
-in daylight he confounded all light colours; yellow
-and its varieties he could distinguish perfectly, but
-he was incapable of seeing any difference between red,
-blue, pink, brown, and white. Another man, who was a
-silk-weaver, had to change his trade, because he could
-not distinguish between red, pink, and sky blue. A
-Genevese artist whom circumstances compelled to paint
-<span class='pageno' id='Page_51'>51</span>a portrait by candle-light, used yellow for pink in laying
-on his flesh tints, with a pleasing result that may be
-readily imagined. In fact, the instances of colour-blindness
-mentioned by physiological writers are almost
-innumerable, and I should only weary my readers if I
-related all the authentic cases of this singular affliction.
-One instance, however, which was very carefully observed
-by Wartmann, a distinguished German oculist,
-merits our attention. The afflicted person, whom Wartmann
-speaks of as D., was thirty-three years old. Those
-of his brothers and sisters whose hair was fair suffered
-from the same infirmity, but those whose hair was dark
-were exempt from it. Like so many others who are colour-blind,
-he could not distinguish between cherries and
-their leaves, and confounded a sea-green piece of paper
-with a scarlet ribbon placed near it. A rose of the
-ordinary hue appeared greenish-blue. Being anxious to
-see if reflected, refracted, and polarized light exercised
-a different action on his retina, Wartmann tried him first
-with the prismatic spectrum, but he could only distinguish
-four colours,—blue, green, yellow, and red. He
-could distinguish perfectly the peculiar black lines seen
-crossing the spectrum in certain places, and known by
-the name of Fraunhofer’s lines. He then placed in his
-hands thirty-seven pieces of differently coloured glass,
-but he could only distinguish four varieties. The colours
-produced by polarized light seemed to give the patient
-quite as much trouble as those produced in the ordinary
-way. Chocolate brown appeared reddish brown; purple,
-dark blue; and violet, a dirty blue. When colours were
-illuminated by sunlight, they seemed to him to be redder
-than usual, even green and blue appearing red.</p>
-
-<p class='c021'>In considering cases of colour-blindness, it is very
-difficult not to be misled into using wrong terms, as applied
-to colour, for we have no possible means of knowing
-what colour it is that is really seen by the patient.
-<span class='pageno' id='Page_52'>52</span>Thus, for instance, Dr. Whewell could not distinguish
-between red and green. But what colour did he really
-see? Did he see the leaves and cherries both red or
-both green, or was it some colour between the two that
-was impressed upon his retina? Again, great care
-must be exercised in placing implicit reliance on the
-statement of persons who are colour-blind, for we must
-recollect that their only means of conveying the results
-of what they experience is by the use of an organ that
-is confessedly defective, and which is quite likely to
-deceive them, and us too, without their being parties to
-the deception.</p>
-
-<p class='c021'>The cause of colour-blindness is completely unknown;
-philosophers and physiologists are still in the realms of
-hypothesis concerning this peculiar optical defect. As
-yet, the most careful observation has failed to detect
-any difference between the eyes of those who are colour-blind,
-and the eyes of ordinary persons, that could in
-any way account for this singular affection of the sense
-of sight.</p>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div class='chapter'>
-  <span class='pageno' id='Page_53'>53</span>
-  <h2 id='ch1-06' class='c012'>CHAPTER VI.<br /><span class='c020'>ILLUSIONS CAUSED BY LIGHT ITSELF.</span></h2>
-</div>
-<p class='c014'><span class='sc'>When</span> playing about the Christmas fire, children
-frequently amuse themselves by whirling round and
-round a piece of wood, one end of which they have
-previously lighted and blown out. In proportion as
-the movement becomes more rapid, the path of the red-hot
-end becomes more and more connected, until at last
-a burning ring is formed, in every part of which the
-shining charcoal appears to be at the same time. The
-only way of accounting for this illusion is by supposing
-that the image formed by the burning stick upon the
-retina remains there for an appreciable period, the impression
-made by it at one part of its journey remaining
-until it returns to its former position. The power
-possessed by the retina of retaining impressions
-explains a large number of illusions of the same kind.
-The chord of a musical instrument, for instance, when
-struck, appears to occupy a longer space during the
-time it vibrates, than when it is at rest. A rapidly revolving
-wheel appears almost solid on account of the
-combined images of the spokes seeming to unite into
-one homogeneous mass.</p>
-
-<p class='c021'>The persistence of luminous impressions upon the
-retina has given rise to the invention of a number of
-well-known optical toys, amongst which may be
-mentioned the <i>phenakistiscope</i>, the <i>thaumatrope</i>, the
-<i>phantascope</i>, and many others.</p>
-
-<div id='i054'  class='figleft id007'>
-<span class='pageno' id='Page_54'>54</span>
-<img src='images/i054.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 3.</span>—The Phenakistiscope.</p>
-</div>
-</div>
-
-<p class='c021'>The phenakistiscope may be described (<a href='#i054'>figs. 3</a> and <a href='#i055'>4</a>)
-as consisting of an iron pin <i>a b</i> turning easily on its
-axis, and passing through two holes in a brass rod <i>t g</i>,
-bent twice at right angles. Attached
-to one end of the pin is a disc of cardboard,
-divided into several equal sectors,
-and pierced near its circumference
-with as many similar sized rectangular
-holes (<a href='#i055'>fig. 4</a>). In each sector the same
-scene is represented, with this difference,
-that the movements of the objects
-are so arranged as to be progressive
-from one extreme to the other. The
-disc being fastened to the pin <i>a b</i> (<a href='#i054'>fig. 3</a>)
-by the screw <i>v</i>, with the figures facing
-outwards towards <i>a</i>, the whole apparatus
-is held before a looking-glass
-by the handle <i>m</i>. If the disc be now
-rotated by the button <i>b</i>, and the eye
-placed opposite one of the square holes
-in the card, the figures on the disc will appear to move
-more or less quickly according to the rate at which it is
-rotated. The three bricklayers in <a href='#i055'>fig. 4</a> will be seen
-to pass their bricks from one to the other with perfect
-regularity if the drawing has been made carefully.
-Numberless other designs may be made for this little
-instrument, such as a windmill in full sail, a man working
-a pump, a conjurer swallowing knives—in fact, any
-scene with objects in motion may be drawn, and will
-cause infinite amusement for the long winter evenings.</p>
-
-<p class='c021'>The time during which the impression of any object
-remains upon the retina appears to be in direct proportion
-to its brilliancy. For a burning coal it is stated
-to be about the tenth of a second; consequently, if the
-stick mentioned at the beginning of the chapter is
-rotated ten times in a second, a continuous luminous
-<span class='pageno' id='Page_55'>55</span>ring will appear to be formed. That the time necessary
-for producing a distinct impression varies with the
-brilliancy of the object, may be readily guessed from
-the fact that an electric spark is perfectly visible,
-although its duration can hardly be measured, while a
-cannon-ball in flight is only perceptible to the practised
-eye of the artilleryman, owing to its reflecting only a
-small quantity of very diffused light.</p>
-
-<div id='i055'  class='figcenter id008'>
-<img src='images/i055.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 4.</span>—Disc of the Phenakistiscope.</p>
-</div>
-</div>
-
-<p class='c021'>The second instrument, the thaumatrope, is constructed
-on the same principle. It consists of a certain
-number of circular discs of card three or four inches
-across, which are capable of being turned on their axes
-<span class='pageno' id='Page_56'>56</span>with great rapidity by means of the finger and thumb and
-a couple of silk threads fixed at opposite sides of their
-circumference. On each of these discs a design is
-painted, one-half appearing on one side, and the other
-half on the other, in such a manner that the two parts
-form a single picture. You may have, for instance,
-Harlequin on one side and Columbine on the other, but
-on turning the card you will see them together. The
-body of a Turk may be drawn on one side and his head
-on the other, and, by rotating the card, the head suddenly
-finds a pair of shoulders to fit it. A sentence
-may be divided in the same way, or the words, or even
-the letters, may be divided between the opposite sides
-of the card: in fact, like the phenakistiscope, the designs
-applicable to this little instrument are endless.</p>
-
-<p class='c021'>The third of these instruments, the phantascope, is
-constructed in accordance with the peculiar power possessed
-by the eyes of adapting themselves to the distance
-of the objects they are looking at. Everybody
-must have noticed that in order to see objects plainly
-that are placed at different distances we insensibly alter
-the position and focus of the eyes, and that, consequently,
-objects even in the same plane as those we are
-looking at are not perceived by us until something calls
-our attention to them, and causes us to alter the position
-and focus of our eyes and fix our gaze on them.
-For instance, in looking at a canary in a cage, we have
-but a confused idea of the wires, which we will suppose
-to be midway between the bird and the observer. But
-if anything attracts our attention to the wires we lose
-sight of the bird, or at any rate see it only as a confused
-mass. If this experiment is made with care, it
-will be perceived that the object seen confusedly is
-always double,—a fact that may be verified by interposing
-the finger between the eyes and any object.
-When we look at the finger, the distant object will seem
-<span class='pageno' id='Page_57'>57</span>to be doubled; if we look at the object, it is the finger
-that undergoes duplication.</p>
-
-<p class='c021'>We know by experience that when we look at an object
-and press one of the eyeballs slightly with the
-finger, the image of it becomes doubled. The explanation
-of this phenomenon is not very easy, but it is generally
-supposed that in the case of ordinary vision the
-two eyes produce the sensation of a single image in
-consequence of the two impressions being formed at
-corresponding parts of each retina, and that habit
-causes us to see only a single object in such a case.
-But when the eyes are so disposed as to be capable
-of seeing distant objects distinctly, the two images
-formed by a near object are no longer found in the corresponding
-portions of each retina, and so produce the
-sensation of double vision. The same thing happens
-when either of the eyes is momentarily displaced.</p>
-
-<p class='c021'>These phenomena have given rise to the construction
-of a very simple instrument, the phantascope, with
-which many interesting experiments may be performed,
-and which was invented some years since by Dr. Lake,
-an eminent physician of New York.</p>
-
-<p class='c021'>In the middle of one of the edges of a thin piece of
-wood, say six inches or a foot in length, which serves
-as a base for the instrument, is fixed a rod fourteen or
-sixteen inches long, upon which slide a couple of ferules
-capable of being fixed at any height by means of
-thumb-screws. Each of these ferules holds a piece of
-cardboard five or six inches long, and of any convenient
-breadth, in a horizontal position. The upper card is
-pierced in a longitudinal direction with a slit rather less
-than a quarter of an inch broad, and about three inches
-long; that is to say, a little wider than the distance between
-the centres of two eyes. The second card has a
-similar slit of the same length, and corresponding vertically
-with the one above it; the width, however, in
-<span class='pageno' id='Page_58'>58</span>this instance being only about the eighth of an inch. In
-addition, the lower card should be marked with a fine
-line drawn across the centre, which we shall call the index.</p>
-
-<p class='c021'>Things being thus arranged, if we place two similar
-objects—two A’s, for instance—upon the wooden stage
-of the instrument, about three inches apart, and look
-at them through the two slits, we shall see them as under
-ordinary circumstances; but on fixing our eyes intently
-on the index of the lower card, and gradually
-raising it, we shall see the two A’s become double, the
-two images of each letter separating themselves more
-and more the nearer the lower card approaches the upper
-one, until the last two of the images will coalesce,
-and appear to be placed on the lower cardboard, the
-other two remaining in their proper place. The eyes
-must be kept firmly fixed upon the index, otherwise the
-illusion disappears immediately, and two A’s only are
-seen in their true position on the base of the instrument.
-This is an instance of the production of an image in a
-place where it certainly does not exist. This illusion
-is seen best when the upper screen is about ten inches
-from the object, the lower screen being just half-way
-between; but, as in most of these cases, the distances
-will differ according to the focus of the observer’s eyes.
-The proper distances once being found, the experiment
-may be varied in a hundred different ways. For example,
-instead of two letters and a line we may have
-two flowers on the stage, and the figure of a flower-pot
-on the intermediate screen. If the two flowers are
-painted different colours, they will unite and form a
-mixed tint. Thus a red and yellow flower will give an
-orange image, a blue and yellow a green image, and so
-on. A perpendicular stroke and a horizontal one will
-give a cross. A few experiments with this little instrument
-will throw a light upon many of the obscurer
-<span class='pageno' id='Page_59'>59</span>points that exist amongst the phenomena of vision, and
-will show conclusively that the two eyes rarely see in
-the same manner, and that it is sometimes one, and
-sometimes the other, that sees most distinctly. A
-couple of pieces of cardboard, pierced with suitable
-slits and held in the hand may be substituted for the
-apparatus above described, but of course they will be
-much more difficult to use, and will give less satisfactory
-results.</p>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div class='chapter'>
-  <span class='pageno' id='Page_60'>60</span>
-  <h2 id='ch1-07' class='c012'>CHAPTER VII.<br /><span class='c020'>THE INFLUENCE OF THE IMAGINATION.</span></h2>
-</div>
-<p class='c014'><span class='sc'>The</span> above facts show plainly that optical illusions
-find their source in the very mechanism of the organs
-of sight, and that without going farther than the eye
-itself we may discover numberless examples of these
-phenomena. We shall presently bring before our readers
-the innumerable means devised by art for deceiving
-the sense of sight and impressing us with sensations that
-are purely imaginary. But before describing these numerous
-pieces of apparatus we must still remain for a
-short time within the domain of man’s faculties, and describe
-some of the illusions that we are subjected to by
-those powers of the imagination that are supposed to
-hold in check the five senses of the body. Our imagination,
-however, plays us as many tricks as our eyes,
-and, like them, is alternately false and true. Touch,
-taste, smell, hearing, and sight, are all supposed to be
-under its powerful influence for good or evil; but they
-are all deceived by it in turn, more especially the sense
-of sight, which we generally boast of as being the most
-trustworthy of them all. Were we to describe all the
-labyrinths into which our imagination is continually
-leading us, we might easily extend this little volume to
-one of treble the size. But our purpose is not so much
-to write a history of all the hallucinations to which the
-imagination is subject, but to cull from those already
-<span class='pageno' id='Page_61'>61</span>existing the most interesting instances in which this
-great faculty is alternately the victim and the tyrant of
-the sense of sight.</p>
-
-<p class='c021'>Amongst many works on this subject we may cite that
-of Brière de Boismont on “Hallucinations, Apparitions,
-Visions, &amp;c.,” from which we shall draw largely in the
-following pages. The examples we shall give will be
-those only in which the victims of the hallucination were
-in the full enjoyment of their mental faculties, and could
-healthily analyze the sensations and impressions to which
-they were subjected.</p>
-
-<p class='c021'>One of the first of these bears upon those diseases of
-the eye to which allusion was made in <a href='#ch1-04'>Chapter IV</a>.
-Towards the end of 1833, a poor washerwoman who was
-tormented grievously with rheumatic pains gave up her
-business, and took to sewing for her livelihood. Being
-but little accustomed to this kind of work, she was compelled
-to sit over her needle late at night in order to save
-herself from starving. The unwonted strain upon the
-eyes soon brought on ophthalmia, which speedily became
-chronic. Nevertheless, she continued her work, and
-fell a prey to <i>diplopia</i>, or double sight in each eye.
-Instead of a single needle and thread, she saw four continually
-at work, everything else about her being similarly
-multiplied. At first she took no notice of the singular
-illusion, but at last both imagination and sight
-joined arms against the judgment, and the poor creature
-imagined that Providence had taken pity on her
-forlorn condition, and had worked a miracle in her
-favour by bestowing on her four pair of hands in
-order that she might do four times her usual amount of
-work.</p>
-
-<p class='c021'>The following is another instance of the passage of
-illusion into hallucination. A man fifty-two years old,
-of a plethoric constitution, after having suffered from a
-defect in his visual functions that caused him to see objects
-<span class='pageno' id='Page_62'>62</span>sometimes double, and at others upside down, suddenly
-showed signs of cerebral congestion, and threatened
-apoplexy. By proper treatment, however, he was
-saved for a time from the latter catastrophe, but he became
-permanently afflicted with strabismus, or squinting,
-and he suffered from a singular hallucination. His
-eyelids would contract, and his eyeballs would roll from
-side to side at more or less distant intervals. On these
-occasions he imagined he saw the figures of different
-persons that he knew moving about, and would even
-follow them outside his door into the other rooms of the
-house. He was perfectly aware that these appearances
-were merely the effect of the imagination, but this did
-not in any way detract from their appearance of reality.
-The man afterwards died from an attack of apoplexy.</p>
-
-<p class='c021'>The following examples are also cases of singular
-optical deception, some of them being so extraordinary
-as to trench upon the supernatural, and in the days of
-ignorance would have given those who were their victims
-the character of unearthly personages.</p>
-
-<p class='c021'>A certain English painter, who in some sort inherited
-the palette of Sir Joshua Reynolds, and believed
-himself superior in many respects to the great master,
-used to boast that in one year he painted over three
-hundred portraits, large and small. This fact seemed
-to Wigan a physical impossibility, and he questioned
-him closely as to the secret of his astonishing rapidity
-of execution, for he never required more than one sitting
-from his patrons. Wigan states that he saw him
-paint a miniature of a well-known personage in eight
-hours, which was incomparable in its fidelity to nature
-and finished execution. Wigan asked him to give him
-some details of the method he adopted, and he gave him
-the following answer: “When a sitter presents himself,
-I look at him attentively for half an hour, sketching
-the outlines of his features on my canvass during the
-<span class='pageno' id='Page_63'>63</span>time. I have no occasion for a longer sitting, and I
-pass on to some one else. When I wish to continue
-the first portrait, I take the sitter in my imagination,
-and I seat him in the chair, where I see him as distinctly
-as if he were really there, and I can even
-heighten a tint, or soften down a clumsy form at will,
-without altering the likeness. I look from time to time
-at the imaginary figure, and I go on painting. I stop
-now and then to examine his position, absolutely as if
-the original were before me; for every time I look
-towards the chair I see the sitter. This method of proceeding
-has rendered me very popular; and as I have
-always succeeded in catching the likeness of my patrons,
-they have been simply enchanted at my sparing
-them the tedious sittings exacted by other painters.
-Little by little I have begun to lose the distinction between
-the real and imaginary sitter, and I have often
-maintained stoutly that my patrons had already sat to
-me on the previous day. At last I became convinced
-that it was the real sitters that I saw, and thenceforth
-all became confusion. I suppose my friends took alarm
-at my hallucinations, for I remember nothing of what
-happened during the thirty years that I remained in the
-madhouse. This long period has left no trace on my
-memory, except indeed the last six months of my confinement.
-It seems to me, however, that when my
-friends talk of having visited me I have some vague recollection
-of the fact; but it is a subject that I do not
-care to pursue.”</p>
-
-<p class='c021'>The most remarkable feature of the case is, that this
-artist after a lapse of thirty years resumed his pencil,
-and painted almost as well as when he was forced by
-madness to abandon his art.</p>
-
-<p class='c021'>This faculty of being able to evoke shadows, with
-which to people one’s solitude, may be carried so far as
-to transform real persons into phantoms. Hyacinth
-<span class='pageno' id='Page_64'>64</span>Langlois, a distinguished artist, living at Rouen, tells
-us that Talma, with whom he was extremely intimate,
-confided to him that, whenever he went upon the stage,
-he had the power, by mere force of will, to cause the
-clothes and flesh of his numerous auditory to disappear,
-and become transformed from living beings into so many
-skeletons. When his imagination had peopled the house
-with these singular phantoms, the emotion he felt was
-so great that it gave his dramatic powers still greater
-force, and enabled him to produce the wonderful effects
-that have made his name so famous.</p>
-
-<p class='c021'>Wigan says, that he once knew a most intelligent and
-amiable man, who could at will evoke his own image.
-He often laughed at seeing his second self standing before
-him, the phantom appearing to laugh as heartily as
-himself. This illusion was for a long time a matter of
-amusement to him, but at last he became persuaded
-that he was haunted by his own double. His second
-self appeared to hold arguments with him continually,
-and beat him frequently on various points of dispute, a
-matter which mortified him excessively, as he was rather
-proud of his powers of reasoning. This gentleman,
-although always considered as being somewhat eccentric,
-was never put under the slightest restraint, and at
-last the creature of his imagination so tormented him,
-that he resolved not to live through another year. He
-consequently paid all his debts, arranged his affairs, and
-waited pistol in hand until the clock struck twelve on
-the 31st of December, and then deliberately blew out
-his brains.</p>
-
-<p class='c021'>In <i>Abercromby on the Mind</i> we read an account of
-the observations made by a gentleman who was the victim
-of illusions during the whole of a pretty long life.
-If he met a friend in the street, he was unable to tell
-at first whether he saw a real human being or only a
-phantom. By close examination he could detect a difference
-<span class='pageno' id='Page_65'>65</span>between the real person and the creature of his
-imagination, the features of the former being sharper
-and more defined than those of the phantom; but in
-general he was obliged to test the reality of the figure
-he saw by the senses of touch and hearing. He was
-able, by concentrating his thoughts upon the appearance
-of any friend, to call up his image; a power which
-extended even to scenes that he had witnessed. Although
-he could produce these hallucinations at will, he was
-powerless in making them disappear; and when once
-he succeeded in calling forth these creatures of his imagination,
-he never could tell how long the delusion
-would last. This gentleman was in the prime of life, a
-good man of business, and otherwise in a perfect state
-of mental and bodily health. A member of his family
-possessed the same faculty, but in a minor degree.</p>
-
-<p class='c021'>In 1806, General Rapp, when returning from the
-siege of Dantzic, having occasion to speak to the Emperor
-Napoleon, walked into his private room without
-being announced, and found him in such a profound
-state of abstraction, that he remained for some time
-unperceived by his imperial master. The General, seeing
-him thus perfectly motionless, fancied he must be
-ill, and purposely made a slight noise. Napoleon instantly
-turned his head, seized the General by the arm,
-and pointing upwards, exclaimed, “Do you see it up
-there?” The General, hardly knowing what to say,
-remained silent; but the Emperor repeated his question,
-and he was obliged to reply, that he saw nothing.
-“What,” said the Emperor, “you don’t see anything?
-You don’t see my star shining before your eyes?” And
-becoming more and more animated, he went on to say,
-that the mysterious visitor had never abandoned him,
-that he saw it throughout all his great battles, that it
-always led him onward, and that he was never happy
-but when he was gazing at it.</p>
-
-<p class='c021'><span class='pageno' id='Page_66'>66</span>That such hallucinations have no real existence as far
-as the eye goes, is proved by the fact of many people
-who have lost their sight, being subject to them. It is
-hardly to be wondered at that those who by accident
-have been deprived of their sight, should wish so
-ardently to see once more the persons and sights they
-have taken pleasure in, that they should at last create
-for themselves illusions of this character. The same
-thing has frequently occurred with those whose sight is
-more or less weak. An old man of eighty, who was
-purblind, never sat down to a table during the last
-years of his life, without seeing around him a number
-of his friends who had long been dead, dressed in the
-costume of fifty years before. This old man had but
-one eye, which was extremely weak, and wore a pair of
-green preservers, in the glass of which he continually
-saw his own face reflected.</p>
-
-<p class='c021'>Doctor Dewar, of Stirling, mentioned to Abercromby
-a very remarkable instance of this species of hallucination.
-The patient, who was quite blind, never walked
-in the street without seeing a little old woman hobbling
-on before him and leaning on a stick. This apparition
-always disappeared when he entered his house.</p>
-
-<p class='c021'>Similar illusions frequently happen to every one, even
-the most healthy amongst us, but a little consideration
-soon puts them to flight. It would be useless to mention
-the numberless cases in which a square tower has
-appeared round, or where the landscape has suddenly
-seemed to recede from the sight. Such illusions as
-these have been long well known, and appreciated at
-their proper value; but there are others whose true
-cause has remained a mystery, until explained by the
-progress of science, such as the Spectre of the Brocken,
-the Fata Morgana, and the mirage.</p>
-
-<p class='c021'>Analogous appearances have been seen in Westmoreland
-and other mountainous districts, the inhabitants
-<span class='pageno' id='Page_67'>67</span>imagining that the air was full of troops of cavalry, and
-whole armies even; such illusions resulting simply from
-the shadows of men and horses passing over an opposite
-mountain being thrown on the fog.</p>
-
-<p class='c021'>A vast number of different circumstances give rise to
-these illusions, such as a strong impression, or the recollection
-of some striking event, which may easily
-cause them, by the association of ideas. Wigan relates,
-that being at a <i>soirée</i> held at the house of M. Bellart,
-a few days after the execution of Marshal Ney, the
-groom of the chamber, instead of calling out the name
-of <i>M. Maréchal aîné</i> (M. Maréchal, senior), announced
-the arrival of M. le Maréchal Ney. A shudder passed
-through the company, and many of them declared, that
-for an instant they saw the face and figure of the dead
-man in place of those of his involuntary representative.</p>
-
-<p class='c021'>When the mind is thus prepared, the most familiar
-objects are transformed into phantoms. Ellis relates
-an anecdote of this kind, which he heard from an eye-witness,
-who was a ship’s captain of Newcastle-on-Tyne.
-During a voyage that he made, the ship’s cook died.
-Some days after the funeral, the chief mate came running
-to him in a great fright, with the news that the
-ship’s cook was walking on the water, astern of the
-vessel, and that all the crew were on deck looking at
-him. The captain, who was angry at being disturbed
-with so nonsensical a tale, answered sharply, that they
-had better put the ship about and race the ghost to
-Newcastle. His curiosity, however, was presently
-aroused, and he went upon deck and looked at the spectre.
-He frankly avowed that for some moments he saw
-what really appeared to be his old shipmate, just as he
-knew him in life, with his walk, clothes, cap and figure
-perfectly resembling those of the dead man. The panic
-became general, and every one was struck motionless
-for a time. He had the presence of mind, however, to
-<span class='pageno' id='Page_68'>68</span>seize the helm and put the ship about, when as they
-neared the ghost, they found the absurd cause of their
-fright was a broken mast from some wreck, which was
-floating after them in an upright position. If the captain
-had not boldly sailed up to the supposed ghost, the
-story of the dead cook walking upon the water would
-have continued to this day to terrify half the good inhabitants
-of Newcastle.</p>
-
-<p class='c021'>Such facts as these are innumerable, and we shall
-mention a few more which will explain a host of stories
-found in various ancient and modern authors.</p>
-
-<p class='c021'>Ajax was so angry at the arms of Achilles being
-awarded to Ulysses, that he became furious, and, seeing
-a herd of pigs, drew his sword and fell upon them,
-taking them for Greeks. He next seized a couple of
-them and beat them cruelly, loading them at the same
-time with insults, imagining one of them to be Agamemnon,
-his judge, and the other Ulysses, his enemy.
-When he came to himself, he was so ashamed at what
-he had done, that he stabbed himself with his sword.</p>
-
-<p class='c021'>Theodoric, blinded by jealousy and yielding to the
-base solicitations of his courtiers, ordered that Symmachus,
-one of the most upright men of his time, should
-be put to death. The cruel order had hardly been executed,
-when the king was seized with remorse, and
-bitterly reproached himself with his crime. One day a
-new kind of fish was put upon the table, when the king
-suddenly cried out that he saw in the head of the fish
-the absolute resemblance of that of his victim. This
-vision had the effect of plunging the king into a state
-of melancholy that lasted his whole life.</p>
-
-<p class='c021'>Bessus once, when surrounded by his guests and giving
-himself up to the enjoyment of the feast, ceased
-suddenly to listen to the flattering speeches of his courtiers.
-He apparently listened with great attention to
-some sound that was heard by no one else, and suddenly
-<span class='pageno' id='Page_69'>69</span>leaping from his couch, mad with rage, he seized his
-sword and rushing at a swallow’s nest that was near,
-beat it down, killing the poor birds inside it, crying out
-that these insolent birds dared to reproach him with the
-murder of his father. Surprised at such a sight, his
-courtiers gradually disappeared, and it became known
-some time afterwards that Bessus was really guilty, and
-that the senseless action he had performed simply resulted
-from the voice of conscience.</p>
-
-<p class='c021'>The illusions of sight and hearing are often found to
-take an epidemic form, and historians relate an immense
-number of anecdotes bearing on this particular phase of
-self-delusion. One of the commonest of them is that
-which transforms the clouds into armies and figures of
-all kinds. Religious prejudices, optical phenomena,
-physical laws that are still unknown, dangerous fevers,
-derangements of the brain, afford a natural explanation
-of these hallucinations.</p>
-
-<p class='c021'>We have borrowed most of these examples from
-Brière de Boismont’s works, for the special purpose of
-showing how easy it is to deceive the imagination, and
-to demonstrate the facility with which the sense of sight
-is led astray without the intervention of complicated
-apparatus. In addition, we may quote instances from
-Brewster, showing the ease with which the imagination
-enables us to see distinct forms in a confused mass of
-flames, or in a number of shadows superposed upon each
-other. This great philosopher gives us an anecdote of
-Peter Heamann, a Swedish pirate and murderer. One
-day that his crew were repairing some unimportant
-portion of the ship, after having pitched the place well
-he took the brush in order to tar the other parts of
-the vessel, which were much in want of such treatment;
-but as soon as he spread the pitch over the timbers of
-the ship, he was thunderstruck at seeing apparently
-reflected in its shining surface the image of a gallows
-<span class='pageno' id='Page_70'>70</span>with a headless man beneath. The head belonging to
-the body was lying before it, and the body itself was
-depicted with every limb—legs, thighs, and arms—perfect.
-He frequently told his crew of these illusions,
-adding that it was evidently a prediction of the fate in
-store for them. He was often in such a state of terror,
-that on calm days he would drop down into the hold
-and wrap himself up in a spare sail in order not to
-catch sight of the horrible image that he constantly saw
-in the shining surface of the tar.</p>
-
-<p class='c021'>The imagination really seems to create for itself a sort
-of mental visual organ which is in intimate relation
-with that of the body, and which often takes its place
-so efficiently—as in the case of dreams—that the mind
-is utterly unable to perceive the substitution. It is on
-account of this that practical opticians are so unsparing
-in their endeavours to predispose their spectators to
-being deceived.</p>
-
-<p class='c021'>When both the body and mind are healthy, the relative
-intensity of the two kinds of impressions is very
-unequally divided, mental images being more evanescent
-and comparatively weak, and with persons of ordinary
-temperament incapable of effacing or disturbing the
-reflections of visible objects. The affairs of life could
-not go on if the memory introduced amongst them brilliant
-representations of the past in the midst of ordinary
-domestic scenes or the objects familiar to us. We may
-account for this by supposing that the set of nerves
-which carries the efforts of the memory to the brain
-cannot execute their functions at the same time as those
-which take cognizance of the images reflected on the
-retina. In other words, the mind cannot accomplish
-two separate functions at one and the same time, and
-the mere act of directing the attention to one class of
-subjects causes all others to become instantly imperceptible.
-The exercise of the mind in these instances is,
-<span class='pageno' id='Page_71'>71</span>however, so rapid that the alternate appearance and
-disappearance of the two different impressions is completely
-unnoticed. Thus, for instance, while looking at
-the dome of St. Paul’s, if our memory suddenly evokes
-the image of some other object, Mont Blanc for instance,
-the picture of the cathedral, although still depicted on
-our retina, is momentarily effaced by the effort of the
-will, although we may not change the position of our
-eyes during the time. While the memory continues
-to dwell on the picture it has called up, it is seen
-with sufficient distinctness, although its details may
-be somewhat misty and its colours confused; but as
-soon as the wish to see it passes away the whole
-disappears, and the cathedral is seen in all its former
-distinctness.</p>
-
-<p class='c021'>In darkness and solitude, when surrounding objects
-produce no images that can interfere with those of the
-mind, these latter are more lively and distinct: and
-when in addition we are half asleep and half awake,
-the intensity of mental impressions approaches that of
-visible objects. In the case of persons of studious
-habits who are continually employed in mental effort,
-these images are more distinct than with those who
-follow the ordinary avocations of life, and during their
-working hours rarely see the objects round them. The
-earnest thinker, absorbed by meditation, is in a manner
-deprived for the time of the use of his senses. His
-children and servants pass in and out of his study
-without his seeing them, they speak to him without
-his hearing them and they may even try to rouse
-him from his reverie without success; and yet his
-eyes, ears, and nerves received the impression of light,
-sound, and touch. In such instances, the mind of the
-philosopher is voluntarily occupied in following out an
-idea which interests him profoundly; but even the most
-unlearned and thoughtless of us sees the images of dead
-<span class='pageno' id='Page_72'>72</span>or absent friends with his mind’s eye, or even fantastic
-figures which have nothing to do with the train of
-thought he may be pursuing. It is with these involuntary
-apparitions as with spectres of the imagination:
-although they are intimately connected with some
-thought that has passed through our mind unperceived,
-it is impossible to trace a single link of the chain connecting
-them together.</p>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='pageno' id='Page_73'>73</span><span class='c006'>PART II.</span></div>
-    <div class='c000'><span class='c015'>THE LAWS OF LIGHT.</span></div>
-  </div>
-</div>
-
-<hr class='c009' />
-<div class='chapter'>
-  <h2 id='ch2-01' class='c012'>CHAPTER I.<br /><span class='c020'>WHAT IS LIGHT?</span></h2>
-</div>
-<p class='c014'><span class='sc'>Everybody</span> knows the effects of the action of light,
-without, however, understanding precisely what constitutes
-light itself. Any formal definition would rather
-puzzle than help the student; we must therefore content
-ourselves with saying that light is that effect of
-force which causes us to perceive external objects.</p>
-
-<p class='c021'>A man who was blind from his birth, and upon whom
-the operation for cataract had been successfully performed,
-had accustomed himself for a long time to imagine
-the nature of those unknown phenomena that his
-affliction had prevented him from observing. He had
-arranged in his mind the various definitions that had
-been given to him as to the nature of light, and having
-combined them, he fancied he had acquired some notion
-of what the sense of vision really meant. But what
-was the astonishment of the surgeon who had restored
-to him his fifth sense, when he asked him to give his
-opinion upon the effects of light, to see him take up a
-lump of sugar and reply that it was under that form
-that he had imagined it to himself.</p>
-
-<p class='c021'><span class='pageno' id='Page_74'>74</span>As for us who have the happiness of possessing the
-sense of sight, we know this mysterious agent more by
-the enjoyment that we have derived from it, than from
-any analysis we have made of its nature. It is an endless
-chain that connects us with the entire universe; a
-bond that laughs at distance and spans the abysses of
-space. By means of light we can appreciate the beauties
-of hue and form, and by its power we touch as it
-were the inaccessible. It constitutes the most intimate
-connexion between ourselves and external objects—a
-connexion that seems even to alter our temper, disposition,
-and character, according to the variations of its
-intensity. The dull and foggy days of winter, those
-days when sleet and rain struggle in the atmosphere,
-spread like a veil over us, and throw a shadow upon
-our life. The return of the bright spring sun, the reappearance
-of light and blue sky, on the contrary, open
-up our hearts and minds, gay nature enchants us once
-more, and a feeling of fresh happiness prepares us for
-the coming glories of the newly risen year.</p>
-
-<p class='c021'>This intimate connexion between the light of heaven
-and the human mind, hallowed as it is by our desire to
-rise towards the Source of all light, might be made the
-subject of many eloquent pages; and it would be an
-interesting and useful task to show the gradual progress
-of mankind from those ancient people who trembled
-at the approach of darkness, and who fervently
-saluted the dawn with prayers and praises, down to the
-philosophers of the present age, who investigate its
-effects with so much reverential joy. But we must
-cease paying any more attention to the superficial action
-of this marvellous force which in these latter days
-has become, in the hands of man, the source of so
-many illusions and the origin of a complete world of
-rich and brilliant pictures, but which after all only exist
-in the imagination.</p>
-
-<p class='c021'><span class='pageno' id='Page_75'>75</span>It was believed for a long time that light was a compact
-mass of tiny particles emitted by luminous bodies,
-which struck our eyes and so produced the phenomenon
-of vision. These particles or molecules were naturally
-thought to be extremely minute, and the objects illuminated
-by them were supposed to throw them off as
-if they were endowed with elasticity. Under this hypothesis,
-light was a material body. The illustrious
-Newton was the first propagator of this theory; the
-last was M. Biot, a French philosopher, lately dead.</p>
-
-<p class='c021'>The undulatory theory has now-a-days completely
-superseded the corpuscular hypothesis. It was first
-started about the year 1660 by the Dutch philosopher
-Huyghens, who has left behind him numerous treatises
-on optics, and the properties of light, as well as a curious
-account of the inhabitants of the other members of
-the solar system, including a minute description of the
-various planetary manners and customs. At the beginning
-of the present century, Fresnel showed, by the
-most brilliant discoveries the superiority of this theory,
-and shortly after Arago confirmed him in his demonstrations.
-According to the undulatory hypothesis,
-light is not a mass of molecules emitted by a luminous
-body, but simply the vibration of an elastic fluid which
-is conceived to fill the whole of space. A comparative
-example may assist you in understanding this theory
-more clearly. If you throw a stone into a smooth
-piece of water, there will form around the point where
-the stone fell, a series of circular undulations, starting
-from the centre and gradually enlarging themselves.
-If a loud noise is suddenly heard, the same effect is
-produced round the point from whence the sound proceeds.
-A series of waves are formed which spread not
-only horizontally, as on the surface of the water disturbed
-by the stone, but in every direction. In fact,
-in the case of sounds, the waves are so many gradually
-<span class='pageno' id='Page_76'>76</span>increasing spheres. In the case of light, when a luminous
-body is placed in space, the ether which surrounds
-it is thrown into a state of vibration, and the motion is
-immediately propagated in all directions, with extreme
-velocity. It is these undulations that produce upon our
-eyes the sensation of light. We may therefore say that
-light, like sound, is movement, while darkness, like silence,
-is absolute rest.</p>
-
-<p class='c021'>Many people still believe that light is propagated instantaneously,
-and cannot bring themselves to imagine
-that we do not see a flame the moment we light it, but
-only an instant after. I have myself spoken to well-educated
-people possessed of good judgment and a certain
-amount of elementary knowledge, who could never
-bring themselves to believe that we see the stars, not as
-they now exist, but as they appeared at the particular
-moment when the luminous wave by which we are enabled
-to perceive them left their surface, and which only
-reaches us after travelling through space a certain number
-of years, days, or hours, according to their distance.
-It is extremely useful and interesting to form a correct
-idea upon the way in which light is propagated.</p>
-
-<p class='c021'>The determination of the prodigious quickness with
-which the waves of light move through space, says
-Arago, is undoubtedly one of the happiest results of
-modern astronomy. The ancients believed that it moved
-with infinite velocity, and their view of the subject was
-not, like so many of the questions relating to physics, a
-mere opinion without proof; for Aristotle, in mentioning
-it, brings forward the apparently instantaneous
-transmission of daylight. This notion was disputed by
-Alhazen, in his <i>Treatise on Optics</i>, but only by meta-physical
-weapons, which were again opposed by several
-very worthless arguments, by his commentator, Porta,
-although he admitted the immateriality of light. Galileo
-seems to have been the first amongst modern philosophers
-<span class='pageno' id='Page_77'>77</span>who endeavoured to determine the velocity of
-light by experiment. In the first of his dialogues, <i>Delle
-Scienze Nuove</i>, he announces by the mouth of Salviati,
-one of the speakers present, the ingenious means he
-had employed, and which he thought quite sufficient to
-solve the question. Two observers with lights were
-placed at the distance of one mile from each other; one
-of them extinguished his light, and the other as soon as
-he perceived it extinguished his. But as the first observer
-saw the second light disappear the instant he
-had extinguished his own, Galileo concluded that light
-was propagated instantaneously through a distance double
-that which separated the two observers. Certain
-analogous experiments that were made by the members
-of the Academy <i>Del Cimento</i>, but at three times the
-distance, led to precisely the same conclusions.</p>
-
-<p class='c021'>These attempted proofs seem at first sight to be absurd,
-when we think of the vastness of the problem to
-be solved; but we must judge these experiments with
-less severity, when we consider that almost at the same
-epoch, men of such well-deserved repute as Lord Bacon
-believed that the velocity of light, like that of sound,
-was sensibly altered by the force and direction of the
-wind.</p>
-
-<p class='c021'>Descartes, whose theories upon light had so much
-analogy with those known under the name of the undulatory
-hypothesis, believed that light was transmitted
-instantaneously throughout any distance, and endeavours
-to prove his position by proofs that he thought he
-had obtained whilst observing an eclipse of the moon.
-It must be acknowledged, however, that his very ingenious
-train of reasoning proves that whether the transmission
-of light is instantaneous or not, it is at least too
-considerable to be determined by experiments made on
-the earth, like those of Galileo, and which he vainly
-hoped would have solved the question.</p>
-
-<p class='c021'><span class='pageno' id='Page_78'>78</span>The frequent occultations of the first satellite of Jupiter,
-the discovery of which was almost consequent
-upon that of lenses, furnished Römer with the first
-means of demonstrating that light was propagated by
-perceptible degrees.</p>
-
-<p class='c021'>In tracing out the history of human knowledge, says
-Dr. Lardner, we have frequently to point out with some
-little surprise, joined to a feeling of profound humility,
-the important part played by chance in the advancement
-of science. In searching zealously after mere trifles
-which, when found, are of no consequence, we frequently
-lay our hands on inestimable treasures. The frequency
-of this fact impresses the mind with the notion that some
-secret and unceasing power exists, in accordance with
-which human knowledge and science are continually
-progressing. It is in physical, as in moral philosophy.
-In our ignorance—like the dog mentioned by Æsop,
-which, seeing in the water the reflection of the prey it
-held in its mouth, dropped the substance and tried to
-seize the shadow—we are continually searching after
-trifles; but, more fortunate than the animal of whom
-we have been speaking, the shadow that we try to seize
-is often transformed into a rich treasure. We can say
-with every confidence that “the Providence which
-shapes our ends,” knows our wants better than we do
-ourselves, and bestows on us the things we <i>ought to have</i>
-asked for instead of those we <i>have</i> asked for. We shall
-find a very simple proof of this in the history of the
-discovery of the velocity of light.</p>
-
-<p class='c021'>A short time after the invention of the telescope and
-the consequent discovery of Jupiter’s satellites, Römer,
-a celebrated Danish astronomer, was engaged in a series
-of observations, the object of which was to determine
-the time which one of these bodies took to revolve
-round its planet. The method employed by Römer was
-to observe the successive occultations of the satellite,
-<span class='pageno' id='Page_79'>79</span>and to notice the interval that elapsed between each of
-them. But it at last happened that the interval between
-the two occultations, which was about forty-five
-hours, became prolonged by periods of 8, 13, and 16
-minutes, during that half of the year when the earth
-was receding from the planet, while it became proportionally
-cut short during the rest of the year. Römer
-was struck by a happy idea; he suspected instantly
-that the moment when he remarked the disappearance
-of the satellite was not always coincident with the instant
-when it really took place, but that it sometimes
-appeared to happen later—that is to say, after an interval
-of time sufficiently long to allow the light that
-had left the satellite immediately after its disappearance,
-to reach the eye of the observer. Hence it became
-evident that the farther off the earth was from the
-satellite, the longer was the interval of time between its
-disappearance and that of the arrival of the last portions
-of its light upon the earth; but that the moment of the
-disappearance of the satellite is that of the commencement
-of the occultation, and that the moment of the
-arrival of the last portions of light is that when the
-commencement of the occultation is observed.</p>
-
-<p class='c021'>It was thus that Römer explained the difference between
-the calculated and observed time of the occultation,
-and he saw that he was on the threshold of a
-great discovery. In a word, he saw that light propagated
-itself through space with a certain velocity, and
-that the fact we have just mentioned furnished the precise
-means of measuring it.</p>
-
-<p class='c021'>Thus the occultation of the satellite was retarded one
-second for every 185,000 miles that the earth is
-distant from Jupiter; the reason being, that a ray of
-light takes a second to travel this distance, or, in other
-words, because the velocity of light is at the rate of
-185,000 miles per second.</p>
-
-<p class='c021'><span class='pageno' id='Page_80'>80</span>It must be remembered when considering this subject,
-that in any system of undulations or vibrations, no
-matter through what medium they are propagated, their
-movement is simply a change of form, and not a transmission
-of matter. The waves which spread round a
-central point when a stone is thrown into the water,
-give one the idea that the water which forms the wave
-really moves towards the observer. But it is not so, as
-may be readily proved by placing on the surface a floating
-body, which we shall find is but little, if at all, influenced
-by the undulations of the water. The appearance
-of rolling waves given on the stage by means of a painted
-cloth, to which an undulatory motion is given, is an instance
-of this apparent movement. In the case of the
-floating body, which would follow the movements of the
-water, we shall find that wave after wave rolls to the
-shore, in the same way as the painted marks on the
-imitation sea keep their place, although the cloth itself
-undulates. The waves of the sea even appear to the eye
-to be endowed with a progressive motion, but an instant’s
-observation will convince us of our error; for if such
-were the case, every object floating on the ocean would
-be gradually carried on shore. A vessel floating on the
-waves is not carried along by them, at least not until it
-reaches within a few yards of the shore, where the
-water is really in motion; but out in the open sea a
-floating body will alternately rise on their crests, and
-fall into the valleys that separate them. The same
-effect may be observed with any object floating on the
-water. If, however, in addition to being in a state of
-undulation the sea is really in motion from the effects
-of a current, or from any other cause, the floating
-object will of course be carried along by it—in fact, the
-two movements are quite independent of each other,
-and may take place in similar or contrary directions.
-It is very important that we should be able to distinguish
-<span class='pageno' id='Page_81'>81</span>at an early period the exact difference between
-true movement and mere undulation; and we must remember
-that although the waves of light are propagated
-at the rate of 185,000 miles a second, still there
-is no transmission of any material substance at this
-marvellous rate. The same observation applies to
-sonorous vibrations transmitted through the air.</p>
-
-<p class='c021'>Thus we are constrained to admit peaceably the truth
-of the undulatory hypothesis as compared with the corpuscular
-theory. I say <i>peaceably</i>, because I am forcibly
-reminded by the contrast I have made between the two
-theories of an anecdote related of one of the greatest
-monsters who ever walked this earth, but who was afterwards
-struck down in the midst of his power by the
-hand of a weak girl. I allude to the infamous Marat,
-who one day presented himself at the house of Dr.
-Charles, a celebrated natural philosopher, of the time of
-the first French Republic, in order to advance certain
-notions of his own against the optical principles that
-Newton has left behind in his <i>Principia</i>, and other
-works—also, to oppose certain theories connected with
-electrical science. Dr. Charles, who did not approve of
-Marat’s wild notions, undertook to convince him of his
-errors. But instead of discussing the matter peaceably,
-Marat allowed himself to be carried away by his temper,
-which was naturally very violent. Every argument advanced
-by his antagonist seemed to increase his rage,
-until at last he lost all control over himself, drew his
-small sword, and rushed upon his opponent. The doctor,
-who was unarmed, had to exercise all his powers to prevent
-himself from being wounded, and being much more
-stoutly built than Marat, he at last succeeded in throwing
-him down, and wresting his sword from him, which he
-immediately took care to break. Whether it was the
-violence of the fall, the shame he felt at being doubly
-beaten, or the effects of his fit of passion, does not
-<span class='pageno' id='Page_82'>82</span>appear, but Marat fainted. Assistance was called, and
-he was carried home to his house, his offence against all
-the laws of propriety being forgiven by his more talented
-and better-tempered adversary.</p>
-
-<p class='c021'>There are many persons, no doubt, whom we should
-astonish, and possibly enrage, by asserting positively
-that we could cause darkness by means of light, that
-silence could be produced by sound, or cold by heat.
-These are daring paradoxes, and at first sight appear
-almost as reasonable as that of Anaxagoras, a Greek
-philosopher, who asserted that snow was black. But as
-I hope that most of my readers do not possess the
-passionate temper of the French tribune, I will confide
-to them a little secret that will make these paradoxes
-plain. It is called by natural philosophers the theory
-of interference.</p>
-
-<p class='c021'>The experiments connected with this subject are
-exceedingly difficult to perform, and require the aid of
-apparatus far beyond the reach of the ordinary student.
-It is a case where theory and description are much easier
-than practice.</p>
-
-<p class='c021'>If a ray of electric light is thrown upon a screen, it is
-possible to direct another ray upon the same spot in
-such a manner that they will extinguish each other
-mutually. The reason of this phenomenon may be
-understood, if we remember that light is caused by
-undulatory movement, and that by opposing two series
-of waves to each other in such a manner that their
-vibrations coming in contact produce rest, we can easily
-see how the waves of light of one ray may be stopped
-by those of a second.</p>
-
-<p class='c021'>Going back to our illustration of the eddies on a
-pool of water, it is easy to prove that by throwing
-a second stone into the water we form another series
-of undulations; which are mutually destroyed when
-they encounter each other. It is the same with the
-<span class='pageno' id='Page_83'>83</span>peculiar fluid which, existing throughout space, is
-thrown in a state of undulation by incandescent bodies;
-by opposing one set of waves to another we obtain rest
-as a result.</p>
-
-<p class='c021'>This fact was first observed by Grimaldi in 1665, and
-Dr. Thomas Young was the first to offer an explanation.
-Fresnel used it with great success at the beginning of
-the century to demonstrate the truth of the undulatory
-theory, by showing that it could not be explained by
-any other.</p>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div class='chapter'>
-  <span class='pageno' id='Page_84'>84</span>
-  <h2 id='ch2-02' class='c012'>CHAPTER II.<br /><span class='c020'>THE SOLAR SPECTRUM.</span></h2>
-</div>
-<p class='c014'><span class='sc'>The</span> white light that the glorious orb of day spreads
-over the face of nature is the original source of all those
-brilliant and sombre colours with which the works of
-the Creator are beautified. To the rays of the sun we
-owe not only the whiteness of the lily, but the scarlet of
-the field poppy, the modest blue of the timid violet, the
-splendour of the peacock’s plumage, the cool green of
-the meadows, and the purple and gold of the distant
-mountains. For, as we have hinted before, this white
-light, which seems of itself so destitute of colour, is
-productive of every hue that the eye of man is capable
-of appreciating.</p>
-
-<p class='c021'>It may seem that I am bestowing too much praise
-upon our own sun; but if you are surprised that I
-should seek to exalt this brilliant globe of ever-burning
-fire, I must ask you to recollect, that though the starry
-heavens are full of suns as vast and important as ours,
-and possibly affording brilliant colourless light to worlds
-full of inhabitants, there are others that give forth rays
-that are far from being white. Some are as green as
-emeralds, others are as blue as sapphires, while others
-give out a warm light like a ruby or topaz. The worlds
-which surround these can only receive light of a certain
-colour, or at any rate they are restricted to a few shades
-and hues. Imagine living in a world where everything
-<span class='pageno' id='Page_85'>85</span>was always <i>couleur de rose</i>, or in which the inhabitants
-were continually looking blue! A residence in either
-of them for a short time would undoubtedly cause us to
-appreciate the relative value of our own little sun, small
-as it is in comparison with some of the mighty orbs
-floating about in space.</p>
-
-<p class='c021'>The fact that the light of the sun is the source of all
-the changing hues to be found on the surface of the
-earth season after season was first discovered by Newton,
-and his experiments are easily repeated with a very few
-and inexpensive appliances.</p>
-
-<p class='c021'>A small round hole is made in the window-shutter of
-a room, facing the sun, and the pencil of light proceeding
-from it is allowed to fall upon the surface of a three-sided
-prism, held in a horizontal position, and placed
-at a distance of a few inches from the aperture (<a href='#i004b'>fig. 5</a>,
-Frontispiece). The pencil of light does not pass through
-the prism as if it were a plate of glass with parallel
-sides, but in virtue of the laws of refraction, of which
-we have already spoken, it is turned out of its natural
-course, and is thrown upon the wall in the direction indicated
-in the figure. The pencil of light is not only
-turned aside, but it is also widened out into a band
-which is truly painted with all the colours of the rainbow,
-every tone and hue being of the most marvellous
-brilliancy. This long coloured stripe, which constitutes
-one of the most beautiful sights that the science of
-optics can afford us, is known to scientific men by the
-name of the solar spectrum.</p>
-
-<p class='c021'>Before going into the causes that produce these
-colours, let us first examine their number and position.
-Beginning at the top, we shall find that they run in the
-following order:—Violet, indigo, blue, green, yellow,
-orange, red. The red being lowest is called the least
-refrangible of them all; or, in other words, in passing
-through the prism it was bent less out of its course than
-<span class='pageno' id='Page_86'>86</span>its companions. Violet, being at the top, is of course
-the most refrangible. The cause of the separation of
-the colours of white light is consequently only the effect
-of their individual character. They were, so to speak,
-so many streams flowing together until an unexpected
-deviation in their course caused them to separate. This
-change in the direction of their flow brought out their
-personal individuality, and they at once became completely
-disunited.</p>
-
-<p class='c021'>Every single tint in the prismatic spectrum is simple,
-and cannot be decomposed. This may be shown by
-passing any of them through another prism, when it
-will be found that no change will take place in the
-colour or size of the pencil. Hence those worlds already
-spoken of, whose light of day is red, blue, or green,
-never see any colours but these. (<a href='#i004a'>Fig. 6</a>, Frontispiece).</p>
-
-<p class='c021'>It is just as easy to reunite the colours into which
-white light is decomposed, by applying a second prism
-in a reversed position to the pencil of coloured light, as
-it is to separate them in the first instance. The method
-of accomplishing this is shown in <a href='#i004c'>fig. 7</a>, Frontispiece.</p>
-
-<div id='i086'  class='figcenter id009'>
-<img src='images/i086.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 8.</span>—The Recomposition of Light.</p>
-</div>
-</div>
-
-<p class='c021'>Another experiment in the same direction consists in
-<span class='pageno' id='Page_87'>87</span>reuniting the colours by causing them to pass through
-a double convex lens, behind which is placed a screen
-of ground glass, or a card (<a href='#i086'>fig. 8</a>). By advancing and
-withdrawing this screen we can easily find the exact
-spot where the rays reunite, and form a dazzling spot
-of white light. This point is called the focus, from a
-Latin word, signifying “fire-place,” a term which will
-put the student in mind of the frequently repeated experiment
-of burning a piece of paper with an ordinary magnifying-glass.</p>
-
-<p class='c021'>Instead of using a lens, you can, if you please, employ
-a concave mirror, using the ground glass or cardboard
-screen, as before. The colours reflected by the
-mirror unite at its focus, and produce a brilliant white
-spot in just as conclusive a manner as in the other experiment.</p>
-
-<div id='i087'  class='figcenter id010'>
-<img src='images/i087.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p>Fig. 9.—Recomposition of Light by means of a Concave Mirror.</p>
-</div>
-</div>
-
-<p class='c021'>A fourth experiment, which is somewhat more difficult
-for the student to accomplish, consists in causing
-every one of the seven different colours to be reflected
-from a separate mirror.</p>
-
-<p class='c021'>The mirrors in this case are concave, and are so
-mounted as to be capable of being moved in any direction.
-By directing each of the seven rays, one by one,
-upon the same point, you may observe the gradual decomposition
-<span class='pageno' id='Page_88'>88</span>of the coloured light. The effect obtained
-by adding the last colour to the mixture is quite magical,
-the white circle being produced from two brilliantly-coloured
-spots.</p>
-
-<div id='i088'  class='figcenter id011'>
-<img src='images/i088.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p>Fig. 10.—Recomposition of Light by means of a number of Mirrors.</p>
-</div>
-</div>
-
-<p class='c021'>A fifth experiment, first devised by Newton, is also
-within the reach of the student. On a disc of cardboard
-the centre and border of which have been previously
-painted black, are pasted seven strips of paper, painted
-as nearly as possible of the same colour as the components
-of the spectrum—or if the student is anything
-of an artist he may paint the disc in imitation of the
-spectrum, carefully shading off the tints into each other.
-If the disc be now rapidly rotated the colours will disappear,
-and a greyish hue will be seen, which will approach
-more closely to white, the nearer the colours on
-the disc are to those of the spectrum. This experiment
-is not precisely the same in principle as the preceding
-ones, for it is evident that the colours on the disc do
-not mix, but only the impressions they form upon the
-retina. We have already said that such impressions
-remain on the eye for one-tenth of a second or there-abouts;
-the disc must therefore revolve at least ten
-times a second, or the effect will not be perceived.</p>
-
-<div id='i089'  class='figcenter id012'>
-<img src='images/i089.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 11.</span>—Newton’s Disc.</p>
-</div>
-</div>
-
-<p class='c021'>From these experiments it follows that the colours
-<span class='pageno' id='Page_89'>89</span>with which all natural substances are clothed, ought not
-to be looked upon as belonging to them absolutely, but
-only as a property dependent on the reflection and
-absorption of light from their surfaces. The leaves of
-plants, for instance, must not be regarded as being
-really green in themselves, but as being capable of absorbing
-certain portions of light, and reflecting others.
-Grown in the dark, the green substance contained in the
-plant and its leaves becomes white, and no longer possesses
-the property of absorbing red light, and reflecting
-green. A green leaf placed in red light becomes almost
-black, from its power of absorbing light of that
-colour; in the blue it reflects a much greater proportion
-of the coloured ray. A very striking experiment may
-be performed with a substance known to chemists as the
-<span class='pageno' id='Page_90'>90</span>iodide of mercury. If a little of this salt, which is of a
-brilliant red, be placed in a watch-glass, and heated over
-a spirit-lamp, it will gradually sublime, and a card held
-over it will be covered with a number of light yellow
-crystals. In this case no change of composition has
-taken place, but simply a change in the power the salt
-possesses of reflecting some rays and absorbing others.
-By simply scratching the surface of the card with a
-pointed piece of wood, the yellow crystals become transformed
-once more into the red variety; not only this,
-the transformation gradually spreads, like a red cloud,
-over the whole of the deposit. There are some other
-salts known to chemists which possess the property of
-dichroism, or double colour. The double cyanide of
-platinum and barium, for instance, appears violet when
-viewed in one direction, and yellow in another. Change
-of temperature is often sufficient to change the colour of
-bodies—white oxide of zinc, for example, becomes bright
-yellow when heated. Such instances might be supplied
-<i>ad infinitum</i>, but enough has been said to prove that
-colour, after all, is only an appearance, and not an essential
-property of bodies.</p>
-
-<p class='c021'>We have already spoken of complementary colours,
-or those which it is necessary to add together in order to
-produce white light. Blue, for instance, is complementary
-to orange, red to green, violet to yellow, and <i>vice
-versa</i>. But it is not by the aid of the palette that this
-can be proved, for in the case of coloured pigments the
-arrangement of their atoms interferes in some way with
-the success of the experiment, and it is only by means
-of the colours of the spectrum that such recompositions
-can be effected.</p>
-
-<p class='c021'>Although most philosophers consider that there are
-seven colours in the spectrum, there are others who do
-not admit it, but assert that there are really only three,
-red, yellow and blue—which by the superposition of
-<span class='pageno' id='Page_91'>91</span>their edges produce the intermediate hues of green and
-orange. Perhaps it would be nearer to the truth to say
-that the spectrum is composed of an infinite number of
-colours of different hues.</p>
-
-<p class='c021'>We have already stated that every one of these
-colours is indecomposable, and that there are certain
-worlds illuminated by a single colour only, instead of
-possessing the infinite number of tints enjoyed by the
-inhabitants of the solar system. An idea of this effect
-can easily be gained in a very simple but surprising
-manner by inserting panes of glass of different colours
-in the hole of the shutter of a dark room. If the light
-is yellow, you will find that all those objects that are
-capable of reflecting yellow light are coloured by it,
-while those which are bright red or blue become almost
-black by absorbing the only light present. If we could
-procure an object which was perfectly complementary
-in colour to the yellow glass, it would appear perfectly
-black. The same experiment may be repeated with
-the other colours. After remaining in this coloured
-light for some time, if you suddenly pass out into daylight
-the complementary colour will tinge everything
-around you.</p>
-
-<p class='c021'>Instead of using a room into which coloured light
-only is admitted, lamps burning with a coloured flame
-may be employed. Brewster mentions the following
-experiment, which is a very striking one:—Fill a spirit-lamp
-with alcohol in which has been dissolved as much
-common salt as the spirit will take up; on being lit it
-will be found to burn with a livid yellow flame. A
-room lighted entirely with one or two lamps of this
-kind will form a laboratory for some very singular experiments.
-It should, if possible, be hung with pictures
-in water and oil colours, and the persons present ought
-to wear nothing but the brightest colours, and the table
-be ornamented with the gayest of flowers. The room
-<span class='pageno' id='Page_92'>92</span>being first lighted with ordinary daylight, the lamps
-above mentioned should be brought in, and the daylight
-carefully excluded, when an astonishing metamorphosis
-will take place. The spectators will be hardly
-able to recognise each other; the furniture of the room,
-and every other object contained in it, will reflect but a
-single colour. The flowers will lose their brilliant
-tints, the paintings will appear as if they were drawn
-in Indian ink. The brightest purple, the purest lilac,
-the richest blue, the liveliest green, will be converted
-into a monotonous yellow. The same change will take
-place in the countenances of those present; a livid
-paleness will spread over their faces, whether young or
-old, and those who are naturally of an olive complexion
-will hardly appear changed at all. Every one will
-laugh at the appearance of his neighbour’s face, without
-thinking that he is just as great a subject of laughter to
-them. If, in the midst of the amusement caused by this
-experiment, the light of day is admitted at one end of
-the room, the other end being still lighted with the
-salt-lamp, every one will appear to be half-illuminated
-with the livid colour which has caused so much surprise,
-the other portion of their figure and clothes being of
-the natural hue. One cheek, for instance, will appear
-animated with its usual brilliancy, while the other will
-be that of a corpse; one side of a lady’s dress will be
-brilliant blue or green, as the case may be, the other a
-colour that it would puzzle an artist to give a name to.
-The experiment may be varied by admitting the white
-light through several small holes in the shutter of the
-room, every luminous spot painting the place where it
-falls in its natural colours, and the yellow spectators
-will become spotted with the most singular tints and
-hues. If a magic lantern is used to throw on the walls
-of the room and the clothes of the company any luminous
-figures, such as those of flowers or animals, they
-<span class='pageno' id='Page_93'>93</span>will be coloured with these figures in the tint of the
-wall or fabric upon which they fall, yellowish colours
-of course escaping the transformation. If nitrate of
-strontia be substituted for the salt, a crimson tint will
-be spread over everything. In fact, a lamp prepared
-in this way will form a source of endless amusement.
-It is not necessary to use alcohol for the purpose;
-wood-spirit or methylated alcohol will serve the purpose
-equally well. If a lamp is not to be had, a few pieces
-of cotton-wool, tied on wires and dipped in the salted
-spirit, will do almost as well.</p>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div class='chapter'>
-  <span class='pageno' id='Page_94'>94</span>
-  <h2 id='ch2-03' class='c012'>CHAPTER III.<br /><span class='c020'>OTHER CAUSES OF COLOUR.</span></h2>
-</div>
-<p class='c014'><span class='sc'>The</span> colours of the spectrum are to the sense of sight
-what the tones of the gamut are to the sense of hearing.
-On the one hand, the differences in the lengths
-of the sonorous waves constitute the variety of note perceptible
-by the ear; on the other, the differences in the
-lengths of the luminous waves constitute the variety of
-colour perceptible by the eye. By and by, we shall
-learn both the length and rapidity of these vibrations,
-but it will be as well first to describe the experiments
-made in this direction by the immortal Newton
-himself.</p>
-
-<p class='c021'>Every one has, doubtless, at one period of his life,
-amused himself with blowing soap-bubbles by means of
-a tobacco-pipe and a little lather—a sufficiently childish
-amusement, you will possibly say, but one narrowly
-connected with the most intricate secrets of the science
-of optics. These little globes, so fragile that they disappear
-in a breath, hardly seem worthy of the attention
-of a thinker, and still less the examination of a philosopher;
-but it is nevertheless true that Newton made experiments
-on the colours shown on the surface of these
-apparently insignificant objects which ended in the
-most brilliant discoveries, just as on seeing an apple fall
-he began a train of thought which only terminated in
-the enunciation of the hypothesis of the earth’s power
-of gravity.</p>
-
-<p class='c021'><span class='pageno' id='Page_95'>95</span>All transparent substances, whether liquid, solid, or
-gaseous, become coloured with the most brilliant hues
-as soon as they are reduced to plates of extreme thinness.
-In the soap-bubble it is the oleaginous particles
-floating on the surface which thus become coloured, but
-Newton showed that thin plates of air were similarly
-capable of showing colour, and that the thinner the
-plates were the more brilliant were the tints. We may
-see this in the soap-bubble, which becomes more beautiful
-as it gets larger and thinner. By placing a convex
-lens of large size on a flat plate of glass, Newton
-observed that rings of different colours were formed
-round the spot where the two pieces of glass touched.</p>
-
-<div id='i095'  class='figcenter id013'>
-<img src='images/i095.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p>Fig. 12.—Newton’s Rings.</p>
-</div>
-</div>
-
-<p class='c021'>By measuring the convexity of the lens and the diameter
-of the various rings, Newton was enabled to tell to
-a minute fraction the exact thickness of the plate of
-air corresponding to the different colours. The glasses
-being placed in position, a ray of a particular colour—red,
-for instance—was thrown upon the surface. The
-result was a black spot at the point where the two surfaces
-touched, and surrounding it at various distances
-were several rings alternately red and black. Calculating
-the thickness of the plates of air at the part
-where the dark rings made their appearance, Newton
-found that their dimensions were in the proportion of
-the even numbers two, four, six, eight, &amp;c.; while the
-red rings showed figures corresponding to the odd numbers.
-Although trammelled by the corpuscular theory,
-Newton’s deductions from these experiments show that
-<span class='pageno' id='Page_96'>96</span>they can only be accounted for by the undulatory hypothesis.
-Thus the thickness of the plate of air at the first
-red ring is that of the red wave, the thickness at the
-second that of two red waves, and so on; so that in order
-to arrive at the thickness of the red wave we need
-only measure the distance between the portions of the
-glasses where the first red ring occurs.</p>
-
-<p class='c021'>This experiment, was applied to the measurement of
-all the waves. Whenever they were reflected on the
-glasses a parallel series of rings was formed, but it was
-found that the first ring was more or less distant from
-the central spot, according to the colour used. The red
-ring was the largest; the orange, yellow, green, blue,
-indigo, and violet, following in the same sequence as in
-the spectrum. The word “thickness” seems hardly fit
-to apply to dimensions arrived at by Newton in his experiments,
-so infinitely small do they appear to be, yet
-their correctness has never been impugned, although
-the experiments have been repeated by the philosophers
-of all countries. The waves of red light are so small
-that 40,000 of them go to an inch, and those of violet
-light situated at the other end of the spectrum are still
-smaller, measuring only the 60,000th part of an inch.</p>
-
-<p class='c021'>The waves of the other colours are between these two,
-while the wave of white light, which is a mixture of
-them all, is just half-way between the two.</p>
-
-<p class='c021'>Thus was the physical cause of the various hues of
-colour discovered by this great man, revealing as it does
-the singular and mysterious analogy between sound and
-light. The rays of light, like the waves of sound, produce
-a different effect, according to their length, by
-causing quicker or slower pulsations in the nerves of
-sight, just as musical sounds vibrate upon the drum of
-the ear with different velocities.</p>
-
-<p class='c021'>This is not all, for the relationship between sound and
-light does not cease here: we have as yet only spoken
-of the size of the undulations, and have only shown
-<span class='pageno' id='Page_97'>97</span>how their dimensions are connected with the sensation
-of colour; but there are other things to be considered,
-for on investigation we find that not only do the different
-coloured waves vary in the length of their undulations,
-but also in the number that take place in a given
-time.</p>
-
-<p class='c021'>The perception of sound is produced by the action of
-the drum of the ear, which vibrates sympathetically
-with the pulsations of the air that have been originated
-by the vibrations of the sounding body; and the perception
-of light is produced in a similar manner by the
-vibrations originating in a luminous body, and propagating
-themselves through the luminous ether until
-they reach the nerves of sight. The number of these
-pulsations taking place in the eye has been accurately
-determined in the following manner. Let us suppose
-that we are looking at a coloured object—let us say, a
-red railway signal-lamp; from the lamp to our eye there
-flows a continuous line of luminous undulations; these
-undulations enter the eye and become depicted on the
-retina. For every wave that passes through the pupil,
-there is a separate and corresponding vibration of the
-optic nerve, and the number of these vibrations that
-take place in the course of a second can be easily calculated
-if we know the velocity of light and the breadth
-of the waves. We have before found that light travels
-at the rate of 185,000 miles per second; it therefore
-follows, that a series of undulations 185,000 miles long
-pass through the pupil every second; consequently the
-number of vibrations per second is arrived at by calculating
-how many waves measuring the 40,000th of
-an inch—that being the length of a wave of red light—are
-contained in 185,000 miles. The following table,
-showing the number of waves passing into the eye
-per second for the different colours, will interest the
-student:—</p>
-<div><span class='pageno' id='Page_98'>98</span></div>
-<div class='std-table'>
-
-<table class='table3' summary=''>
-<colgroup>
-<col width='33%' />
-<col width='66%' />
-</colgroup>
-  <tr>
-    <td class='c022'>Extreme red</td>
-    <td class='c023'>458,000,000,000,000 waves per second.</td>
-  </tr>
-  <tr>
-    <td class='c022'>Red</td>
-    <td class='c023'>477,000,000,000,000 &nbsp;&nbsp;&nbsp;&nbsp; "</td>
-  </tr>
-  <tr>
-    <td class='c022'>Orange</td>
-    <td class='c023'>506,000,000,000,000 &nbsp;&nbsp;&nbsp;&nbsp; "</td>
-  </tr>
-  <tr>
-    <td class='c022'>Yellow</td>
-    <td class='c023'>535,000,000,000,000 &nbsp;&nbsp;&nbsp;&nbsp; "</td>
-  </tr>
-  <tr>
-    <td class='c022'>Green</td>
-    <td class='c023'>577,000,000,000,000 &nbsp;&nbsp;&nbsp;&nbsp; "</td>
-  </tr>
-  <tr>
-    <td class='c022'>Blue</td>
-    <td class='c023'>622,000,000,000,000 &nbsp;&nbsp;&nbsp;&nbsp; "</td>
-  </tr>
-  <tr>
-    <td class='c022'>Indigo</td>
-    <td class='c023'>658,000,000,000,000 &nbsp;&nbsp;&nbsp;&nbsp; "</td>
-  </tr>
-  <tr>
-    <td class='c022'>Violet</td>
-    <td class='c023'>699,000,000,000,000 &nbsp;&nbsp;&nbsp;&nbsp; "</td>
-  </tr>
-  <tr>
-    <td class='c022'>Extreme violet</td>
-    <td class='c023'>727,000,000,000,000 &nbsp;&nbsp;&nbsp;&nbsp; "</td>
-  </tr>
-</table>
-
-</div>
-<p class='c024'>Whatever theory we may adopt to explain the phenomena
-of light, we arrive at conclusions that strike
-the mind with astonishment and admiration. According
-to the corpuscular hypothesis, it was supposed that
-the molecules of light were endowed with the power of
-attraction and repulsion, that they possessed poles and
-centres of gravity like the earth, and that they had
-other physical properties that could only be given to
-ponderable matter. Starting with these notions, it is
-difficult to divest oneself of the idea of sensible size, or
-to induce the mind to conceive particles so extremely
-small as those of light would necessarily be if the theory
-of emission were accepted. If a particle of light weighed
-a grain, it would produce by means of its enormous velocity
-the effects of a cannon-ball weighing 120 lbs.,
-travelling at the rate of 300 yards per second. How
-infinitely small would be these particles, seeing that the
-most delicate optical instruments are submitted to their
-action for years without being injured!</p>
-
-<p class='c021'>If we are astonished at the extreme smallness and
-prodigious rapidity of the luminous molecules whose
-existence is necessitated by the corpuscular theory, the
-numerical results of the undulatory hypothesis are not
-less surprising. The extreme smallness of the distance
-between the waves, and the inconceivable quickness of
-their undulations, although both are easily calculated,
-must raise in the mind of the student feelings of the
-utmost wonder and admiration.</p>
-
-<p class='c021'><span class='pageno' id='Page_99'>99</span>Colour, then, simply results from the difference in
-the rate of vibration of the rays, as Professor Tyndall
-observes in his lectures on the “Analogy between Sight
-and Sound,” the impression of red being produced by
-waves that undulate a third less rapidly than those
-which produce the sensation of violet.</p>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div class='chapter'>
-  <span class='pageno' id='Page_100'>100</span>
-  <h2 id='ch2-04' class='c012'>CHAPTER IV.<br /><span class='c020'>LUMINOUS, CALORIFIC, CHEMICAL, AND MAGNETIC PROPERTIES OF THE SPECTRUM.</span></h2>
-</div>
-<p class='c014'><span class='sc'>The</span> solar spectrum may be compared to a battle-field
-with an army drawn up upon it ready for action. In
-the centre we find the luminous rays, on one side the
-light troops which produce chemical effect, and on the
-other the heating rays, which may be compared to squadrons
-of heavy cavalry. Close by the light brigade are
-the magnetic rays, which are a corps of skirmishers,
-sometimes appearing, and at others hiding themselves
-from view in a very mysterious manner.</p>
-
-<p class='c021'>But to drop metaphor, we shall find on examination
-of the spectrum that the three great forces—heat, light,
-and chemical effect—are regularly distributed over three
-different portions of this wonderful band of colour.</p>
-
-<p class='c021'>Before Fraunhofer the intensity of the light of different
-parts of the spectrum remained undetermined
-with any degree of accuracy; but this philosopher, by
-the use of a very delicate photometer, obtained the results
-given below.</p>
-
-<p class='c021'>The maximum of luminous effect is situated just at
-the junction of the yellow and orange. Taking this
-spot as its starting-point, it gradually decreases on each
-side until it ceases altogether at the extreme red and
-violet.</p>
-
-<p class='c021'>With respect to the calorific portion of the spectrum
-<span class='pageno' id='Page_101'>101</span>it was for a long time supposed that the heat-giving
-properties of any part were in direct proportion to the
-amount of its luminous effect; but Sir John Herschel
-proved by a long series of experiments that the heat of
-the spectrum gradually increased from the extreme
-violet to the extreme red, and that passing this point it
-still further increased until it attained its maximum at
-a point where not a single ray of light existed. From
-these grand experiments he adduced the important conclusion,
-that in solar light there existed invisible rays,
-which produced heat, and which possessed even a less
-degree of refrangibility than the extreme red rays. Sir
-John Herschel then tried, but unsuccessfully, to determine
-the exact refrangibility of the invisible heat
-rays.</p>
-
-<p class='c021'>Sir Henry Englefield compared these results, and
-obtained the following figures:——</p>
-<div class='std-table'>
-
-<table class='table4' summary=''>
-<colgroup>
-<col width='44%' />
-<col width='55%' />
-</colgroup>
-  <tr>
-    <td class='c022'>Blue</td>
-    <td class='c023'>56 deg. Fahr.</td>
-  </tr>
-  <tr>
-    <td class='c022'>Green</td>
-    <td class='c023'>58&nbsp;&nbsp;&nbsp;&nbsp; "</td>
-  </tr>
-  <tr>
-    <td class='c022'>Yellow</td>
-    <td class='c023'>62&nbsp;&nbsp;&nbsp;&nbsp; "</td>
-  </tr>
-  <tr>
-    <td class='c022'>Red</td>
-    <td class='c023'>72&nbsp;&nbsp;&nbsp;&nbsp; "</td>
-  </tr>
-  <tr>
-    <td class='c022'>Beyond the red</td>
-    <td class='c023'>79&nbsp;&nbsp;&nbsp;&nbsp; "</td>
-  </tr>
-</table>
-
-</div>
-<p class='c024'>Bérard obtained similar results, but he at first found
-that the maximum of heat was just at the end of the
-extreme red, and that beyond it the air was only about
-one-fifth warmer than the ordinary temperature. Sir
-John Herschel attributed these discordant results to
-Bérard having used a thermometer with too large a
-bulb; he accordingly repeated his experiments with
-other instruments with long narrow bulbs, and arrived
-at similar results to those obtained by the English
-philosopher.</p>
-
-<p class='c021'>We will now pass on to the physical properties of the
-other end of the spectrum. Towards the end of the last
-century, Scheele, a Swedish philosopher, remarked that
-<span class='pageno' id='Page_102'>102</span>chloride of silver was blackened more quickly by the
-violet portion of the spectrum than by any other. In
-1801, Ritter of Genoa, in repeating certain experiments
-made by Herschel, found that a much stronger blackening
-effect was produced at a point beyond the violet,
-and that the discoloration was produced with less intensity
-by the violet and still less so by the blue, the
-change gradually decreasing till the red ray was reached.
-He also found that when slightly blackened chloride of
-silver was exposed to the effects of the red rays, or
-even in the space beyond, its colour was restored to it.
-From these facts he drew the conclusion that in the
-solar spectrum there existed two kinds of rays, one at
-the red extremity, which favoured oxygenation; the
-other, at the blue end, which possessed the contrary
-properties. He also found that when phosphorus was
-placed in the invisible rays beyond the red, it gave off
-fumes of oxide, which were immediately extinguished
-when it was transferred to the other end.</p>
-
-<p class='c021'>On repeating the experiment with chloride of silver,
-Lubeck found that the tint varied according to the
-colour in which it was placed. Beyond or in the violet
-ray it became brownish red, in the blue it became
-bluish or bluish grey, in the yellow it remained white,
-or became slightly yellow and reddish in or beyond the
-red ray. When he used prisms of flint glass, the chloride
-of silver was discoloured beyond the visible limits
-of the spectrum.</p>
-
-<p class='c021'>Without being aware of Ritter’s experiments, Dr.
-Wollaston obtained the same results by acting on
-chloride of silver with violet light. In continuing his
-researches he discovered that gum guaiacum was also
-influenced by the chemical rays of light.</p>
-
-<p class='c021'>The magnetic influence supposed to be exerted by the
-solar rays still remains without positive proof, although
-numbers of philosophers have experimented in this
-<span class='pageno' id='Page_103'>103</span>direction. More than fifty years ago Dr. Morichini
-announced that the violet rays of the solar spectrum
-possessed the property of magnetizing steel needles
-that were previously free from magnetism. He produced
-this effect by concentrating the violet rays upon
-one-half of each needle with a convex lens, taking care
-to keep the other half concealed beneath a screen.
-After having continued this experiment for more than
-an hour, the needles were found to be quite magnetic.</p>
-
-<p class='c021'>Dr. Somerville tested Morichini’s experiments by
-covering one-half of an unmagnetized needle an inch
-long with a piece of paper, and exposing the uncovered
-half to the violet rays of the spectrum, and found that
-the needle became magnetic in the course of a couple
-of hours, the exposed end being the north pole. The
-indigo rays produced almost the same effect, but the
-blue and green rays were much less powerful. When
-the needle was exposed to the yellow, orange, red, and
-invisible rays beyond the red, no magnetic effect was
-produced, although the experiment was continued for
-three days. Pieces of chronometer and watch springs
-were submitted to the same influences with a similar
-result; but when the violet rays were concentrated upon
-the needles and pieces of spring with a lens, the time
-necessary for magnetizing them was greatly reduced.</p>
-
-<p class='c021'>Baumgartner of Vienna and Christie of Woolwich
-also repeated these experiments. The latter philosopher
-found that when a needle of magnetized steel, copper,
-or even glass, vibrated by force of torsion in the rays
-of the sun, the arc of vibration diminished much more
-quickly than when the experiment was conducted in the
-shade. The sun’s rays appeared to have the greatest
-effect upon the magnetized needle. From these results
-Christie concluded that the solar rays were capable of
-exerting a certain amount of magnetic influence.</p>
-
-<p class='c021'>These experiments were afterwards fully confirmed
-<span class='pageno' id='Page_104'>104</span>by those of Barlocci and Zantedeschi. The former
-found that a natural magnet which was capable of supporting
-a pound weight, had its power almost doubled
-by exposure to strong sunlight for four-and-twenty
-hours. Zantedeschi exposed a magnet which would
-carry fifteen ounces to the sun for three days, and increased
-its power two and a half times. These experiments
-seem almost to decide the fact of the power of
-white and violet light to induce magnetic force; but a
-series of researches by a philosopher who without doubt
-is greater than any of those already mentioned, seems
-to throw some doubt on the facts we have related above.</p>
-
-<p class='c021'>Before concluding, we must add a few more facts relating
-to the existence of invisible rays at both ends of
-the spectrum. “The visible portion of the spectrum,”
-says Dr. Tyndall, in one of his Royal Institution lectures,
-“simply marks an interval of radiant action, the
-rays existing in which bear such a relation to our visual
-organs, as to be capable of exciting in them the sensation
-of light. Beyond this interval, in both directions,
-right and left, the radiant action continues to exercise
-itself, but the rays emitted are dark, in consequence of
-their exerting no influence on our eye. Those that exist
-beyond the red ray are capable of producing heat,
-while those that are beyond the violet excite chemical
-action. These invisible violet rays can be actually
-made perceptible to the eye, or, in other words, the undulations
-or waves proceeding from this end of the
-spectrum can be made to strike against certain substances
-and induce luminous vibrations, so as to connect
-the dark space beyond the violet with a brilliantly illuminated
-band. I have here a substance capable of
-effecting this change. The lower half of this sheet of
-paper has been moistened with a solution of sulphate
-of quinine, the other half being left in its ordinary
-condition. I will now hold the paper in such a manner
-that the line that separates the prepared half from the
-<span class='pageno' id='Page_105'>105</span>other shall cut the spectrum in two halves horizontally.
-The upper half will remain unaltered and may be
-readily compared with the lower half, upon which you
-will see the spectrum prolonged beyond its ordinary
-limits. The effect produced is the addition of a splendid
-band of fluorescent light, which extends over a space of
-several inches, which but an instant before was a dark
-mass. I withdraw the prepared paper, and the light
-disappears; I replace it, and the light shines forth once
-more; showing us in the most brilliant way that the
-visible limits of the ordinary spectrum are not the limits
-of radiant action.</p>
-
-<p class='c021'>“I plunge a pencil into the solution of sulphate of
-quinine, and I pass it over the paper. You see that
-wherever the solution falls, the light bursts forth. The
-existence of these rays has been known for a long time.
-Young was familiar with them, and subjected them to
-experiment; but it is to Professor Stokes that we are
-indebted for a complete series of researches on this subject.
-It was he who first made those invisible rays
-visible, as we have done.”</p>
-
-<p class='c021'>In the same way the Professor proceeded to show that
-the heat rays were invisible by passing a beam of sunlight
-through a solution of iodine in spirits of wine,
-which, although it completely stopped all light, allowed
-the heat rays to pass uninterruptedly. By collecting
-these invisible rays into a focus by means of a lens,
-Dr. Tyndall was enabled to ignite various combustible
-bodies.</p>
-
-<p class='c021'>Thus we see the reason why certain rays produce
-certain effects on the eye, each particular degree of refraction
-causing a different set of vibrations, resulting
-in a different sensation for every part of the spectrum,
-and reproducing the effect of various colours on the
-optic nerve. In the following chapters we shall conclude
-our account of the different colours in the spectrum
-and of the laws of light.</p>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div class='chapter'>
-  <span class='pageno' id='Page_106'>106</span>
-  <h2 id='ch2-05' class='c012'>CHAPTER V.<br /><span class='c020'>THE LAWS OF REFLECTION.—MIRRORS.</span></h2>
-</div>
-<p class='c014'><span class='sc'>When</span> a ray of light falls obliquely on any polished
-surface, as that of a mirror, a piece of water, a plate
-of burnished metal, or any other reflecting substance,
-the ray, like an elastic ball, is immediately projected in
-a contrary direction to that in which it fell. Moreover,
-the direction in which it is reflected is at right angles
-to the surface, and in the same plane as that of the ray
-in the first instance. This experiment may be tried
-very easily, and will show the reason for the two following
-laws.</p>
-
-<p class='c021'>1. The angle of incidence is equal to the angle of reflection,
-and <i>vice versâ</i>.</p>
-
-<p class='c021'>2. Reflection can only take place in one direction—in
-that of the incident rays, both of which are always
-in a plane perpendicular to the reflecting surface.</p>
-
-<p class='c021'>The following figure will assist the student in performing
-experiments on the reflection of light from flat
-surfaces.</p>
-
-<p class='c021'>The ray <span class='fss'>A B</span> falling obliquely on the horizontal
-mirror, is reflected upwards at the same angle in the
-direction <span class='fss'>B C</span>. This may be proved geometrically by
-placing a graduated circle in a vertical position in the
-plane <span class='fss'>A B C</span>, when we shall find that the angle <span class='fss'>A B D</span>
-formed by <span class='fss'>A B</span> (the incident ray) with the perpendicular
-<span class='fss'>D B</span> is equal to the angle formed by this perpendicular
-line and the reflecting ray <span class='fss'>B C</span>. You may also prove
-<span class='pageno' id='Page_107'>107</span>in the same way that these three lines are all in the
-same vertical plane.</p>
-
-<div id='i107'  class='figcenter id014'>
-<img src='images/i107.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 13.</span>—Reflection from Plane Surfaces.</p>
-</div>
-</div>
-
-<p class='c021'>Let us now examine the effects of light reflected
-from plane surfaces. We must first, however, notice a
-certain optical illusion to which we are continually
-falling a prey, almost without our knowledge. We
-always fancy objects to be in reality in the place where
-we see them, and, in spite of our having already enumerated
-a large number of these deceptions, we must
-still add one more to the list. In reality we rarely see
-objects in the place where they really are; for if by
-the effect of reflection, refraction, or any other cause,
-the rays of light are made to deviate from their course,
-we no longer see the object from which they proceed in
-its real position, but in the direction taken by the luminous
-pencil at the moment of entering the eye.</p>
-
-<div id='i108'  class='figcenter id015'>
-<img src='images/i108a.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 14.</span>—Refraction.</p>
-</div>
-</div>
-
-<div id='i108b'  class='figcenter id016'>
-<img src='images/i108b.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 15.</span>—Experimental Proof of Refraction.</p>
-</div>
-</div>
-
-<p class='c021'>For instance, if the ray <span class='fss'>A B</span> is bent during its passage
-to the eye at <span class='fss'>B</span>, and consequently reaches it in the
-<span class='pageno' id='Page_108'>108</span>direction <span class='fss'>B C</span>, it is at <span class='fss'>A´</span>, and not at <span class='fss'>A</span>, that we shall see
-the object from which it proceeds. Every ray of light
-which passes out of a medium of a certain density into
-another of a different density is bent from its primary
-course, or, in scientific language, it is refracted. The
-experiments we made in a former chapter on the
-properties of the prism are founded on this principle.
-<span class='pageno' id='Page_109'>109</span>The law may be easily illustrated by allowing a ray of
-light to fall upon the surface of a vessel of water, as
-shown in the preceding figure.</p>
-
-<div id='i109'  class='figcenter id017'>
-<img src='images/i109.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 16.</span>—The Effects of Plane Mirrors.</p>
-</div>
-</div>
-
-<p class='c021'>The light of the stars and planets undergoes a similar
-deviation when passing in its course through the earth’s
-atmosphere; and at the moment we see the rising of
-the sun, the moon, or a star, they are in reality still
-below the horizon. Our eyes consequently are still
-deceiving us, no matter what part of the domain of optics
-we may enter.</p>
-
-<p class='c021'><span class='pageno' id='Page_110'>110</span>There are two kinds of mirrors—plane and curved.
-We will first examine the properties of the former sort,
-being those which are ordinarily applied to the usages
-of every-day life.</p>
-
-<div id='i110'  class='figcenter id018'>
-<img src='images/i110.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 17.</span>—Reflection from the Surface of Water.</p>
-</div>
-</div>
-
-<p class='c021'>In the figure in the preceding page we have a young
-lady looking at her reflection in a tall cheval glass.
-Every point upon the surface of her clothes and face is
-reflected back to her eye from the surface of the tin
-amalgam which has been applied to the back of the mirror
-by the looking-glass maker, for the purpose of rendering
-the image of the object more brilliant than if the
-glass alone were used. The rays which proceed from
-every one of these points strike upon the surface of this
-metallic layer, are stopped by its opacity, and are reflected
-back to the eye at an angle equal to that at which
-they strike the surface. The image seen by the eye is
-formed, consequently, by the reflection of every one of
-<span class='pageno' id='Page_111'>111</span>these rays; and as we always see objects in the direction
-taken by the luminous ray at the moment it enters
-the eye, we fancy we see objects before us that are
-really behind, or on each side of us. For instance, the
-ray starting from the left foot of the young lady in the
-figure is reflected from the point indicated on the surface
-of the glass, but the eye does not stop here, but
-sees the foot at an equal distance beyond the mirror.</p>
-
-<p class='c021'>The same thing takes place, not only with glass, but
-with all substances having polished surfaces. Still water,
-which to all intents and purposes has a polished surface,
-reflects the objects within its range as perfectly as
-a mirror.</p>
-
-<p class='c021'>The preceding observations apply to all plane reflecting
-surfaces; but there are other sorts of mirrors,
-whose effects are of a more interesting nature, and
-which we must hasten to describe—we allude to those
-whose surfaces are either convex or concave.</p>
-
-<p class='c021'>Curved mirrors are made of a great variety of shapes,
-but for the present we shall only describe those which
-are spherical. Spherical mirrors may of course be
-either concave or convex.</p>
-
-<div id='i111'  class='figcenter id019'>
-<img src='images/i111.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 18.</span>—Concave Mirror.</p>
-</div>
-</div>
-
-<p class='c021'>Suppose the arc <span class='fss'>M N</span> (<a href='#i111'>fig. 18</a>) to be movable round
-the point <span class='fss'>O</span>, this revolution will describe the surface of
-the mirror. The central point <span class='fss'>C</span> of the hollow sphere
-<span class='pageno' id='Page_112'>112</span>of which the mirror forms part, is called the centre of
-curvature, the line <span class='fss'>O L</span> the principal axis. By remembering
-these very simple definitions, we shall be able
-to understand the action of these mirrors without the
-slightest difficulty.</p>
-
-<p class='c021'>To understand how the rays of light are reflected
-from the surface of the mirror <span class='fss'>N M</span> at the point <span class='fss'>F</span>, which
-is called the focus, we have only to consider the mirror
-as consisting of an infinite number of facets, all inclined
-towards that particular point, and forming by reason of
-their immense numbers a regular spherical surface. In
-considering the mirror from this point of view, we can
-immediately see that, on account of the inclination of
-the supposed facets, the rays that they receive are all
-reflected back again at the same point; and it may be
-proved geometrically, that when the incident rays are
-parallel the focus will be situated somewhere on the line
-<span class='fss'>O C</span>, its position depending on the curvature of the
-mirror.</p>
-
-<p class='c021'>If, therefore, we receive on a spherical mirror a pencil
-of sunlight, the rays which compose it may be regarded
-as parallel, the sun being at so great a distance from the
-earth; it follows that these rays will all be reflected
-together in a particular point, viz., at <span class='fss'>F</span>, and if any
-object be placed there it will be illuminated with great
-brilliancy. The laws governing the reflection of heat
-being nearly similar to those regulating the action of
-light, the rays reflected from a burning body will ignite
-any inflammable substance placed at the point <span class='fss'>F</span>. The
-focus for parallel rays is called the principal focus of a
-mirror. Having described the effects of parallel rays,
-let us now see what happens when the source of light is
-close to the mirror. If it is placed at a very small
-distance, the luminous rays are divergent instead of
-parallel, and their meeting point becomes changed in
-accordance with the laws laid down at the beginning of
-<span class='pageno' id='Page_113'>113</span>this chapter. That is to say, the focus will approach
-more or less to the centre of curvature <span class='fss'>C</span>, according as
-the source of light is placed nearer to or further from the
-mirror; consequently, in the case of the candle in <a href='#i113'>fig. 19</a>,
-instead of uniting at <span class='fss'>F</span>, the rays will meet at <i>f</i>, a
-point situated somewhat nearer the mirror than the
-principal focus. If, instead of placing the light at <span class='fss'>A</span>, we
-place it at <i>f</i>, we shall find the rays will be concentrated
-at the point <span class='fss'>A</span>. Thus the foci are consequently related
-to each other, and are hence called <i>conjugate foci</i>. It
-will be readily seen that a spherical mirror may have an
-infinite number of conjugate foci, according to the distance
-of the source of light. It is also clear, that if we
-cause the light to approach the mirror, the focus will
-also approach it.</p>
-
-<div id='i113'  class='figcenter id020'>
-<img src='images/i113.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 19.</span>—Conjugate Foci.</p>
-</div>
-</div>
-
-<p class='c021'>Continuing our experiment, we shall find that when
-the candle passes the principal focus so as to be between
-it and the mirror, the reflected rays first become parallel
-and then divergent, and cannot consequently produce
-any focus beyond the mirror, but are reflected in the
-way shown in <a href='#i115'>fig. 20</a>.</p>
-
-<p class='c021'>In experimenting on the plane mirror, we imagined
-we saw the object at a certain distance behind it; the
-same thing happens when we see ourselves reflected in
-<span class='pageno' id='Page_114'>114</span>a concave mirror, and the particular point at which we
-suppose we see our reflection is called the virtual focus.</p>
-
-<div id='i114'  class='figcenter id021'>
-<img src='images/i114a.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 20.</span>—Virtual Focus.</p>
-</div>
-</div>
-
-<p class='c021'>If instead of a candle we place our head before a
-concave mirror, we shall see ourselves magnified as in
-<a href='#i114b'>fig. 21</a>.</p>
-
-<div id='i114b'  class='figcenter id022'>
-<img src='images/i114b.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p>Fig. 21.—Concave Mirror.</p>
-</div>
-</div>
-
-<p class='c021'>We shall easily see how this happens by tracing the
-paths of the rays in <a href='#i115'>fig. 22</a>.</p>
-
-<div id='i115'  class='figcenter id021'>
-<span class='pageno' id='Page_115'>115</span>
-<img src='images/i115a.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p>Fig. 22.—Magnifying effect of Concave Mirrors.</p>
-</div>
-</div>
-
-<div id='i115b'  class='figcenter id021'>
-<img src='images/i115b.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p>Fig. 23.—The Reversal of real Images.</p>
-</div>
-</div>
-
-<p class='c021'>The rays, for instance, which proceed from the forehead
-at the point <i>a</i> are reflected from the point <i>o</i> to the
-<span class='pageno' id='Page_116'>116</span>eye in such a way as to appear to proceed from a point
-beyond the mirror, <span class='fss'>A</span>. In the same manner the rays reflected
-from the chin appear to take their origin from
-the point <span class='fss'>B</span>. If, on the other hand, we place ourselves
-at a distance from the principal focus, we shall produce
-a reversed and diminished image of our face. This
-image is not illusory, like the preceding ones, but is
-real, and may be received upon a screen, as shown in
-<a href='#i115b'>fig. 23</a>.</p>
-
-<p class='c021'>We may easily follow the path of the rays as shown
-in the figure, and we shall see that the rays forming
-the images of the church-tower and the terrace below,
-cross at a certain point.</p>
-
-<p class='c021'>Convex mirrors produce precisely opposite effects, and
-give a diminished image instead of a magnified one, as
-may be perceived on examining <a href='#i116'>fig. 24</a>.</p>
-<div id='i116'  class='figcenter id021'>
-<img src='images/i116.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 24.</span>—Diminishing power of Convex Mirrors.</p>
-</div>
-</div>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div class='chapter'>
-  <span class='pageno' id='Page_117'>117</span>
-  <h2 id='ch2-06' class='c012'>CHAPTER VI.<br /><span class='c020'>METALLIC BURNING MIRRORS.</span></h2>
-</div>
-<p class='c014'><span class='sc'>The</span> classical student will remember that Archimedes
-burned the fleet of Marcellus, by means of burning-glasses,
-from the heights of the fortifications of his
-native city of Syracuse. Unfortunately, any account of
-the system of catoptrics, or the science of reflections,
-employed by the ancient Syracusan in their construction
-is lost to us, and many modern writers have
-gone so far as to doubt the fact altogether. The
-knowledge of the properties, however, of concave
-mirrors which we have just been acquiring, will enable
-us to form a pretty good guess as to the means adopted
-by Archimedes for the destruction of the enemy’s fleet.
-The ancients, not having the means of either casting or
-grinding such enormous mirrors, must have constructed
-them of a large number of small ones, so arranged
-that the images of the sun reflected by them would all
-fall in the same place, or nearly so. In this case, the
-larger the number of mirrors, the greater would be the
-burning effect. In order to explain the reflection of
-rays incident upon the surface of concave mirrors, we
-supposed them to consist of an immense number of
-plane mirrors placed in a curve, so that the reflected
-rays might all meet in one point; but on examining
-into the history of burning mirrors, we find that the
-plan has been adopted in reality in a great number of
-<span class='pageno' id='Page_118'>118</span>instances. We have also said, that the reflection of the
-heating rays was governed by similar laws to those
-influencing the rays of light; consequently, by directing
-a pencil of sunlight upon the surface of a concave
-mirror, we obtain the maximum of light and heat at
-the focal point.</p>
-
-<p class='c021'>Many modern writers give the ancients too little
-credit for their knowledge of optical principles, and
-late investigations seem to prove that the old school of
-philosophers were much more learned in these matters
-than has been generally supposed. The discovery of a
-rock crystal double convex lens in an Egyptian tomb of
-great antiquity is an instance of this. Descartes wrote
-a little treatise to prove that the stories related of the
-burning mirrors of Archimedes were pure fabrications,
-although many Latin authors have described them both as
-being used by that philosopher and in more modern
-times; Dion, for instance, who lived in the early part
-of the sixth century, states that at the siege of Constantinople,
-Proclus burnt the fleet of Vitalian with
-mirrors of brass; but the opinion of Descartes seemed
-to outweigh all other testimony. Buffon, who wished
-to sift the matter thoroughly, constructed for himself,
-after many previous experiments on the laws of reflection,
-a series of mirrors that closely imitated those
-ascribed to Archimedes. His first memoir, “On the
-Invention of Mirrors capable of burning at a great Distance,”
-was published in the Transactions of the French
-Academy of Sciences for 1747. A few years later he
-combated both theoretically and practically the opinion
-of Descartes, in a memoir containing an account of an
-immense number of experiments. Before speaking of
-the extraordinary effects of burning mirrors, it will be
-as well to do justice to the predecessors of the learned
-naturalist we have just mentioned, by quoting a passage
-from the works of Father Kircher, who, 128 years previously,
-<span class='pageno' id='Page_119'>119</span>experimented in this direction with great patience
-and perseverance, and tried to prove that the
-stories related of Archimedes were true. “The larger
-the surface of a mirror,” says this philosopher (who, like
-Huyghens, was a practised astronomer), “the more light
-it reflects from the objects opposite to it. If it is only
-a foot square, it will throw a square foot of light upon
-any wall or screen placed before it. Experiment shows
-that this light is composed of an infinite number of rays
-reflected from different points on the surface of the mirror.
-Direct the rays from a second mirror upon the
-same place as those from the first, and the light and
-heat will clearly be doubled. They will become trebled
-if you direct the rays from a third mirror upon the
-same spot, and so on <i>ad infinitum</i>. In order to prove
-that the intensity of the light and heat is in direct proportion
-to the number of reflecting surfaces employed,
-I took five mirrors, and found that on exposing them to
-the sun I obtained with only one, less heat and light
-than if I used direct sunlight. With two the light and
-heat increased considerably; three gave as much heat
-as an ordinary fire, and four gave me a still greater
-effect. I therefore concluded that by multiplying these
-plane mirrors, I not only obtained greater effects than
-those got by using parabolic, hyperbolic or elliptical mirrors,
-but that I could use them upon objects at a much
-greater distance. With five mirrors I could obtain
-these effects at a distance of 100 feet, but what terrible
-phenomena would have taken place had I used one
-thousand instead of five?” He ends by begging mathematicians
-to experiment in this direction with greater
-care than they had hitherto done.</p>
-
-<p class='c021'>After Kircher we may cite as an experimentalist with
-these terrible instruments the French philosopher Villette,
-who constructed several mirrors, in direct imitation
-of those of Archimedes, for Louis XIV. and other
-<span class='pageno' id='Page_120'>120</span>sovereigns. The <i>Journal des Savants</i> for 1679 gives
-an account of his principal metallic burning mirror in
-the most eulogistic terms, adding an instance of ignorance
-which is singularly quaint and curious. It is of
-the fourth and most perfect of Villette’s mirrors that
-the <i>Journal des Savants</i> speaks, the first having been
-bought by Tavernier, and presented to the Shah of
-Persia, who considered it as one of the rarest and most
-precious curiosities that he possessed: the second was
-sold to the King of Denmark, and the third was given
-by M. Villette to Louis XIV., from whom he received
-the praises and rewards that were due to his talent and
-perseverance. “It was thirty-four inches in diameter,
-and vitrified flints and bricks almost instantaneously,
-no matter how large they were. It consumed the greenest
-wood, burning it to ashes in an instant, and fused
-the most refractory metals with equal ease and quickness.
-Steel, no matter how hard, resisted its power no
-more than other metals, and melted so quickly that one
-part burnt away in inconceivably brilliant sparks, some
-of them forming stars as large as a franc piece, leaving
-a flowing mass of metal behind. The last made by
-Villette was still more powerful, being larger and more
-carefully made. It was forty-four inches in diameter,
-and three inches and a line deep. Its burning point, or
-focus, was situated at a distance of three feet seven
-inches from the surface, and was apparently as large as
-a five-sou piece; and it was at this spot, where the rays
-of light and heat were concentrated into so small a
-space, that the wonderful effects of its violent power
-became manifest, the spot of light being of such brilliancy
-that the eyes could no more withstand its brightness
-than that of the sun. Besides the property of
-burning which it possessed in so wonderful a degree, it
-was capable of exhibiting other effects just as curious
-as those already related. It had the power of sending
-<span class='pageno' id='Page_121'>121</span>the images of objects to a distance of fifteen feet or
-more, so that a man looking at himself in this mirror
-with a stick or sword in his hand, saw the image of them
-suspended in the air, apparently ready to strike the observer.
-On seeing such an effect for the first time, the
-observer could hardly fail to experience the greatest
-surprise, and even fear; and it is stated that the king
-having placed himself, sword in hand, before one of
-these mirrors, in order to observe the effect, was surprised
-to find himself face to face with an armed hand
-apparently directed against him. When he advanced,
-the hand seemed to spring forward to meet him. The
-king could not conceal his surprise and fright, and afterwards
-felt so ashamed at being terrified with a mere
-shadow that he ordered the mirror to be taken away,
-and could never be prevailed upon to look into it again.”
-The <i>Journal des Savants</i> then goes on quaintly to remark
-on the various startling effects produced by these
-mirrors, winding up by stating that its powers of reflection
-were so great, that at night the light of a torch or
-flambeau was reflected so perfectly that an observer
-placed at four hundred feet distant could read the smallest
-print.</p>
-
-<p class='c021'>It also mentions a curious piece of superstition on
-the authority of a scientific writer of the name of Robertson,
-who states that it happened at Liége. In reading
-the accounts of these experiments we can see how
-easily the minds of individuals were affected in those
-days by the wonderful. It happened while one of Villette’s
-mirrors was at Liége, that the latter end of the
-summer was somewhat rainy, and great fears were entertained
-that a bad harvest and dear bread would be
-the result. Certain evil-minded people, who had taken
-a fancy to the mirror and wished to possess it by unfair
-means, spread the report that the continual rain was
-entirely caused by its action on the clouds and sun, and
-<span class='pageno' id='Page_122'>122</span>that the coming famine must be laid upon the shoulders
-of its owner and inventor. This absurd idea took such
-forcible possession of the minds of the populace of
-Liége, that great mobs collected together, uttering all
-kinds of maledictions against the mirror and its inventor,
-and at last became so violent that they attacked
-Villette’s house with the intention of smashing his great
-work, and administering to the unfortunate philosopher
-the chastisement they supposed he deserved. Happily,
-however, for M. Villette and his mirror, Liége was
-governed in those days by the Prince Bishop of Cologne,
-who was a man of great enlightenment. He
-put the crowds round M. Villette’s to flight by armed
-force, but he found that the conviction that all the coming
-mischief would result from the unlucky mirror was
-so strong, that he was obliged to issue a pastoral peremptorily
-declaring that the idea had originated with a
-number of malicious people, who spared no pains to
-propagate it for their own bad purposes, and that it
-was a mischievous and dangerous error to ascribe to
-a mirror a power which only belonged to the Almighty.</p>
-
-<p class='c021'>In 1747, Buffon performed many extraordinary experiments
-with burning mirrors, which were more surprising
-than any that had hitherto been described.
-They were mostly performed at the <i>Jardin des Plantes</i>,
-at Paris, of which institution Buffon was director; and
-many of them are worth describing.</p>
-
-<div id='i124'  class='figcenter id021'>
-<img src='images/i124.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 25.</span>—Burning Mirror.</p>
-</div>
-</div>
-
-<p class='c021'>On the 3rd of April, at about two o’clock in the afternoon,
-the great mirror was mounted on its stand, and
-was found to be capable of setting a plank of wood on
-fire at a distance of 138 feet, when 128 glasses were
-used, although the light was weak at the time, and the
-sun was covered with mist. In pursuing these experiments
-great care had to be taken to prevent the by-standers
-placing themselves within range of its terrible
-power, for several were nearly blinded by looking
-<span class='pageno' id='Page_125'>125</span>at the brilliant focal point of the instrument. The
-next day, at eleven in the forenoon, although the sun
-was still covered with mist and fleecy clouds they were
-able to produce such a heat at 150 feet distant, with
-154 glasses, that a pitched plank began to smoulder
-and would have burnt into flame had not the sun disappeared
-at that particular moment. On the fifth of
-April, at three in the afternoon, with the light much in
-the same weak condition as it was on the other days,
-they succeeded in igniting at 150 feet distant, a heap of
-shavings of deal mixed with charcoal and sulphur,
-in less than a minute and a half, with 154 glasses.
-When, however, the sun shone with its natural power,
-a few seconds were sufficient to effect these results.</p>
-
-<p class='c021'>On the 10th, when the sun was shining pretty powerfully,
-a pitched pine plank was easily fired with 128
-glasses, at 150 feet distant. In this case the ignition
-was very sudden, and extended over the whole of the
-radiant spot forming the focus, which at the distance
-named measured 16 inches in diameter. The same day
-at half-past two, a pitched elm plank covered in some
-places with chopped wood, was set fire to with extreme
-rapidity, and burnt with such violence that it had to
-be dipped in water before it could be put out. In
-this experiment 148 glasses were used, at a distance of
-150 feet.</p>
-
-<p class='c021'>On the 11th of April, the burning point was fixed at
-20 feet distant from the mirror, and combustible substances
-were easily burnt with only 12 glasses. With
-21 glasses a half-burnt elm plank was set fire to, and
-with 45 a piece of tin weighing six pounds was almost
-immediately melted. Silver sheet was fused, and an
-iron plate was made red-hot with 117 glasses. In
-giving an account of these interesting experiments,
-Buffon expresses his conviction that at 50 feet it would
-have been easy to have melted metals if all the glasses
-<span class='pageno' id='Page_126'>126</span>of the mirrors had been used. When used at that distance,
-the burning spot was six to seven inches in
-diameter. He also noticed that when metals were
-melted, part of them were dissipated in brilliant vapour,
-which was so thick as to cast a shadow on the ground,
-although it seemed to be as bright as the sun itself.
-When the sun was at its full strength, and all the
-glasses were brought into requisition, wood was set on
-fire at a distance of over 200 feet, and metals and minerals
-were fused at 40 and 50 feet. Hence the possibility
-of making and using these mirrors as Archimedes
-was said to have done, was proved practically by the
-great naturalist. <a href='#i124'>Fig. 25</a> represents a burning mirror
-in action.</p>
-
-<p class='c021'>Robertson, an English philosopher, residing in France
-during the days of the first Republic, reconstructed
-the mirrors described by historians as being used by
-Archimedes, and the results he obtained were thought
-sufficiently important by the Council of the Department
-of Ourthe to merit an attentive examination by two
-members of their body, who reported in favour of their
-being used as instruments of war.</p>
-
-<p class='c021'>It would be possible to pursue this subject still further,
-and give an account of numerous experiments
-made on burning mirrors by various philosophers, but
-we must not forget that it is light and heat that we
-have more especially to deal with in the present work.
-Already we have possibly strayed from our path a little
-too far, but the two influences are so closely connected
-with each other that it is almost impossible to speak of
-them separately when reflection is in question.</p>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div class='chapter'>
-  <span class='pageno' id='Page_127'>127</span>
-  <h2 id='ch2-07' class='c012'>CHAPTER VII.<br /><span class='c020'>LENSES.</span></h2>
-</div>
-<p class='c014'><span class='sc'>The</span> word lens is derived from the Latin name of the
-seed of the <i>Ervum lens</i>, or ordinary lentil. When eating
-this wholesome vegetable, almost every one has noticed
-that its shape is exactly that of a double convex
-lens, as represented in the following figure:—</p>
-
-<div id='i127'  class='figcenter id022'>
-<img src='images/i127.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 26.</span>—Double Convex Lens.</p>
-</div>
-</div>
-<p class='c025'>Perhaps it would be more correct if we were to say that
-a double convex lens is like a lentil, rather than turn
-the comparison the other way, seeing that this little
-seed has given its name not only to the particular-shaped
-glass depicted above, but also to some five others
-more or less analogous to it.</p>
-<p class='c021'>In <a href='#i128'>fig. 27</a> we have the different forms of lenses
-<span class='pageno' id='Page_128'>128</span>shown in section. The first is the <i>double convex lens</i>,
-the second the <i>plano-convex</i>, the third and sixth the
-<i>concavo-convex</i>, the fourth the <i>double concave</i>, and the
-fifth the <i>plano-concave</i>. A <i>crossed lens</i> is a double
-convex lens whose one side is more convex than the
-other. The third lens is also called <i>meniscus</i>.</p>
-
-<div id='i128'  class='figcenter id015'>
-<img src='images/i128.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 27.</span>—Forms of Lenses.</p>
-</div>
-</div>
-
-<p class='c021'>The properties of the first, second, and third are
-similar; that is to say, they cause parallel rays of light
-passing through them to converge at a certain point,
-called their focus; while the three others have a divergent
-action on rays passing through them. By examining
-the path of the rays through these lenses, we
-shall find that the first three magnify objects seen
-through them, while the latter have the contrary effect.</p>
-
-<p class='c021'>As in the case of the curved mirrors, the rays falling
-on the surface of a convex lens may be either parallel,
-divergent, or convergent. In the case of parallel rays,
-as depicted in the following figure, they are represented
-as meeting at a point beyond the lens, which is called
-the sidereal focus, or the focus for parallel rays. It is
-generally found by causing the image of the sun or of
-some distant object to be thrown by the lens upon a
-screen, or by knowing the curvature of the faces, and
-the refractive power of the glass.</p>
-
-<p class='c021'>Every ray on striking the surface of the lens is refracted
-inwards, until it meets with its companions at
-<span class='pageno' id='Page_129'>129</span>the focus <span class='fss'>F</span>, in accordance with the law of refraction,
-by which a ray of light passing from one transparent
-medium, such as air, to another which in this instance
-is glass, becomes refracted or bent in proportion to the
-relative density of the two mediæ. The nearer the ray
-passes to the edge of the lens, the more it is refracted,
-the angle of incidence being greater; the ray through
-the exact centre being uninfluenced by the form of the
-glass. Hence they all meet in a single point. Figs.
-29 and 30 show the path of the rays when they are
-divergent and convergent.</p>
-
-<div id='i129'  class='figcenter id021'>
-<img src='images/i129a.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 28.</span>—Path of a Ray through a Convex Lens.</p>
-</div>
-</div>
-
-<div id='i129b'  class='figcenter id021'>
-<img src='images/i129b.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 29.</span>—Path of divergent Rays through a Convex Lens.</p>
-</div>
-</div>
-
-<p class='c021'>If the rays of light are not parallel, as in the case
-of the source of light being near the lens, they do not
-converge so rapidly as when they proceed from a distant
-object, consequently the focus for near objects is
-longer in proportion to their distance. In <a href='#i129b'>fig. 29</a> for
-<span class='pageno' id='Page_130'>130</span>instance, if a candle be placed as shown, and a screen
-on the other side of the lens, a point will be found
-where the image of the candle is seen upon it in a reversed
-position. The distance between these two
-points is always relative, and they are called conjugate
-foci. Thus, the candle may change places with the
-screen with a similar effect, as long as the exact position
-of the two points is preserved. If the candle is placed
-farther off, we must diminish the distance between the
-screen and the lens, and <i>vice versâ</i>. In fact, the nearer
-the object, the longer the focus; the farther it is off,
-the shorter the focus. Half an hour’s experiment
-with a double convex lens, a piece of white cardboard,
-and a small candle, will teach the student more about
-the properties of convex lenses than a chapter of explanation.
-A common magnifying-glass, or even an
-old spectacle lens, will serve the purpose of more expensive
-instruments.</p>
-
-<div id='i130'  class='figcenter id021'>
-<img src='images/i130.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 30.</span>—Conjugate Foci.</p>
-</div>
-</div>
-
-<p class='c021'>We now proceed to speak of the images formed
-by lenses. In <a href='#i131'>fig. 31</a> we have a flower placed on one
-side of a lens. As it is not at an infinite distance, the
-rays sent out by its various parts are convergent, and
-not parallel, consequently they do not meet at the
-sidereal focus, but at a point beyond it, according to the
-rule already laid down. The rays proceeding from the
-exact centre of the flower striking the lens exactly in
-<span class='pageno' id='Page_131'>131</span>the middle at right angles, suffer no change, the others
-being refracted in proportion to their angles of incidence.</p>
-
-<div id='i131'  class='figcenter id021'>
-<img src='images/i131.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 31.</span>—Images formed by Convex Lenses.</p>
-</div>
-</div>
-
-<p class='c021'>The rays proceeding from the flower cross each other
-at a certain point: hence the image on the screen is
-reversed. The dimensions of the image will depend on
-the distance of the object from the lens. This is a fact
-we meet with every day, when using an opera-glass or
-a telescope. Images formed by convex lenses upon a
-screen are called by opticians <i>real images</i>, in contradistinction
-to those which are the result of mere reflection,
-as in the case of plane mirrors. These latter are known
-as <i>virtual images</i> and are produced by convex lenses
-as well as by plain reflecting surfaces. In <a href='#i132'>fig. 32</a>, for
-instance, the unreversed image of the insect seen by the
-eye is not a real image, but a virtual one,—a fact that
-might be easily proved by placing a screen in the position
-of the eye, when it would be found that no image
-would be formed.</p>
-
-<p class='c021'>When using an ordinary magnifying-glass we see the
-virtual image of the object we are looking at, but in the
-case of a telescope or opera-glass we see the real image
-of the object, formed by the large lens in front, and
-<span class='pageno' id='Page_132'>132</span>reversed again by the arrangement of small lenses next
-to the eye.</p>
-
-<div id='i132'  class='figcenter id015'>
-<img src='images/i132a.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 32.</span>—Magnifying Property of Convex Lenses.</p>
-</div>
-</div>
-
-<p class='c021'>Double concave lenses produce effects which are just
-the reverse of those we have been considering. Instead
-of increasing in thickness from the edges to the centre,
-they follow the contrary plan, and increase from the
-centre to the edges. Consequently, instead of the rays
-meeting at the focus, they diverge from each other, and
-gradually spread out, as shown in <a href='#i132b'>fig. 33</a>.</p>
-
-<div id='i132b'  class='figcenter id015'>
-<img src='images/i132b.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 33.</span>—Diminishing Effect of Concave Lenses.</p>
-</div>
-</div>
-
-<p class='c021'>The above figure shows the path of the rays proceeding
-from the vase, and meeting the eye at such an angle
-<span class='pageno' id='Page_135'>135</span>that the virtual image is greatly diminished. Concave
-lenses, as the student has no doubt already guessed, do
-not give real images.</p>
-
-<div id='i134'  class='figcenter id021'>
-<img src='images/i134.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 34.</span>—Cannon of the Palais Royal.</p>
-</div>
-</div>
-
-<p class='c021'>The effects produced by the action of concave mirrors
-may be produced with just as much facility by convex
-lenses. If a body is placed in a focus of a lens which
-receives the direct rays of the sun, the heat as well as
-the light will be concentrated at one point; and if the
-object is combustible, it will take fire sooner or later,
-according to the size of the lens. All the experiments
-mentioned by Buffon as being produced by a concave
-mirror are equally obtainable with a concave lens.
-When of sufficient diameter, the most refractory metals,
-such as platinum or iridium, may be melted and dissipated
-into vapour. Before lucifer matches and vesuvians
-were as common as they are now, it was not at
-all unusual to find smokers carrying a small burning-glass
-and a piece of tinder, for the purpose of lighting
-their pipes or cigars; and there hardly exists a boy who
-has not lighted a bonfire in the fields or playground by
-means of an old spectacle lens or telescope glass.</p>
-
-<p class='c021'>Amongst other applications of this property of lenses
-may be mentioned that of causing guns to fire at a
-certain time, by arranging a small burning-glass above
-the touch-hole. In the Gardens of the Palais Royal, at
-Paris, there is such a gun, so arranged that on sunny
-days it fires exactly at noon, or, in other words, at the
-moment the sun comes to the meridian. Every fine
-day towards twelve o’clock, crowds of Parisians who
-have nothing to do may be seen bending their steps
-towards the Palais Royal to set their watches by the
-gun, which they believe to be superior as a time-keeper
-to the finest chronometer in the world. There they
-stand, most of them old fellows with a scar or two about
-their faces, showing that they have nobly won the rest
-they appear to enjoy so innocently and calmly with
-<span class='pageno' id='Page_136'>136</span>watch in hand, leaning against the railings, and waiting
-with impatience the moment when true solar noon is
-indicated by the sharp report of the little piece. Their
-belief in the correctness of solar time is something
-astonishing; and if a bystander were to insinuate, no
-matter how delicately, that solar time varied slightly
-every now and then, he would either receive a smile of
-pitying contempt, or else he would be called out upon
-the spot. <a href='#i134'>Fig. 34</a> gives a pretty view of the celebrated
-cannon of the Palais Royal.</p>
-
-<div id='i136'  class='figcenter id023'>
-<img src='images/i136.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 35.</span>—Fresnel’s Lighthouse Apparatus.</p>
-</div>
-</div>
-
-<p class='c021'>We now come to another application of the refracting
-power of lenses, in the way of concentrating rays,
-which is infinitely more valuable to humanity than
-either of those we have just mentioned; we mean the
-construction of enormous refracting apparatuses for
-lighthouse purposes. The first lighthouse of which we
-<span class='pageno' id='Page_137'>137</span>have any record is that which was erected on the island
-of Pharos, by Ptolemy Philadelphus, in the year 470
-of the foundation of Rome. This was merely a tower,
-upon the top of which fires were kept burning at night;
-but as the world progressed, the blazing tar-barrel or
-wood fire gave place to the carefully-constructed lamp
-and silvered reflector apparatus, which are fast disappearing
-in their turn before the electric or Drummond light
-and the refracting apparatus constructed by Fresnel,
-who was the first to endeavour to abolish the old-fashioned
-and inefficient metallic mirror from the
-lanterns of lighthouses. <a href='#i136'>Fig. 35</a> shows a section of
-Fresnel’s apparatus. <span class='fss'>A</span> is a plano-convex lens of about
-a foot in diameter, whose focus corresponds with those
-of the concentric lenticular rings of glass which surround
-it, and which are seen more plainly in <a href='#i140'>fig. 36</a>.
-These rings, which are ground and polished with the
-greatest accuracy, are somewhat in the shape of an
-ordinary quoit, and are equivalent to a plano-convex
-lens with the centre portion cut out. This arrangement
-is so powerful that the distance at which a light provided
-with it can be seen is only limited by bad weather,
-the state of the atmosphere and the distance of the
-horizon. It is common for such lights to be seen at a
-distance of between fifty and sixty miles. The apparatus
-is mostly arranged in the form of an octagon,
-and is generally provided with additional reflecting
-mirrors at those parts above the light which are out of
-the range of the lenses. The light shining fully in eight
-directions at one time, can scarcely be missed by any
-ship within range; but in order to guard against any
-possibility of accident, the optical apparatus is often
-made to revolve by clockwork, so that every point of
-the ocean is illuminated in turn. By using coloured
-glasses, or by causing the light to disappear at distinct
-intervals, different lighthouses may be identified by
-<span class='pageno' id='Page_138'>138</span>ships that are out of their reckoning. <a href='#i140'>Fig. 36</a> represents
-the interior of the lantern of a first-class lighthouse,
-showing the arrangement of the lenticular rings
-round the central lens. If ever the student should
-pass through Havre, he should not miss the opportunity
-of seeing this noble apparatus, which is one of
-the finest ever manufactured.</p>
-<div id='i140'  class='figcenter id021'>
-<span class='pageno' id='Page_140'>140</span>
-<img src='images/i140.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 36.</span>—Lantern of a First-class Lighthouse.</p>
-</div>
-</div>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div class='chapter'>
-  <span class='pageno' id='Page_141'>141</span>
-  <h2 id='ch2-08' class='c012'>CHAPTER VIII.<br /><span class='c020'> OPTICAL INSTRUMENTS.—THE SIMPLE AND COMPOUND MICROSCOPE. THE SOLAR AND PHOTO-ELECTRIC MICROSCOPE.</span></h2>
-</div>
-<p class='c014'><span class='sc'>The</span> lenses and mirrors whose properties we have
-been considering in the previous chapters, have been
-combined in different ways for the purpose of examining
-objects too small or too distant to be perceived by
-the human eye. To instruments used for the former
-purpose the name of microscope has been given, from
-two Greek words signifying <i>small</i> and <i>to see</i>. In like
-manner the name of telescope is also derived from two
-Greek words, meaning <i>distant</i> and <i>to see</i>. Besides
-these two classes of optical instruments, others have
-been devised to facilitate the depicting of natural
-objects, either by means of the pencil or of photography,
-or to amuse the eye by optical illusions. Thus
-we have the camera obscura, the camera lucida, the
-magic lantern, the phantasmagoria, and numberless
-other instruments of the same sort, most of which will
-be described in the latter part of this book.</p>
-
-<p class='c021'>There are two sorts of microscopes, the simple and
-the compound; the one consisting of a single convex
-lens, and the other of several combinations of both
-convex and concave lenses.</p>
-
-<p class='c021'>When speaking of convex lenses, we described the
-properties of the ordinary magnifying-glass, or simple
-<span class='pageno' id='Page_142'>142</span>microscope. The uses of this instrument are almost too
-well known to need description. It is used by old
-people, the lenses of whose eyes have become flattened
-by old age, by watchmakers for examining the minute
-portions of their work, by jewellers for the same purpose,
-and by most people for examining maps, engravings,
-and photographs. Simple microscopes are
-generally mounted in horn, ivory, or metal handles for
-convenience’ sake. Some simple microscopes consist
-of two or more lenses mounted together in order to increase
-the magnifying power. The student must distinguish
-between several lenses mounted together in
-this way, and the true compound microscope, which is a
-comparatively complicated optical arrangement, as we
-shall see presently. When two single lenses are thus
-mounted together, the power of the combination is equal
-to the powers of each added together.</p>
-
-<p class='c021'>There is good reason for supposing that the simple
-microscope is a comparatively ancient invention. Seneca,
-who lived in the first century, declares that in his
-time it was well known that, when writing was looked
-at through a globe full of water, it appeared larger and
-blacker. In the eighth century we find the use of magnifying
-spectacles for old people common in most countries,
-and yet it was only at the beginning of the seventeenth
-century that a true optical instrument, in the
-form of a telescope, was invented. It only needed the
-placing of two magnifying glasses in a line to discover
-the principle of the telescope, but nearly a thousand
-years elapsed after the first introduction of these glasses
-before an accident rendered the principle evident.</p>
-
-<p class='c021'>In <a href='#i143'>fig. 37</a> we see the commonest form of microscope
-in the hands of an observer; and by examining the
-following figure and tracing out the path of the rays,
-we shall easily discover the principles on which its
-action depends.</p>
-
-<div id='i143'  class='figcenter id021'>
-<span class='pageno' id='Page_143'>143</span>
-<img src='images/i143.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 37.</span>—The Compound Microscope.</p>
-</div>
-</div>
-
-<p class='c021'>The object to be looked at is placed at <i>a</i> (<a href='#i144'>fig. 38</a>), on
-a piece of thin glass usually called a <i>slide</i>. A small
-converging lens placed at <i>b</i> collects the rays proceeding
-from the object, and transmits them as far as <i>c d</i>, where
-they come under the influence of a second converging
-<span class='pageno' id='Page_144'>144</span>lens <span class='fss'>B</span>, which causes them to spread out still more before
-they reach the eye. Consequently we not only
-see the image of the object magnified
-by the lens <i>b</i>, but still more enlarged
-by the action of the lens <span class='fss'>B</span>,
-and appearing considerably enlarged
-at <span class='fss'>C D</span>. The lens placed in front of
-the object is called the <i>objective</i> or
-<i>object-glass</i>; that placed nearest the
-eye, the <i>eye-piece</i>. These names
-apply equally to the similar lenses
-used in telescopes and other optical
-instruments. The instrument shown
-in <a href='#i144'>fig. 38</a> is the simplest possible
-compound microscope, and is very
-rarely used. The eye-piece is generally
-constructed of two lenses, and
-the object-glass of as many as eight;
-the object in multiplying the lenses
-being, not only to increase the magnifying
-power, but to decrease certain
-defects inherent in all lenses
-whose surfaces are parts of spheres.</p>
-
-<div id='i144'  class='figleft id024'>
-<img src='images/i144.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 38.</span>—The Theory of the Compound Microscope.</p>
-</div>
-</div>
-
-<p class='c021'>The amplification depends mainly
-upon the power of the objective,
-but different eye-pieces are also used
-to increase the apparent size of the
-objects to be examined. Thanks to
-the investigations of modern philosophers, we are enabled
-to magnify objects to 2,000 times their diameter
-with perfect distinctness; that is to say, the surface of
-the object appears to occupy 4,000,000 times its natural
-extent. Under such a power a hair would appear
-about six inches thick, a fine needle would look like a
-street post, and a grain of sand like a mass of rock.
-Although it is possible to employ compound microscopes
-<span class='pageno' id='Page_145'>145</span>of such a high magnifying power in the investigation
-of certain classes of objects, all ordinary preparations
-are best seen under a power of 500 or 600 diameters.
-It would be utterly impossible to give our readers the
-slightest idea of the benefits conferred on the human
-race by this marvellous instrument. Suffice it to say,
-that no naturalist or surgeon ever attempts the most
-simple investigation into the structure of any body
-without the aid of the microscope. It has already
-shown us that a world of creatures exists which, although
-invisible to the eye of man, are possessed of wonderful
-forms, colour, and beauty of structure, and is daily
-adding to our knowledge in this direction. We can
-hardly submit any substances to this marvellous instrument
-without discovering animal or vegetable life of the
-most vivid character. A drop of scum from the surface
-of a stagnant pool is instantly seen to be peopled with
-animal and vegetable life, when submitted to microscopic
-examination. At one moment a rolling ball glistening
-like glass slowly revolves past our view; then a little
-fellow like a piece of spiral spring screws his way along,
-backing when he meets with an obstacle; or a shuttle-shaped
-vegetable, apparently made of glass, with green
-balls inside him, slowly works his way from side to side,
-or, possibly, a mad battledore-shaped being dashes past
-at an inconceivable rate.</p>
-
-<p class='c021'>As it is indispensable that the object should be well
-lighted, a concave mirror is placed below it to reflect
-the rays of light from a lamp or white cloud, through the
-object when it is transparent. When it is opaque, it is
-illuminated by the rays of light being concentrated
-upon it by means of a convex lens. The name microscope
-appears by common consent to be applied more particularly
-to the compound instrument, the epithet of
-magnifier or magnifying-glass being kept for simple
-<span class='pageno' id='Page_146'>146</span>microscopes, although they are all, strictly speaking,
-<i>microscopes</i>.</p>
-
-<p class='c021'>In the ordinary compound microscope, it is only
-possible for one person to see the object to be examined
-at once; for popular exhibitions of microscopic objects
-the reflecting microscope has been devised, by means of
-which the images of the objects to be looked at are
-thrown upon a screen. The principle of this instrument
-is the same as that of the magic lantern and phantasmagoria,
-of which we shall speak presently. <a href='#i147'>Fig. 39</a>
-(see next page) represents the photo-electric microscope,
-so called from the objects being reflected by the electric
-light.</p>
-
-<p class='c021'>The jars seen on the ground are the cells of a voltaic
-battery, by which the electricity is generated. The
-luminous rays starting from the incandescent charcoal
-points are reflected through the tube and its lenses by
-the reflector placed at the back of the instrument, and
-are concentrated upon the object to be magnified. The
-image thus produced passes through a second system of
-converging lenses, and is projected upon the screen
-magnified some millions of times according to the power
-of the object-glass employed.</p>
-
-<div id='i147'  class='figcenter id021'>
-<img src='images/i147.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 39.</span>—Photo-Electric Microscope.</p>
-</div>
-</div>
-
-<p class='c021'>“The experiments made with the photo-electric microscopes,”
-says M. Ganot, “are amongst the most curious
-and pleasing to be found in the whole range of physical
-science. With this instrument it is possible to show the
-smallest objects magnified almost indefinitely to an unlimited
-number of spectators. A human hair will appear
-as large as a broomstick, an ordinary flea will look the
-size of a sheep, and the tiny cheese mite, as well as the
-smallest animalcules, will be visible in all their beauty of
-form and colour as clearly as if they were seen with the
-naked eye. One of the most remarkable experiments to
-be made with this instrument is that which shows the
-circulation of the blood. The tail of a live tadpole
-<span class='pageno' id='Page_147'>147</span>is inserted between two plates of glass, or on an instrument
-specially made for the purpose, and placed in the
-microscope armed with a somewhat low power. The
-spectator immediately perceives upon the screen a mass
-of rivers and rivulets, all flowing with the red corpuscles
-forming the blood of the animal, and rushing through
-<span class='pageno' id='Page_148'>148</span>its veins and arteries with inconceivable rapidity.
-Another interesting experiment consists in dissolving a
-small quantity of sal-ammoniac in warm water, and
-passing a small portion of the solution across a warm
-glass slide. When placed in the microscope the water
-gradually evaporates, leaving behind a mass of feathery
-crystals, whose growth may be watched atom by atom,
-each crystalline molecule grouping itself around the
-others in forms resembling a mass of fern-leaves.”</p>
-
-<p class='c021'>The apparatus we have been describing is sometimes
-illuminated with the rays of the sun, as in the following
-figure.</p>
-
-<div id='i148'  class='figcenter id021'>
-<img src='images/i148.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 40.</span>—Solar Microscope.</p>
-</div>
-</div>
-
-<p class='c021'>It is then called the solar microscope, and exhibits
-objects with great beauty and clearness. The use of the
-sun’s rays, however, has, in our own country at least,
-been entirely superseded by the electric and lime light.
-The latter method of illumination, which consists in
-projecting a stream of oxygen and hydrogen upon a
-ball of lime, is cheaper and more certain than the electric
-<span class='pageno' id='Page_149'>149</span>light, although the latter is possibly the more brilliant
-of the two. The construction of the solar microscope
-differs but little from the instrument already
-described, and may be readily understood from the
-foregoing figure. The large mirror is placed outside
-the window of the room in which the microscope stands,
-so that the solar rays are reflected upon the surface of
-a series of convergent lenses, and from thence on to
-another mirror, from which it is again reflected through
-the microscope. As the position of the sun is constantly
-changing, it is necessary to connect the outside mirror
-with a train of clockwork. It may be mentioned that
-an instrument of this kind, for reflecting the sun’s rays,
-is called a heliostat.</p>
-
-<p class='c021'>The student will, no doubt, at once perceive that if
-we concentrate the light of the sun upon an object, we
-shall also concentrate the heat, and either melt or consume
-it. A screen is therefore used in such cases,
-which will allow the light to pass while holding back
-the rays of heat. A solution of alum is found to answer
-the purpose admirably.</p>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div class='chapter'>
-  <span class='pageno' id='Page_150'>150</span>
-  <h2 id='ch2-09' class='c012'>CHAPTER IX.<br /><span class='c020'>THE TELESCOPES OF GALILEO, GREGORY, NEWTON, HERSCHEL, LORD ROSSE, AND FOUCAULT.</span></h2>
-</div>
-<p class='c014'><span class='sc'>If</span> history has failed to furnish us with the name of
-the inventor of the microscope, we have very exact information
-as to the first experimenters upon the powers
-of the telescope.</p>
-
-<p class='c021'>“In the archives of the Hague,” says Arago, “we
-find documents, by the aid of which Van Swieten and
-Moll have come to a decisive conclusion as to the first
-and true inventor of the telescope.”</p>
-
-<p class='c021'>We read in these documents that a spectacle-maker
-of Middleburg, named John Lippershey, addressed a
-petition to the States-General on October 2, 1606, in
-which he asked leave to take out a patent, which should
-constitute him the only maker of this instrument, or
-which should confer upon him an annual pension, on
-the condition of not manufacturing them for other nations.
-The petition qualifies the instrument as serving
-to see distinct objects, as had already been explained
-to the members of the States-General.</p>
-
-<p class='c021'>On the 4th of October, 1608, the States-General
-appointed a deputy from each province to experiment
-on the new instrument, which was placed on a tower of
-the palace belonging to the Stadtholder. Huggard
-says that the first telescopes experimented on were a
-foot and a half in length.</p>
-
-<p class='c021'><span class='pageno' id='Page_151'>151</span>On the 6th of October, the commission declared the
-instrument of Lippershey to be useful to the nation, but
-demanded that it should be made for two eyes instead
-of one.</p>
-
-<p class='c021'>On the 9th of December, Lippershey, having announced
-that he had solved the problem, Van Dorth,
-Magnus, and Van der Au were ordered to verify the
-fact, which they did by making a very favourable report
-on the 11th of the same month. The binocular
-instrument was therefore found to answer.</p>
-
-<p class='c021'>In reading the extracts from the archives of the
-Hague, given by Moll, we may remark with great pleasure
-the promptitude with which the commissioners of
-the States-General examined Lippershey’s instruments.
-But their satisfaction soon gave way to displeasure,
-when they found a large number of opticians making
-these instruments, and selling them to foreigners, like
-so much spice from the East Indies. Later on one feels
-indignant at finding the commissioners of the States-General
-to be so wanting in proper feeling as to decide
-that the telescope must be considered imperfect until it
-could be used with both eyes, without either winking or
-seeing the reflection of the pupils in the eye-pieces.
-Consequently, instead of being permitted to expend his
-talent on perfecting the optical powers of the single
-telescope, Lippershey saw himself condemned to waste
-his time upon the double instrument. The States-General
-finished by giving Lippershey 900 florins; but
-they refused him a patent, on the ground that it was
-already notorious that other opticians had commenced
-the manufacture of similar instruments.</p>
-
-<p class='c021'>Amongst others who were rivals of Lippershey, we
-must mention John Adrian Metius, the son of Adrian
-Metius, of Amsterdam, who discovered that the nearest
-relation of the circumference of a circle to its diameter
-was 355 to 113. He addressed a letter to the States-General
-<span class='pageno' id='Page_152'>152</span>on the 17th of October, 1608, conceived in the
-following terms:—</p>
-
-<p class='c026'>“After two years’ labour and thought I have succeeded
-in making an instrument, by the aid of which objects
-which are too distant to be visible by the eye, are seen
-plainly. The one I show, although constructed out of
-bad materials, and simply as an experiment, is, in the
-judgment of the Stadtholder and of several other persons,
-as good as the one lately presented to the States-General
-by a citizen of Middleburg. I am sure of improving
-it still further in the course of time, and I beg
-to ask for a patent by which any person who is not
-already in possession of this invention will be forbidden,
-under pain of a heavy fine and confiscation, to make or
-sell similar instruments for twenty-two years.”</p>
-
-<p class='c024'>The States-General refused to grant the patent in
-this case also, but enjoined Metius to perfect his instrument,
-reserving to themselves the power to reward him
-in the future if they thought fit.</p>
-
-<p class='c021'>In Italy, Galileo is generally supposed to have discovered
-independently the method of making a telescope
-on the principle of the Dutch philosophers, about the
-beginning of 1609, having received a very imperfect
-account of these instruments somewhere about that
-time. It may be remarked that in his letter to the
-chiefs of the Venetian Republic, giving an account of
-the properties of these new instruments, Galileo states
-that, if necessary, they could be made specially for the
-use of the navy and army belonging to the state. But
-secrecy was useless, for telescopes were already made
-and sold in Holland at a cheap rate. Besides, Galileo
-makes no allusion to the labours of his Dutch predecessors,
-either in a prior letter handed down to us by
-Venturi, or in the decree of the Venetian Senate, dated
-August 5, 1609.</p>
-
-<p class='c021'>The Italian commentators are in error when they
-<span class='pageno' id='Page_153'>153</span>attribute the second discovery of the telescope to the
-knowledge that Galileo possessed of the laws of refraction,
-and that it was by deductions therefrom that he
-was enabled to construct his first instruments.</p>
-
-<p class='c021'>Huyghens says, in his <i>Treatise on Dioptrics</i>, “I will
-unhesitatingly place that man above all mortals, who,
-by the aid of his own reflections and without the aid of
-accident, first succeeded in constructing a telescope.”</p>
-
-<p class='c021'>“Let us see,” says Arago, when speaking on this
-subject, “if Lippershey and John Adrian Metius were
-men of unparalleled powers.”</p>
-
-<p class='c021'>Hieronymus Saturnus tells us that an unknown man
-of genius called upon Lippershey, and ordered from him
-a number of convex and concave lenses. At the time
-agreed upon the man returned, and chose two, one convex
-and the other concave, and, placing them one before
-his eye and the other at some distance from it, drew
-them backwards and forwards, without giving any explanation
-of his manœuvres, paid the optician, and left
-the place. As soon as he was gone, Lippershey began
-immediately to imitate the experiments of the stranger,
-and soon found that distant objects were brought apparently
-nearer, when the lenses were placed in certain
-positions. He next fastened them to the ends of a tube,
-and lost no time in presenting the new instrument to
-Prince Maurice of Nassau.</p>
-
-<p class='c021'>According to another version, Lippershey’s children
-were playing in their father’s shop, and were looking
-through two lenses, one convex and the other concave,
-when they found to their surprise that the vane on the
-clock-tower of Middleburg Church was greatly magnified
-and apparently brought nearer. The surprise expressed
-by the children having awakened the attention
-of Lippershey, he tried the experiment of fixing the
-lenses on a piece of board; afterwards he tried it again
-by fixing them at the ends of two pieces of tube, sliding
-<span class='pageno' id='Page_154'>154</span>in each other, and succeeded in making the first telescope
-on record.</p>
-
-<p class='c021'>The principal documents from which the above facts
-touching Lippershey have been extracted, are to be
-found in a memoir on the subject by Olbers, printed in
-Schumacher’s <i>Astronomical Annual</i> for 1843.</p>
-
-<p class='c021'>It was said in the time of Galileo that he had in his
-possession a telescope by the aid of which he could see
-the birds flying at Fiesole from the window of his palace
-in Florence. This story does not in the least detract
-from the merit of the illustrious astronomer, who not
-only constructed a telescope for himself, but was the
-first to direct it heavenwards, and that too by purely
-theoretical researches; for in spite of all the documents
-adduced above, there is little or no proof that he had
-ever seen or heard of the Dutchman’s telescope. It is
-only right, therefore, that the instrument constructed
-on this principle should be called the Galilean telescope.
-He afterwards increased its power from four to
-thirty times, beyond which he could not get with the
-means at his command. With his imperfect instruments
-Galileo discovered the satellites of Jupiter, the mountains
-of the moon, and the spots on the sun, and earned
-for himself the name of Lynceus, who according to the
-ancients was one of the Argonauts, possessed of the
-power of seeing through a wall. Towards the end of
-his life, when the old man was blind, and the Academy
-of the Lincei treated his hypotheses with disdain, he
-would laugh sadly at the name bestowed on him, and
-the obstinate Academy. <a href='#i155'>Fig. 41</a> (see next page)
-shows the path of the rays in a Galilean telescope. The
-object-glass <i>O</i> is double convex, and the eye-piece o bi-concave.
-The image is formed between these lenses,
-and the eye appears to see it at that point. The States-General
-complained of being obliged to shut one eye
-when looking through a telescope, but in 1671 a good
-<span class='pageno' id='Page_155'>155</span>Capuchin monk, whose name was Cherubino, placed two
-telescopes together, little thinking that the moderns
-would imitate him in that very worldly instrument, the
-opera-glass.</p>
-
-<div id='i155'  class='figcenter id021'>
-<img src='images/i155.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p>Fig. 41.—The Galilean Telescope.</p>
-</div>
-</div>
-
-<p class='c021'>Everybody has noticed that when objects are close to
-us they appear larger than when they are at a distance;
-it accordingly amounts to the same thing whether, in
-speaking of the power of telescopes, we say they magnify
-twice, four times, or a hundred times, or that they
-are brought within half, a quarter, or a hundredth of
-their distance. Thus there is a telescope at Lord Rosse’s
-Observatory, at Parsonstown in Ireland, which is the
-finest yet constructed. Its highest magnifying power
-is 6,000, therefore every object we look at with it is
-brought within the 6,000th of its distance from us.
-Looking at the moon, for instance, we know that our
-satellite is distant some 240,000 miles from us; we
-have, therefore, only to divide that number by 6,000 to
-find that by means of this wonderful instrument the
-moon is brought within 40 miles of the earth. This
-statement, however, is not strictly true, for it supposes
-the whole of the apparatus used to be theoretically perfect.</p>
-
-<p class='c021'>Kepler, whose great name is now-a-days always associated
-with that of Galileo, but who during their life-time
-was somewhat his rival, substituted for the single
-lens forming the eye-piece a combination consisting of
-two convex lenses, in order to obtain a larger field for
-<span class='pageno' id='Page_156'>156</span>observation than that given by the single bi-concave.
-This combination is commonly known as the astronomical
-eye-piece. It reverses the object looked at, but
-for astronomical purposes this defect is of no consequence.</p>
-
-<div id='i156'  class='figcenter id021'>
-<img src='images/i156.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 42.</span>—The Astronomical Telescope.</p>
-</div>
-</div>
-
-<p class='c021'>The instrument shown in the above figure represents
-an astronomical telescope reduced to its simplest
-form.</p>
-
-<p class='c021'>Fixed parallel to the axis of the larger telescope is
-<span class='pageno' id='Page_157'>157</span>the finder, a small telescope of low power and large
-field, used for finding celestial objects not easily visible
-to the naked eye. It is so arranged, that when the
-object is found and carried to its centre, it is also in the
-centre of the field of the larger instrument. The handle
-and the two toothed wheels serve to raise or lower the
-telescope, which is movable on the horizontal axis,
-which supports it in front, so that it may be directed to
-any part of the heavens the observer may desire.</p>
-
-<p class='c021'>The following figure shows the arrangement of the
-lenses, and the path of the rays through them, in telescopes
-of this form.</p>
-
-<div id='i157'  class='figcenter id021'>
-<img src='images/i157.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 43.</span>—Section of an Astronomical Telescope.</p>
-</div>
-</div>
-
-<p class='c021'>The convex lens which serves as an object-glass, gives
-at <i>a b</i> a reversed image of the star <span class='fss'>A B</span>. The small convex
-lens which acts the part of an eye-piece, enlarged
-this reversed image without changing its position, and
-causes it to be seen in the line <span class='fss'>A´ B´</span>. This eye-piece is
-fixed at the extremity of a tube, which is smaller than
-that containing the object-glass, and slides easily backwards
-and forwards from the spot where the image <i>a b</i>
-is found. The latter is an indispensable condition, for
-it is rare to meet two persons whose eyes are of the
-same focus; besides, the image <i>a b</i> will fall at a different
-<span class='pageno' id='Page_158'>158</span>spot for objects at different distances: thus, if you are
-looking at the moon, and suddenly turn the instrument
-on to a distant nebula, you will find that the eye-piece
-requires adjusting. In showing ordinary observers an
-object in the telescope, it is well to insist on their moving
-the eye-piece backwards and forwards until distinct
-vision is obtained, for it often happens that people will
-say they see an object quite distinctly, when it is in
-reality misty, and will generally refuse to allow the
-focus to be altered. It is very singular how human
-vanity or complaisance will step in when some persons
-are looking through a telescope. They seem to think
-that there is some disgrace or rudeness involved in their
-not being able to see what their predecessors at the
-instrument have seen. Poor John Leech leaves us an
-amusing instance of this in a comic cut inserted in
-one of the early numbers of our old friend <i>Punch</i>. A
-gentleman is endeavouring to show a lady a distant
-steamboat through a telescope, but she has it accidentally
-pointed at two swans that are swimming on
-the margin of the lake below; consequently when he
-asks her if she sees the steamer, she replies that
-“she sees it most distinctly, and there are two of
-them,” a pretty good proof that the instrument was
-not only pointed at the wrong object, but was out of
-focus as well.</p>
-
-<p class='c021'>In constructing a telescope similar to the one described
-above, the object-glass ought to be of considerable
-diameter and of long focus; the eye-piece, on the
-contrary, should be comparatively small and of short
-focus. A little consideration will show the reason of
-this. An object-glass of long focus will form a large
-image at the point <i>a b</i>, and the eye-piece of short focus
-will magnify this image more than another lens of less
-convexity. It is, however, on the size, length of focus,
-and perfection of workmanship of the objective that the
-<span class='pageno' id='Page_159'>159</span>excellence of the telescope depends; large object-glasses
-are consequently rare, and are only to be found in observatories
-of the first class. The object-glass of the
-large telescope at the Observatory at Paris is nearly
-fifteen inches in diameter, and the highest magnifying
-power capable of being employed with it is 3,000. The
-Observatory of Pulkowa, near St. Petersburg, possesses
-a similar instrument, and the Observatory at Chicago,
-United States, a still larger one, measuring between
-eighteen and nineteen inches in diameter. But the
-largest of all is an objective in the possession of Mr.
-Buckingham, an amateur astronomer, who has an observatory
-near London, which is twenty inches in diameter,
-and twenty-eight feet in focal length.</p>
-
-<p class='c021'>The eye-pieces of astronomical telescopes are of different
-powers, and are changed according to the class
-of object to be observed. Thus, in taking a general
-view of the moon, a low power would be used. If you
-wished to examine any particular mountain, you would
-raise the magnifying power by inserting a stronger eye-piece.
-The power used also depends on the state of the
-atmosphere. For instance, on warm evenings, when the
-air is charged with moisture, the tremulousness of the
-atmosphere is so great, that it is often only possible to
-use the very lowest power. By combining four convex
-lenses together, we obtain what is called a terrestrial or
-erecting eye-piece, which has the property of <i>re-reversing</i>
-the image formed by the objective. The eye-pieces of
-all telescopes for use on land or at sea are made on this
-principle. The same effect may be obtained, as we have
-already shown in <a href='#i155'>fig. 41</a>, by using a concave lens, but
-in this the field of view is much diminished.</p>
-
-<p class='c021'>Hitherto we have only spoken of refracting telescopes
-or those instruments provided with a convex object-glass,
-to collect and refract the rays of light given off
-by the object we are desirous of examining; but there is
-<span class='pageno' id='Page_160'>160</span>another and very important class of instruments, in
-which the object-glass is replaced by a reflecting mirror.
-The first reflecting telescope was invented by Dr. Gregory,
-an English philosopher, about 1650. It consisted
-of a brass tube, at the lower extremity of which
-was fixed a concave mirror made of metal, and provided
-with a hole in its centre for the insertion of the small
-tube containing the eye-glass. Towards the other
-end of the telescope was placed a second and smaller
-mirror, which reflected the image formed by the large
-mirror, through the eye-piece to the eye. The following
-figure will show the path of the rays in the Gregorian
-telescope.</p>
-
-<div id='i160'  class='figcenter id021'>
-<img src='images/i160.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 44.</span>—Section of the Gregorian Telescope.</p>
-</div>
-</div>
-
-<p class='c021'>The rays <span class='fss'>A B</span>, proceeding from the object at which
-the instrument is pointed, are first reflected from the
-surface of the principal mirror <span class='fss'>M M</span> on to the small
-mirror <i>m</i>, whence they proceed to form a magnified
-image at <i>a b</i>, which is then again enlarged by the eye-piece
-appearing to the eye as if placed at <span class='fss'>A´ B´</span>. The
-focus in the Gregorian is altered, not by sliding the
-eye-piece backwards and forwards but by moving the
-mirror <i>m</i>, which is provided with a long screw, to which
-is attached a handle. At first sight a reflecting telescope
-<span class='pageno' id='Page_161'>161</span>has the appearance of a very stumpy-looking refracting
-instrument, but one instant’s examination will
-show the observer that the usual object-glass is absent
-at the end of the tube. In <a href='#i161'>fig. 45</a> we have a Gregorian
-telescope, mounted on a tripod stand.</p>
-
-<div id='i161'  class='figcenter id015'>
-<img src='images/i161.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 45.</span>—Gregorian Telescope.</p>
-</div>
-</div>
-
-<p class='c021'>Whilst experimenting on the Gregorian telescope,
-Newton made certain improvements in its construction,
-which we shall proceed to describe. A glance at <a href='#i162'>fig. 46</a>
-<span class='pageno' id='Page_162'>162</span>will show that the path of the rays is much more
-simple than in the instrument we have just noticed.</p>
-
-<div id='i162'  class='figcenter id021'>
-<img src='images/i162.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 46.</span>—Section of a Newtonian Telescope.</p>
-</div>
-</div>
-
-<p class='c021'>The rays of light <span class='fss'>A B</span> are first reflected from the
-concave mirror <span class='fss'>M</span> on to the surface of the small plane
-mirror <i>m</i>, which is placed at an angle of 45°, and reflects
-them as far as the point <span class='fss'>A´ B´</span>, where they form
-the image to be magnified by the eye-glass. It is
-therefore at the <i>side</i> of the instrument, and not at the
-end, as hitherto, that the observer is placed, and at
-right angles to the path of the rays. Observers looking
-at an object through a Newtonian telescope for the
-first time are generally sufficiently astonished to find
-that there is really no difficulty after all in seeing round
-a corner. We shall presently return to the subject of
-Newtonian telescopes, which were abandoned by astronomers
-for many years, until they were brought into
-use again by M. Foucault, a distinguished French philosopher.</p>
-
-<p class='c021'>Towards the end of the last century Sir William
-Herschel invented and constructed the reflecting telescope
-which bears his name. His great object was to
-avoid the loss of light consequent on the double reflection
-which took place in all instruments constructed up
-<span class='pageno' id='Page_163'>163</span>to that time, and he succeeded at last in making a telescope
-in which the observer looked directly through the
-eye-piece at the image formed by the mirror, which was
-inclined in such a manner that the rays were reflected
-to the lower edge of the open end of the tube. In
-using this kind of telescope the observer is placed with
-his back to the object he wishes to examine, a position
-that is even more astonishing to those unaccustomed to
-the use of a Herschellian telescope than the one assumed
-when employing an instrument of the Newtonian construction.
-This position has the defect of causing a
-small portion of the rays proceeding from the object to
-be intercepted by the head of the observer, but the
-amount of light lost is so small in comparison to the
-size of the mirror that in practice it amounts to nothing.</p>
-
-<p class='c021'>The dimensions of the telescope constructed by Herschel
-were enormous for that day. It measured 40 feet
-long, and the mirror was 4 feet in diameter. It was
-supported by a complicated system of scaffolding, pulleys,
-and cords, and was capable of magnifying an object
-6,000 times. It was by means of this splendid instrument
-that Sir William Herschel made those wonderful
-discoveries in astronomy which are inseparably associated
-with his name. With it he discovered the planet
-Uranus, many of the double stars, and a large number
-of nebulæ, which up to that time were unknown. His
-son, Sir John Herschel, inherits his father’s talents as
-an astronomer, and has enriched science with numberless
-observations and discoveries of the greatest importance
-made with this fine instrument. <a href='#i164'>Fig. 47</a> shows
-the construction of the Herschellian telescope, and the
-path of the rays may be easily followed by the student
-without any help from us.</p>
-
-<p class='c021'>The vulgar, ever prone to make mountains out of
-molehills, magnified the power of Sir William Herschel’s
-telescope beyond all bounds. Stories were circulated
-<span class='pageno' id='Page_164'>164</span>about his having given a dinner in the interior of the
-tube to a select party of friends, but as the diameter
-of the telescope was only a little more than 4 feet, the
-entertainment, to say the least of it, would have proved
-somewhat inconvenient to the guests. Another story,
-which was credited by great numbers of people, was
-that he had discovered inhabitants in the moon, but that
-he hesitated to make the matter public for fear he should
-be prosecuted for spreading atheistical notions. In fact,
-the tales told of Sir William Herschel’s telescope were
-endless, and caused the astronomer great inconvenience
-by attracting crowds of idle people to the neighbourhood
-of Slough, where he vainly endeavoured to carry
-on his investigations in peace and quietness. It was in
-vain that these silly assertions were disproved again
-and again. Having once believed them, people were
-slow to reject them, and the story of the dinner was
-told over and over again for many years.</p>
-
-<div id='i164'  class='figcenter id021'>
-<img src='images/i164.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 47.</span>—The Herschellian Telescope.</p>
-</div>
-</div>
-
-<p class='c021'>The instrument above described is one of those known
-as <i>front view telescopes</i>, on account of the image of the
-star being reflected from the surface of the mirror,
-which was placed obliquely at the bottom of the tube
-in front of the observer, who examined it by means of
-the eye-piece without any other reflection taking place,
-<span class='pageno' id='Page_165'>165</span>thereby effecting a saving of light, which fully compensated
-for any loss caused by the mirror being placed
-askew. The concave mirror made by Herschel alone
-weighed a ton, to say nothing of the enormous tube and
-its fittings. Herschel had consequently to invent a
-special apparatus for holding and moving this gigantic
-instrument. The moving gear consisted of a mass of
-beams, pulleys and cords, reminding one more of the
-rigging of a ship than of a philosophical instrument.
-The apparatus for moving the telescope appeared so
-complicated to the casual observer, although in reality
-it was very simple, that it doubtless contributed in no
-small degree to the propagation of the fanciful stories
-we have already spoken of.</p>
-
-<p class='c021'>The performances of this splendid instrument hardly
-came up to the expectations of those who saw it in progress.
-Herschel, it is true, was enabled by its means
-to use a power of from 3 to 6,000, but he could only
-use these amplifications on a few objects—the planets,
-for instance, giving so little light under a high power
-as to become indistinct and misty. In 1802 Baron von
-Zach, in his <i>Monthly Astronomical Compendium</i>, went
-so far as to say that this colossal instrument was not of
-the slightest utility, that no discovery had ever been
-made with it, and that it ought to be considered merely
-as an optical curiosity. Subsequent events, however,
-proved very conclusively that Baron von Zach was utterly
-wrong in his statements and prophecies.</p>
-
-<p class='c021'>The telescope constructed by Herschel, although
-very wonderful for the day in which it was made, has
-long since been eclipsed by that belonging to Lord Rosse,
-and erected by his late father at Birr Castle, near
-Parsonstown in Ireland. It is superior to Herschel’s
-instrument both in point of size, and workmanship.
-The late Lord Rosse, not fearing that his dignity
-would be compromised by such an act, went boldly to
-<span class='pageno' id='Page_166'>166</span>work, and learned to polish mirrors like an ordinary
-workman, the consequence of which was that he could
-bestow unusual pains upon the finishing of the speculum.
-His Lordship not only learnt the mere handicraft of
-speculum polishing, but went deeply into the engineering
-difficulties of the operation, and succeeded in inventing
-many improvements for diminishing labour and rendering
-the form of the surface more perfect. The
-specula ground and polished under Lord Rosse’s method
-are almost entirely free from what is called spherical
-aberration,—that is to say, all rays proceeding from
-a single point of light, such as a star, are collected into
-a single point instead of being scattered in a round
-mass. This freedom from spherical aberration is of
-course necessary to produce perfectly distinct images.
-In his <i>Life of Newton</i> Sir David Brewster calls it one
-of the most marvellous combinations of art and science
-yet seen in the world.</p>
-
-<p class='c021'>The tube of Lord Rosse’s instrument is 55 feet long,
-and weighs 6½ tons. In form it may be compared to
-the chimney of a steamboat of enormous size. At one
-end it terminates in a kind of square box, within which
-is contained the mirror, whose diameter is 6 feet, and
-which weighs nearly 4 tons. The weight of the whole
-apparatus is consequently nearly 10½ tons, or four
-times as much as Herschel’s. It is erected on an
-oblong mass of masonry, 75 feet in length from north
-to south, between two solid walls nearly 50 feet high,
-which serve as supports for the mechanism intended to
-move this enormous tube in all directions. To the
-walls are also fixed movable staircases with platforms
-that can be brought up to the eye-piece with the greatest
-facility, no matter in what position the telescope
-may be placed. This noble instrument has penetrated
-space to a distance perfectly unattempted before its existence,
-and has resolved numerous nebulæ into masses
-<span class='pageno' id='Page_167'>167</span>of stars that until then were supposed to be mere
-clouds of luminous matter. The exact forms of other
-nebulæ have also been accurately determined by this
-telescope, which fully deserves the glowing eulogium
-passed upon it by the Duke of Argyle in his presidential
-address at the meeting of the British Association
-at Glasgow, in 1855. “This instrument,” said his
-Grace, “in extending the range of astronomical science
-as it has done, has been the means of throwing certain
-doubts upon the laws that govern the motions of the
-heavenly bodies, and render it possible that certain of
-the far-distant nebulæ are regulated in their movements
-by other laws than those to which the members of our
-own system are subjected.”</p>
-
-<p class='c021'>The clearness with which this telescope exhibits every
-object within its range is so great that the most distant
-nebulæ are seen with as great distinctness as the nearest
-planet. On directing it towards the moon, which is
-only distant from us about 240,000 miles, the surface
-of our satellite may be explored with a facility almost
-as great as that with which we examine the details of a
-landscape with an ordinary telescope.</p>
-
-<p class='c021'>Maedler, a German astronomer, who has measured
-nearly every mountain and valley on the moon’s surface
-with the greatest exactitude, stated some years before
-Lord Rosse’s telescope was perfected that if a monument
-as large as one of the Pyramids existed on the
-surface of the moon it could have been readily distinguished
-by the instruments then in use. With Lord
-Rosse’s telescope we can see the surface of our satellite
-so much enlarged that a space 220 feet square could be
-readily perceived by a good observer. This enormous
-eye, measuring 6 feet in diameter, would hardly show
-us a lunar elephant; but it is certain that if a troop of
-buffaloes, or animals analogous to them, crossed the
-field of vision, they would undoubtedly be perceptible.
-<span class='pageno' id='Page_168'>168</span>Masses of troops marching backwards and forwards
-would also be plainly visible, and we may assert with
-something like absolute certainty that there are neither
-towns nor villages in the moon, nor any buildings as
-large as St. Paul’s of London or the colossal railway
-stations of that metropolis.</p>
-
-<p class='c021'>This telescope, as we have said before, is the largest
-hitherto constructed, and cost its noble constructor
-more than 25,000<i>l.</i> It must also be recollected that it
-was not a mere scientific toy belonging to an amateur
-philosopher, but a real working instrument in the
-possession of a true man of science, who did work with
-it that will render his name famous while civilization
-lasts. The present Lord Rosse seems worthy in every
-way of his father’s great name, and has already enriched
-astronomical science with numerous valuable
-observations.</p>
-
-<p class='c021'>We shall finish this chapter by a description of the
-Newtonian telescope constructed by M. Léon Foucault.
-The mirror, instead of being made of speculum metal,
-which is an alloy of tin and copper, is made of glass
-from the famous manufactory of St. Gobain. The first
-rough grinding having been finished, it passed into the
-workshops of M. Secrétan, the optician to the Paris
-Observatory, to receive its final polish and finishing
-touches from the hand of M. Foucault himself, the most
-careful optical tests being applied to it before the commencement
-of each operation.</p>
-
-<p class='c021'>The glass mirror having reached the degree of perfection
-desired, was then silvered on its concave surface
-by being plunged into a bath of nitrate of silver,
-dissolved in water, and mixed with certain proportions
-of gum galbanum, nitrate of ammonia, and oil of cloves.
-Half an hour in this bath was sufficient for the deposition
-of a film of silver of sufficient thickness to bear
-polishing. When finished, the mirror was found to
-<span class='pageno' id='Page_169'>169</span>reflect 92 per cent. of the light incident on its surface,
-the loss in the case of achromatic object-glasses and
-metal specula being 20 and 35 per cent. respectively.
-The substitution of a parabolic glass mirror for the
-ordinary metal speculum offers the triple advantage of
-greater lightness, increased distinctness, and more
-<span class='pageno' id='Page_170'>170</span>brilliant images. <a href='#i169'>Fig. 48</a> represents the large silvered
-glass telescope constructed under M. Foucault’s
-direction for the observatory at Marseilles. It measures
-32 inches in diameter, and has a focal length of a little
-more than 16 feet, and is put in motion by clockwork of
-a very perfect description, so that when once pointed at
-a star or planet it follows the object, which would otherwise
-disappear on account of the rotation of the earth.
-The path taken by the rays is precisely the same as in
-Newton’s telescope, the eye-piece being placed at the side
-of the tube, which is provided with a movable platform
-and staircase for the observer.</p>
-
-<div id='i169'  class='figcenter id021'>
-<img src='images/i169.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 48.</span>—Foucault’s Large Telescope.</p>
-</div>
-</div>
-
-<p class='c021'>The optician to whose talent in his art this fine instrument
-is due, has recently executed several small
-telescopes upon the same model, at such a price as to
-bring them within the reach of amateurs with slender
-purses. The principal part of these telescopes, one of
-which is represented in <a href='#i171'>fig. 49</a>, (see next page), is the
-mirror, which is about 4 inches in diameter, and 24
-inches’ focal length. The body, which is cylindrical, is
-made of brass, and revolves on two pivots placed horizontally
-at about one-third of its length from the bottom.
-The bearings on which the pivots move consist of two
-upright standards of metal, which are connected at the
-bottom, and revolve on a pin in the middle of the plate
-of the tripod stand. They are made of such a height
-that the lower portion of the instrument may pass between
-them, when it is necessary to observe objects in
-the zenith. By the turn of a screw the whole of the
-upper portion of the instrument may be dismounted and
-fixed on a lower standard, so that the observer may work
-sitting down if necessary. The body of the telescope
-is provided with a finder. One of the great advantages
-of this form of instrument is that it can be used for
-observations on the zenith without giving the observer
-those unpleasant cricks in the neck so inseparable from
-<span class='pageno' id='Page_171'>171</span>the use of ordinary telescopes in a nearly upright condition.
-The mirror will bear a power of 220 diameters,
-and shows the mountains of the moon, the phases of
-Mercury and Venus, Saturn and his ring, Jupiter and
-<span class='pageno' id='Page_172'>172</span>his satellites, and a large number of double stars and
-nebulæ. It is provided with a set of eye-pieces, so that
-any power almost from 50 to 220 diameters may be
-used at will. The figure on the opposite page will give
-the amateur a good idea of the form and size of this
-instrument.</p>
-<div id='i171'  class='figcenter id015'>
-<img src='images/i171.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 49.</span>—Foucault’s Small Telescope.</p>
-</div>
-</div>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-
-<div class='nf-center-c0'>
-<div class='nf-center c003'>
-    <div><span class='pageno' id='Page_173'>173</span><span class='c006'>PART III.</span></div>
-    <div class='c000'><span class='c015'>NATURAL MAGIC.</span></div>
-  </div>
-</div>
-
-<hr class='c009' />
-<div class='chapter'>
-  <h2 id='ch3-01' class='c012'>CHAPTER I.<br /><span class='c020'>THE MAGIC LANTERN.</span></h2>
-</div>
-<p class='c014'><span class='sc'>The</span> illusions of which we have spoken in the first
-part of this work depended principally on the nature of
-man’s vision, who, we found, was the constant and heedless
-victim of his own powers of sight. We shall now
-examine a series of illusions that are still more extraordinary,
-but which have nothing to do with the deceptions
-practised on us by our visual organs. Instead of
-being deceived by ourselves, we shall find that we are
-led astray by others whose knowledge of the laws of
-optics is greater than our own, enabling them to construct
-instruments capable of amusing us or imposing
-on us, according to our ignorance of natural laws. Let
-us hope, however, that the science of optics has now
-become so familiar to most educated people, that no such
-thing as a real imposition can take place, although at
-the present day there are so many exhibitions of the
-marvellous that ordinary observers have the greatest
-<span class='pageno' id='Page_174'>174</span>difficulty in accounting for them. In former ages, when
-the knowledge of science was confined to a certain class,
-the commonest optical facts of the present day were
-taken advantage of to delude the ignorant. The deceptions
-practised by the ancient priests of Egypt,
-Greece, and Rome were undoubtedly many of them of
-this description. It is a well known fact that both plane
-and concave metallic mirrors were used by the ancients,
-and a passage in Pliny gives an account of certain glass
-mirrors that were made at Sidon. Aulus Gellius, quoting
-Varro, speaks of the reflecting properties of hollow
-mirrors, and we shall see, as we go on, what a number
-of illusions may be practised by means of a series
-of plane mirrors arranged in a particular way. But we
-will first devote a short time to the curious historical
-facts connected with the principle of the magic lantern
-which took place long before the modern invention of
-this instrument by Father Kircher.</p>
-
-<p class='c021'>Brewster says, when treating of this subject, that
-there can be little doubt that the concave mirror was
-the principal instrument used in connexion with the
-pretended apparitions of the gods and goddesses in the
-ancient temples. In the meagre history of these
-apparitions that has come down to us, we can easily
-perceive the traces of an optical illusion. In the
-ancient temple of Hercules at Tyre, there existed a
-certain seat made of consecrated stone, out of which
-the gods rose, apparently at the will of the priests.
-Æsculapius appeared frequently to his worshippers in
-his temple in Tarsus, and the temple of Eugenium was
-famous for the number of gods and goddesses which
-were constantly visiting its sacred precincts. Iamblicus
-tells us that the priests showed the gods to the people
-in the midst of smoke; and when the great magician
-Marinus terrified his auditory by suddenly showing
-them the statue of Hercules in the midst of a cloud of
-<span class='pageno' id='Page_175'>175</span>incense, it was undoubtedly a woman who performed
-the part, dressed up in man’s robes for the occasion.</p>
-
-<p class='c021'>The character of these spectacles in the ancient temples
-is admirably described by Damasius, and there is
-no difficulty in seeing that optical illusions were the
-means employed to delude the audience. He describes
-the apparition on the wall of a large spot of white,
-which at first appeared at a distance, but gradually
-came nearer and nearer until at last it assumed the
-form of a divine or supernatural being, of severe yet
-mild aspect and of great personal beauty. This being
-the Alexandrians immediately honoured as Osiris or
-Adonis.</p>
-
-<p class='c021'>Amongst more modern examples of this illusion may
-be mentioned that of the Emperor Basil of Macedonia.
-Inconsolable at the loss of his son, this potentate had
-recourse to the prayers of the Pontiff Theodore Lantabaren,
-who was celebrated for his power of working
-miracles. The conjurer showed the Emperor the image
-of his dead son magnificently attired and mounted on a
-splendid war-horse. The young man dismounted, and,
-going up to his father, threw himself into his arms and
-disappeared. Salvertius, in speaking of this story,
-observes judiciously, that the deception could only take
-place through the agency of some person who closely
-resembled the Emperor’s son, and that the trick would
-have been easily discovered when the person embraced
-the Emperor. A better explanation of the affair is,
-however, afforded by supposing that the Emperor saw
-an aërial image of a person resembling his son, and
-that when he rushed forward to embrace him it disappeared.</p>
-
-<p class='c021'>The accounts of the operations of the ancient magicians
-are too meagre to give us any idea of the splendour
-of some of these ancient ceremonies. A system
-of deception such as this, employed as a means of
-<span class='pageno' id='Page_176'>176</span>government, must have brought into requisition not
-only the talents of all the learned men of the day, but
-a crowd of accessories calculated to astonish and confound
-the judgment, fascinate the senses, and facilitate
-imposture.</p>
-
-<p class='c021'>An account of an instance of modern necromancy
-has been left us by Benvenuto Cellini, who played a
-prominent part in a case of this sort.</p>
-
-<p class='c021'>He accidentally made the acquaintance of a Sicilian
-priest, a man of great genius and acquirements, and
-well versed in Greek and Latin classical lore. One day
-the conversation turned on necromancy, and the great
-goldsmith told him that he had the greatest desire to
-know something about this wonderful art, and that he
-had felt all his life a great curiosity to penetrate its
-mysteries.</p>
-
-<p class='c021'>The priest replied, that a man ought to have a very
-resolute and fearless character to study this art; but
-Benvenuto answered he had both resolution and courage.
-The priest went on to say, that if he had the heart to
-try, he would be the means of obtaining the fulfilment
-of his wishes. They consequently agreed upon a plan
-of necromantic study. One evening, Benvenuto invited
-one of his companions, Vincenzio Romoli, to take part
-in some experiments that were to be made amongst the
-ruins of the Coliseum. They there met the Sicilian
-priest, who after the manner of the ancients began to
-describe a number of circles in the air in the most imposing
-manner. He had brought with him various gums
-and perfumes, and had made a fire, into which his
-assistant necromancer was to throw them at the proper
-time. He commenced his conjurations, the ceremony
-continuing about an hour, when there appeared legions
-of demons, in such numbers that the whole of the ruins
-seemed filled with them. Benvenuto was nearly fainting
-with the perfumes, when the priest roused him by
-<span class='pageno' id='Page_177'>177</span>telling him to ask for something. He replied, that
-he wished to be transported to the side of his Sicilian
-mistress; but the demons were evidently unpropitious,
-for nothing came of it. His instructor, however, told
-him that they must repeat their experiments a second
-time, and that Benvenuto must bring with him a child
-that had never committed sin. The next time Benvenuto
-took with him a boy of twelve years old whom he
-had in his service, and his friends Romoli and Guddi.
-When they arrived at the place of meeting, they found
-the priest had made the same preparations as before.
-This time, however, he used more powerful conjurations,
-calling on a number of demons by their names, in Hebrew,
-Greek, and Latin; so that the ruin was filled
-with a still greater mass of them than on the other occasion.
-The fire and perfumes were put under the
-charge of Guddi and Romoli, and he gave Benvenuto a
-magic picture to hold in a certain direction, the boy
-being placed underneath it. The priest told him again
-to wish to be in the company of his lady love, but on his
-expressing the wish, the magician told him that the demons
-still refused to do his bidding in this way, but that
-he should visit her once more in a month’s time. The
-poor boy underneath the magic picture was seized with
-a terrible fright, and exclaimed, that he saw millions of
-ferocious spirits and four giants, all endeavouring to
-break through the magic circle the priest had formed.
-All there were evidently in a most abject state of terror,
-and remained in the place until the church bells began
-to ring for morning prayers, when they returned home,
-the boy declaring that two of the demons preceded them,
-dancing and gambolling before them, and sometimes
-running along the housetops.</p>
-
-<p class='c021'>The priest then advised him to try another spiritual
-<i>séance</i>, and endeavour to induce the demons to point
-out sundry pots of buried gold, so that they all might
-<span class='pageno' id='Page_178'>178</span>become rich, but it does not appear that the priest’s
-advice was followed.</p>
-
-<p class='c021'>It is impossible to read the foregoing description of
-what happened, without being convinced that the whole
-affair was an optical illusion, and not the mere result of
-the imagination of those who took part in it. The smoke
-was evidently caused in order to afford a field for the
-exhibition of painted images reflected by concave mirrors,
-and the circle was formed in order that those
-within it might be within range of the images formed on
-the smoke. The mirrors reflecting the images of the
-demons had undoubtedly already been arranged so that
-they would fall just above the fire, and become visible
-when the gums began to burn with a smoky flame. The
-perfumes were simply to help to stupify the spectators,
-and aid in working on their imaginations for those occurrences
-which were beyond the reach of optics, for the
-poor unfortunate boy saw things that his companions
-did not, even to a couple of demons dancing through
-the streets in broad daylight. In fact, it is somewhat
-difficult to draw the line between reality and imagination
-in this case. No doubt the story is considerably
-exaggerated by Cellini, who was a fervid Italian, and
-prone to believe in wonders, as is instanced by his wish
-to study the black art. The priest, too, whom he describes
-as a man of genius, no doubt had a great influence
-over the famous artist, and made him see a great
-deal more than was really there.</p>
-
-<p class='c021'>The introduction of the magic lantern provided the
-magicians of the seventeenth century with a very powerful
-instrument with which to continue their deceptions.
-The use of the concave mirror, which does not appear to
-have had any accessories worth speaking of, required a
-separate apartment, or at least a hiding-place of some
-sort that was difficult to discover under ordinary circumstances;
-but the magic lantern, inclosing as it did the
-<span class='pageno' id='Page_179'>179</span>lamp, the optical apparatus, and the figures in a comparatively
-small spice, was particularly appropriate to
-the wants of the Homes and Davenports of the day, who
-until then had never possessed anything so convenient
-and portable.</p>
-
-<div id='i179'  class='figcenter id021'>
-<img src='images/i179.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 50.</span>—Section of the Magic Lantern.</p>
-</div>
-</div>
-
-<p class='c021'>The magic lantern shown in figures 50 and 51 consists
-of a dark box, containing a lamp and a concave
-metallic mirror, constructed in such a way that the
-whole of the rays proceeding from the lamp are reflected
-through the aperture holding the optical portion of the
-apparatus. In front of the box is fixed a double tube
-<span class='fss'>C D</span>, one-half of which (<span class='fss'>D</span>) slides in the other. A large
-plano-convex lens <i>c</i> is fixed at the inner extremity of the
-double tube, and a small one at its outer end. To the
-fixed tube <span class='fss'>C E</span> is attached a groove <i>b b</i>, which serves to
-hold the painted glass. These glasses, or slides as they
-are generally called, are painted with strong transparent
-colours.</p>
-
-<p class='c021'>The direct light of the lamp <span class='fss'>G</span>, as well as that reflected
-by the mirror and passing through the lens <i>c</i>, is
-<span class='pageno' id='Page_180'>180</span>so concentrated as to project a brilliant beam of light
-through the painted slide, which being in the conjugate
-focus of the large plano-convex lens <i>d</i>, the pictures on
-the glass are refracted in a magnified form on the white
-cloth <span class='fss'>P Q</span>.</p>
-
-<p class='c021'>The magic lantern, therefore, consists of a box to hold
-the lamp, a concave mirror, and a convex lens to concentrate
-the light on the slide, and a second convex lens to
-throw the image on the screen.</p>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div id='i182'  class='figcenter id021'>
-<span class='pageno' id='Page_182'>182</span>
-<img src='images/i182.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 51.</span>—Magic Lantern.</p>
-</div>
-</div>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div class='chapter'>
-  <span class='pageno' id='Page_183'>183</span>
-  <h2 id='ch3-02' class='c012'>CHAPTER II.<br /><span class='c020'>THE PHANTASMAGORIA.</span></h2>
-</div>
-<p class='c014'><span class='sc'>The</span> phantasmagoria may be described as a perfected
-magic lantern, and bears the same relation to its prototype
-that a shilling telescope bought in the Lowther
-Arcade does to one of Dollond’s or Ross’s field glasses.
-The position of the spectators, too, is different, being on
-the other side of the scene which receives the magnified
-pictures, already described when speaking of the magic
-lantern.</p>
-
-<div id='i184'  class='figcenter id015'>
-<img src='images/i184.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 52.</span>—The Phantasmagoria.</p>
-</div>
-</div>
-
-<p class='c021'>The phantasmagoria lantern is generally mounted on
-a stand provided with castors so that it may be moved
-about at will. It consists of a box as represented in
-<a href='#i184'>fig. 52</a>, inclosing a lamp with a metallic reflector, the
-bundle of rays being sent through the centre of the
-tube containing the slide and lenses, as before described.
-The chimney serves to carry off the products of combustion
-generated by the lamp. In <a href='#i185'>fig. 53</a> we have
-shown the interior of the tube containing the lenses.
-Between this tube and the body of the lantern there is a
-space within which slide the glasses whereon are painted
-the figures and landscapes that are to be thrown on the
-white screen. The luminous rays given off by the reflector
-in the interior of the lantern pass through a
-plano-convex lens placed with the flat side outwards.
-In front comes the double convex lens, or object-glass,
-which can be moved backwards and forwards by means
-<span class='pageno' id='Page_184'>184</span>of a rack and pinion. There is also a movable diaphragm,
-which is worked with a couple of cords, by pulling which
-the aperture is made larger or smaller at will. By
-moving the lantern backwards and forwards, working
-the rack and pinion and the diaphragm at the same
-time, the view seen by the spectator seems to advance
-and recede. The pictures are painted on glass with
-transparent colours, the glasses being generally about
-five inches in diameter. To render the illusion perfect
-it is necessary that the spectator should be placed in a
-partially dark room, being separated from the operator
-by the screen already mentioned. Everything being
-ready, the spectators having but little notion of the
-<span class='pageno' id='Page_185'>185</span>distance of the screen, a very small picture is shown to
-them first, the illumination being reduced to a minimum
-by pulling the cords which act on the diaphragm.
-The little picture first seen by them will appear to be
-situated at an enormous distance; but as the lantern is
-brought almost imperceptibly nearer to the screen, the
-image appears to advance towards them in a very surprising
-manner, at last appearing almost as if it were
-going to fall upon the spectators.</p>
-
-<div id='i185'  class='figcenter id015'>
-<img src='images/i185.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 53.</span>—The Phantascope.</p>
-</div>
-</div>
-
-<p class='c021'>Robertson, an English optician who was settled in
-Paris some fifty years since, was one of the first to exhibit
-the phantasmagoria with success. In order to
-obtain the best results he used a room some sixty or
-eighty feet long, and twenty-four wide, which he hung
-entirely with black. Of this a strip twenty-five feet long
-<span class='pageno' id='Page_186'>186</span>was cut off and devoted to the manipulation of the
-phantasmagoria. This portion of the apartment was
-separated from the spectators by a white calico screen,
-tightly strained from side to side, and at first concealed
-from view by a black curtain. The calico screen,
-which was about twenty feet square, was well soaked in
-a mixture of starch and fine gum arabic, in order to render
-it semi-transparent. The floor was raised about
-four or five feet at one end in order that the whole of the
-spectators might have a free and uninterrupted view of
-what was going on.</p>
-
-<p class='c021'>It is undoubtedly to Robertson that we owe most of
-the improvements in the phantasmagoria. The success
-of his performances in Paris during the first Revolution
-has never been equalled by any similar exhibition. The
-enthusiasm excited amongst the Parisian public at the
-time surpassed that awakened even by Cagliostro and
-Mesmer. The spirit which guided Robertson in exhibiting
-these wonders was totally opposed to that which
-animated the two charlatans just mentioned. Robertson,
-unlike them, sought to spread the notion that there was
-nothing occult or supernatural in the marvels he exhibited,
-but that they resulted simply from the application
-of a few simple laws of optics. We shall presently
-give an account of one these famous <i>séances</i>, which
-were powerful enough to distract the attention of the
-people of that day from the stormy events that were
-going on around them; but we will first allow our
-author to tell the story of his experiments in optics in
-his own words.</p>
-
-<p class='c021'>“From my very earliest infancy,” he says in his
-Memoirs, “my lively and passionate imagination caused
-me to be dominated over by the marvellous in a very
-powerful manner. Anything that seemed to go beyond
-nature in any way, excited in me an ardour which then
-appeared to me capable of overcoming all obstacles in
-<span class='pageno' id='Page_187'>187</span>order to realize the effects I had conceived. Father
-Kircher, it was said, believed that the magic lantern
-was the invention of the Evil One. All the worse for
-Father Kircher, who was gifted with a great intellect,
-and many persons were tempted to say that he might
-possibly have some cause for believing in the diabolical
-origin of a simple optical instrument. But as the writer
-who has thus reproached Father Kircher with too much
-credulity has not cited those passages of the work in
-which this statement may be found, I did not think seriously
-of the matter. Who has not in his younger days
-believed in witches, hobgoblins, and compacts with the
-devil? I know I did, and worse; for I imagined and
-fully believed that an innocent old woman who was a
-neighbour of ours, really had dealings with Lucifer, as
-every one asserted. I even went so far as to envy her
-the power of conferring with the Evil One, and once
-shut myself up in my room with an unhappy live cock,
-whose head I cut off in the most barbarous manner,
-having heard that that was the most approved manner
-of summoning into one’s presence the great head of all
-the demons. I waited for him several hours, calling
-on him to appear, threatening to deny his existence for
-the future if he did not appear, but all to no purpose.
-The books on magic and the black art that I had read
-had completely turned my head. I believed everything
-that was in them, and I desired ardently to perform the
-wonders they described, even with the aid of the devil.
-The <i>Magia Naturalis</i> of Porta, and the <i>Recreations</i> of
-Midorge, which treated simply of natural phenomena,
-had no effect upon me, but I was at last obliged to fall
-back on the principles involved in them, in order to
-create the diabolical appearances I had sought after in
-what I considered a truly supernatural manner, until at,
-last my dwelling became a true Pandemonium.</p>
-
-<p class='c021'>“It is only our grandmothers, it has been said for a
-<span class='pageno' id='Page_188'>188</span>long time, who believe in magic, witches, and supernatural
-appearances; but the statement is hardly true,
-seeing how easily the country people fall a prey to the
-first cheat who chooses to invest himself with supernatural
-powers. We have sufficiently ridiculed the
-superstitions of the ancients, and numberless instances
-may be adduced which are a shame to their intelligence,
-and which gives, so to speak, a denial to the stories we
-have heard of their high state of civilization. But I
-believe, if we were to make a collection of all the stories
-of ghosts, of mysterious appearances, of communications
-between the living and the departed, of the discoveries
-of hidden treasures, &amp;c., &amp;c., which have taken place
-even since the Revolution, before whose power so many
-dark things have been brought to light, the collection
-would hardly be less bulky than that of the ancient superstitions
-now happily passed away.”</p>
-
-<p class='c021'>Robertson then goes on to take great credit to himself
-for showing the world that all the superstitions concerning
-ghosts, spectral appearances, and other illusions
-of a similar nature, were to be easily accomplished, by
-simply studying natural laws. He appears first to have
-begun his optical experiments with the solar microscope,
-and we hear of his landlord taking an action against him
-to recover damages for having pierced the doors of his
-rooms with innumerable holes. He studied the subject
-both theoretically and practically for many years, in
-company with his friend Villette, and at last announced
-a public <i>séance</i> at the Pavillon de l’Echiquier at Paris.
-A multitude of advertisements and prospectuses,
-written in the high-flown style of the time, were issued,
-and distributed throughout the city. The newspapers
-of the day are full of accounts of the extraordinary
-impression made on the minds of the Parisians by
-Robertson’s wonderful exhibition. The old-fashioned
-word magic lantern was quite abandoned, and the new
-<span class='pageno' id='Page_189'>189</span>and high sounding Greek appellation, “phantasmagoria,”
-was heard issuing from every one’s mouth. There is
-an amusing account given of Robertson’s exhibition in
-one of the contemporary journals, written by Poultier,
-one of the representatives of the people. He says: “A
-decemvir of the republic has said that the dead return
-no more, but go to Robertson’s exhibition and you will
-soon be convinced of the contrary, for you will see the
-dead returning to life in crowds. Robertson calls
-forth phantoms, and commands legions of spectres. In
-a well-lighted apartment in the Pavillon de l’Echiquier
-I found myself seated a few evenings since, with some
-sixty or seventy people. At seven o’clock a pale thin
-man entered the room where we were sitting, and
-having extinguished the candles he said: ‘Citizens and
-gentlemen, I am not one of those adventurers and impudent
-swindlers who promise more than they can perform.
-I have assured the public in the <i>Journal de
-Paris</i> that I can bring the dead to life, and I shall do
-so. Those of the company who desire to see the
-apparitions of those who were dear to them, but who
-have passed away from this life by sickness or otherwise,
-have only to speak, and I shall obey their commands.’
-There was a moment’s silence, and a haggard
-looking man, with dishevelled hair and sorrowful eyes,
-rose in the midst of the assemblage and exclaimed, ‘As
-I have been unable in an official journal to re-establish
-the worship of Marat, I should at least be glad to see
-his shadow.’ Robertson immediately threw upon a
-brasier containing lighted coals, two glasses of blood, a
-bottle of vitriol, a few drops of aquafortis, and two
-numbers of the <i>Journal des Hommes Libres</i>, and there
-instantly appeared in the midst of the smoke caused by
-the burning of these substances, a hideous livid phantom
-armed with a dagger and wearing a red cap of
-liberty. The man at whose wish the phantom had
-<span class='pageno' id='Page_190'>190</span>been evoked seemed to recognise Marat, and rushed
-forward to embrace the vision, but the ghost made a
-frightful grimace and disappeared. A young man next
-asked to see the phantom of a young lady whom he had
-tenderly loved, and whose portrait he showed to the
-worker of all these marvels. Robertson threw on the
-brasier a few sparrow’s feathers, a grain or two of
-phosphorus, and a dozen butterflies. A beautiful
-woman, with her bosom uncovered and her hair floating
-about her, soon appeared, and smiled on the young man
-with the most tender regard and sorrow. A grave-looking
-individual sitting close by me suddenly exclaimed
-‘Heavens! it’s my wife come to life again,’ and he
-rushed from the room, apparently fearing that what he
-saw was not a phantom.</p>
-
-<p class='c021'>A Swiss asked to see the shade of William Tell. The
-phantom of the great archer was evoked with apparently
-as much ease as the others. Delille, who was
-present, called for Virgil, whose Georgics he had lately
-translated. The poet appeared, having in his hand a
-laurel crown, which he held out to his French commentator.
-Many other equally extraordinary apparitions
-were shown at the will of various individuals in the
-audience, and towards the end of the evening Robertson
-showed his judgment, and under very difficult circumstances.
-A royalist who was present asked for the
-phantom of Louis XVI., the appearance of which would
-no doubt have raised a tumult amongst so many red-hot
-Republicans, had not Robertson replied that before
-the 18th Fructidor, the day on which the French republic
-declared that royalty was abolished for ever, he
-had had a receipt for bringing dead kings to life again,
-but that same day he lost it, and feared that he should
-never recover it again. The answer was said to have
-been whispered to Robertson by his friend Ponthieu,
-who saw the difficulty he was in. It was supposed that
-<span class='pageno' id='Page_191'>191</span>the demand was prompted by an agent of the police,
-who for some cause had a spite against Robertson. In
-any case the affair made such a noise that the next day
-the exhibition was prohibited by those in authority,
-and seals were placed upon the optician’s boxes and
-papers. The exhibition was, however, afterwards allowed
-to be continued, and was so successful that it had
-to be transferred to the old Capuchin convent near the
-Place Vendôme.</p>
-
-<p class='c021'>The whole of Paris rang with eulogiums upon Robertson’s
-wonderful exhibition at the Capuchin Convent.
-He had purposely chosen the abandoned chapel, which
-was in the middle of a vast cloister crowded with tombs
-and funereal tablets. It was approached by a series of
-dark passages, decorated with weird and mysterious
-paintings, and the very door was covered with hieroglyphics.
-The chapel itself was hung with black, and was
-feebly illuminated by a single sepulchral lamp. The
-whole assembly involuntarily remained grave and silent,
-and it was only when the first preparations for the exhibition
-were made, that the audience broke into a low
-murmur. Robertson commenced with an address on
-sorcery, magicians, witches, ghosts, and phantoms, and,
-having worked the spectators up to the proper pitch, he
-suddenly extinguished the single antique lamp already
-mentioned, plunging the assembly into perfect darkness.
-Then there arose a storm of rain, wind, thunder, and
-lightning. The bells tolled lugubriously as if summoning
-the dead from their tombs beneath the feet of those
-present; the wind whistled mournfully, the rain fell in
-torrents, the thunder rolled, and the lightnings flashed.
-But suddenly above all this confusion were heard the
-sweet notes of a harmonium, and in the far-off distance
-the sky was seen clearing gradually. A luminous point
-then made its appearance in the midst of the clouds,
-which gradually became the figure of a man, increasing
-<span class='pageno' id='Page_192'>192</span>in size every instant, until it seemed to be about to precipitate
-itself on to the spectators. A man in the front
-row was so frightened, that he uttered a scream of terror,
-when the phantom instantly disappeared. A series
-of spectres then issued suddenly from a cave. The
-shades of great men crowded together round a boat
-floating on a black and sluggish river, which the spectators
-had no difficulty in identifying as the Styx. The
-shadows gradually disappeared in the distance, getting
-smaller and smaller until they became invisible.</p>
-
-<p class='c021'>Robertson was extremely careful in all his entertainments
-to flatter the popular ideas of the day. For instance,
-one of his most famous exhibitions consisted in
-a picture of a tomb, in the middle of which Robespierre
-issued. The figure, as usual, walked towards the spectators;
-but when apparently within a few yards of them,
-it was struck down by lightning. Voltaire, Lavoisier,
-Rousseau, and other popular favourites then appeared
-on the scene, and disappeared again in the usual manner.
-Robertson generally ended his entertainment with
-an address something like the following:—</p>
-
-<p class='c026'>“We have now seen together the wonderful mysteries
-of the phantasmagoria. I have unveiled to you the
-secrets of the priests of Memphis. I have shown you
-every mystery of optical science; you have witnessed
-scenes that in the ages of credulity would have been considered
-supernatural. You have, perhaps, many of
-you, laughed at what I have shown you, and the gentler
-portion of my audience have possibly been terrified at
-many of my phantoms; but I can assure you, whoever
-you may be, powerful or weak, strong or feeble, believers
-or atheists, that there is but one truly terrible spectacle—the
-fate which is reserved for us all;”</p>
-<p class='c027'>and at that instant a grisly skeleton was seen standing in the
-middle of the hall (<a href='#i194'>fig. 54</a>).</p>
-<div id='i194'  class='figcenter id021'>
-<img src='images/i194.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 54.</span>—Phantasmagoria (<span class='sc'>Robertson</span>).</p>
-</div>
-</div>
-
-<p class='c021'>Even in those unbelieving days, when scepticism of
-<span class='pageno' id='Page_195'>195</span>every sort was riding rough-shod over the French people,
-Robertson had the greatest difficulty in disclaiming all
-approach to the possession of supernatural powers.
-Day after day he received applications from all quarters
-to reveal the secrets of the past, present, and future, to
-describe events that were passing in other countries;
-and it frequently happened, that after his entertainments,
-he would be asked by several members of his
-auditory to assist them in recovering property that had
-been lost or stolen from them. In the latter kind of
-cases he generally used to adopt the excellent plan of
-sending his would-be clients to the nearest police-office.</p>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div class='chapter'>
-  <span class='pageno' id='Page_196'>196</span>
-  <h2 id='ch3-03' class='c012'>CHAPTER III.<br /><span class='c020'>OTHER OPTICAL ILLUSIONS.</span></h2>
-</div>
-<p class='c014'><span class='sc'>By</span> varying the disposition of mirrors, prisms, lenses,
-and light, an infinite number of the most surprising
-effects may be shown, with a comparatively small
-amount of trouble and expense. We shall, therefore,
-devote this chapter to the explanation of a large number
-of allusions, which have been devised by Robertson and
-other adepts in the art of honest deception.</p>
-
-<p class='c021'>One of Robertson’s most famous delusions was the
-“Dance of Demons,” an effect he discovered quite accidentally.
-One evening, while experimenting with the
-phantasmagoria, he suddenly found himself in the dark,
-when two persons, each bearing a light, crossed the room
-on the other side of the screen. A little window which
-happened to be between the lights and the screen, immediately
-threw its double image on the cloth, and the
-method of multiplying shadows was discovered.</p>
-
-<div id='i198'  class='figcenter id021'>
-<img src='images/i198.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 55.</span>—Wizard Dance.</p>
-</div>
-</div>
-
-<p class='c021'>The figures used in this experiment are cut out of
-fine cardboard, and may be made a foot high or there-abouts.
-They are placed on a second screen in front of
-the principal one, and by multiplying the lights, as
-shown in <a href='#i198'>fig. 55</a>, you may have as many shadows as
-you please. The effect is much heightened if the figures
-are cut out so as to show as lights when thrown on the
-screen. A little ingenuity shown in the arrangement of
-the distance and movements of the lights, will produce an
-<span class='pageno' id='Page_199'>199</span>endless amount of amusing effects. Thus, a small image
-of the principal figure may be produced by carrying the
-second light to a great distance, and the lesser figure
-may be easily made to jump over the former, by moving
-the candle in a semicircle over the light that is stationary.
-It is only necessary to recollect that whatever
-movements are made by the lights, the shadows of the
-figures follow their example. With a little ingenuity
-the heads and limbs of the figures may be made moveable;
-and if one assistant attends entirely to the working
-of the figures, and the rest to the lights, an infinite
-number of changes may be carried out. If mounted in
-a frame, they may be made to throw somersaults, fall
-down, or jump up in the air at will.</p>
-
-<p class='c021'>A knowledge of optics will often serve to explain
-with great ease the tricks played by conjurers and impostors
-on princes and other great people, for their own
-vile ends. It is well known that Nostradamus, on being
-consulted by Marie de Médicis on the future destiny of
-France, was shown by him in a mirror events that left
-no doubt on her mind that she would one day share the
-throne of the Bourbons. These illusions were possibly
-effected in the following manner, and may be readily
-understood by reference to <a href='#i201'>fig. 56</a>.</p>
-
-<p class='c021'>The throne in the first chamber is reflected in a mirror
-concealed in the canopy overshadowing a second mirror,
-placed carelessly on a table in the room in which the
-Princess and astrologer are standing. The arrangement
-of the mirrors is such that, on looking into the smaller
-glass, the Princess sees all that is going on in the adjoining
-chamber. The very fact of her consulting Nostradamus
-on her future fate, shows that under certain circumstances,
-at least, this clever woman was as silly as
-a child. It is not, therefore, to be supposed that she
-would notice that the mirror she was looking into was
-inclined at such an angle that it could not reflect her
-<span class='pageno' id='Page_200'>200</span>beautiful face. Nothing could be more natural, either,
-than that this magic looking-glass should be placed on a
-daïs, and shaded by a canopy. Nostradamus, who was
-a shrewd man, could no doubt pretty well see the course
-that events would take, and must consequently have
-felt quite safe in showing the Princess the throne of
-France occupied by Henry of Navarre. This was not
-the first time that the rulers of the earth were duped by
-so-called magicians, who possessed the knowledge that
-the angle of reflection was always equal to the angle of
-refraction.</p>
-
-<p class='c021'>We may also mention, while speaking on this subject,
-the adventure of the Emperor Alexander of Russia, <i>à
-propos</i> of a singular optical experiment at which he was
-present, which had for its end the changing of a man
-into a wild animal, or <i>vice versâ</i>. Certain cynics will
-possibly say that this is by no means difficult, and that
-it is an event that happens every day; but the clever
-trick at which Alexander was so astonished was not
-moral but purely physical. After having gained much
-money and fame in France, Robertson directed his steps
-towards Hamburg, where the Emperor was at that time
-stopping. He performed before the Czar an experiment
-that puzzled his Majesty beyond endurance. He
-showed him a man upon whose shoulders he saw successively
-the head of a calf, a lion, a tiger, a bear, and a
-whole menagerie of other animals. At last, the Czar
-could stand it no longer, and he suddenly rose, put his
-shoulder against the partition, and brought the whole
-to the ground with a loud crash, just at the moment
-that the confederate was assuming the form of a goat.
-If our readers would like to join the Czar in his discovery
-of the manner in which the trick was performed,
-they can easily do so.</p>
-
-<div id='i201'  class='figcenter id021'>
-<img src='images/i201.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 56.</span>—Nostradamus and Marie de Médicis.</p>
-</div>
-</div>
-
-<p class='c021'>The room in which this trick is to be performed should
-have a smaller one adjoining it, about eight feet square.
-<span class='pageno' id='Page_203'>203</span>The magician in the first place shows the small apartment
-to the spectator, who perceives that it contains
-nothing but an empty chair placed against the wall.
-The partition between the two rooms is provided with
-a small hole, covered with glass, exactly opposite the
-chair, and at about the ordinary height of the eyes.
-On the inner side there are two grooves, in which slide a
-block of wood containing a prism, as shown in <a href='#i203'>fig. 57</a>,
-which may be quickly and easily replaced by a piece of
-plane glass. On looking through this opening, the
-spectator sees a man sitting in a chair, but suddenly,
-without any apparent cause, the man changes into a goat,
-a sheep or some other animal. The sudden replacing
-of the prism, which takes place without the spectator
-perceiving it, causes him to see, not the floor with the
-man and chair upon it, but the ceiling, which is carpeted
-exactly in the same way, and is provided with a precisely
-similar chair, upon which is placed a goat or any
-other animal.</p>
-
-<div id='i203'  class='figcenter id015'>
-<img src='images/i203.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 57.</span>—The Arrangement of the Reversing Prism.</p>
-</div>
-</div>
-
-<p class='c021'>While looking at the goat, the plane glass is substituted
-for the prism, and the man reappears; another
-<span class='pageno' id='Page_204'>204</span>movement of the prism, and he changes into a sheep,
-a figure of a sheep having in the meantime replaced that
-of the goat. Of course it is necessary not merely to have
-the walls, floors, and chairs precisely alike, but they
-must each occupy the same relation to each other. If
-it is desirable only to change the head, it is simply
-necessary to have a lay figure with a moveable head,
-dressed precisely in the same manner as the living
-operator, in the upper portion of the chamber. At the
-end, by the substitution of the empty chair, the individual
-may be made to disappear entirely.</p>
-
-<p class='c021'>There may often be seen in the streets of London, a
-man showing a wonderful instrument, consisting of a
-telescope cut in two, the two portions being separated
-from each other by an interval of three or four inches.
-On looking through the instrument, the spectator of
-course sees the object at which it is pointed; but what
-is his astonishment to find, that when the showman
-places a brick between the two halves of the instrument
-he sees just as well as before. The showman generally
-informs him that the instrument in question has such
-powerful lenses, that it will not only see through a
-brick, but even through a policeman’s head if it happened
-to be in the way; and the spectator, having paid
-his penny, goes away perfectly mystified, until, like the
-young lady who believed that all machinery was worked
-“by a screw, somehow,” he comforts himself with the
-idea that the trick is performed “by a mirror, somehow.”
-The following figure will, however, soon clear
-up the mystery.</p>
-
-<div id='i205'  class='figcenter id021'>
-<img src='images/i205.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 58.</span>—The Goat Trick.</p>
-</div>
-</div>
-
-<p class='c021'>Let <span class='fss'>F M</span>, <span class='fss'>L G</span> be an ordinary telescope tube, to be
-separated in the middle by an interval large enough to
-insert a brick, the hand, or some other opaque object.
-The whole is fixed on a stand, consisting of a square
-tube with a couple of elbows to it. Between <span class='fss'>G</span> and <span class='fss'>L</span>
-a mirror (<span class='fss'>A</span>) is placed diagonally, which receives the
-<span class='pageno' id='Page_207'>207</span>image of the objects to be looked at. This mirror sends
-the image downwards to another placed diagonally at
-<span class='fss'>C</span>, a third being placed at <span class='fss'>D</span>, and a fourth at <span class='fss'>B</span>. The
-horizontal ray, meeting the mirror at <span class='fss'>A</span>, is consequently
-bent downwards to <span class='fss'>C</span>, then travels horizontally to <span class='fss'>D</span>,
-when it is reflected upwards to <span class='fss'>B</span>, in which it is seen by
-the eye. Of course a simple tube without any lenses
-at all would serve the same purpose, but the fact of its
-being a telescope serves to distract the attention of the
-too curious observer.</p>
-
-<div id='i207'  class='figcenter id015'>
-<img src='images/i207.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 59.</span>—How to see through a Brick.</p>
-</div>
-</div>
-
-<p class='c021'>Another illusion of the same kind is often practised
-at fancy fairs and bazaars, when a spectator looking
-into what he supposes to be an ordinary looking-glass,
-sees his companions instead of himself. The way in
-which this is effected is very simple. A looking-glass
-is placed diagonally across a square box, the apertures
-in the sides being so arranged that the spectator does
-not perceive that he is looking into a glass that is placed
-at an angle. Of course the exhibitor endeavours to
-<span class='pageno' id='Page_208'>208</span>show the illusion to two persons at once; and if they
-are strangers to each other, and of the opposite sex, a
-great deal of fun is made out of the trick. A showman
-at Greenwich made an immense harvest by showing two
-such mirrors, one to all the young girls who wished to
-see their future husbands, and the other to all the young
-men who wished to see their future wives. Of course
-he had a tolerably good-looking male and female confederate
-to help him. With a couple of mirrors placed
-back to back in a square case, with an opening on
-each side, the illusion is still more perfect, as on looking
-through any of the holes the box seems to be quite
-empty.</p>
-
-<p class='c021'>The “Speaking Head” trick is performed on this
-principle. When the curtain is drawn up, the audience
-perceive an apparently living head placed on a small
-three-legged table, the curtain at the back of the stage
-being quite visible through the legs. By and by the
-bodiless head, which is generally painted in a very fantastic
-manner, begins to speak, answers questions, and
-ends by singing a song. The trick is performed in the
-following way: The spaces between the legs are filled
-with a looking-glass; consequently, the spectators see
-the reflection of the curtains at the <i>sides</i> of the stage,
-which are made exactly like those at the back, thus
-giving the table the appearance of standing on three
-slim legs, with nothing between. Behind the looking-glass
-there is of course plenty of space for the body of
-the man belonging to the magical head. The exhibitor
-naturally takes especial care never to pass in front of
-the table, otherwise the lower part of his body would be
-reflected in mirrors.</p>
-
-<div id='i210'  class='figcenter id021'>
-<img src='images/i210.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 60.</span>—The Polemoscope.</p>
-</div>
-</div>
-
-<p class='c021'>The polemoscope (from two Greek words signifying
-“war” and “to see”) is another instance of double
-reflection. It was said to have been invented by Helvetius,
-<span class='pageno' id='Page_211'>211</span>about 1637. <a href='#i210'>Fig. 60</a> will show the principle of
-this instrument.</p>
-
-<p class='c021'>The luminous rays coming from a distant object are
-received upon an inclined mirror, which is elevated
-above the parapet of a fortification, and are reflected
-downwards to a second, which is placed at a corresponding
-angle. If necessary, lenses can be interposed, so as
-to give a magnified view of the distant object that is
-being examined. By means of such an instrument, the
-movements of the enemy can be followed without danger,
-the apparatus being generally of small size, and not
-attracting notice. Amongst the varieties of this instrument,
-is one whose use is readily seen by inspecting <a href='#i213'>fig. 61</a>,
-by which it seems to be perfectly possible to see
-with safety all that is going on outside the door of the
-house without being perceived. The line of the mirrors
-in this case is at right angles to that of the polemoscope
-in <a href='#i210'>fig. 60</a>. Amongst the different varieties of polemoscope
-which have been invented, may be mentioned a reflecting
-opera-glass, which was greatly used by the
-beaux and dandies of the last century. In the tube of
-this instrument was inserted an inclined mirror, which
-allowed the spectator to point his glass in quite a different
-direction to that of the object he was really
-looking at. In fact, it was constructed somewhat on
-the same principle as the Herschellian or Newtonian
-telescope, and enabled the possessor, while apparently
-enjoying the play, to observe all that was going on in
-the boxes or pit of the theatre. Years ago, there was
-a little instrument of a similar kind, sold for a penny
-in the streets of London, which consisted of a morsel of
-looking-glass set at an angle, in a pill-box, and which
-gave the possessor the power of seeing all that was
-going on behind him. Persons who wear dark preservers
-are often in the habit of observing all that is
-<span class='pageno' id='Page_212'>212</span>going on behind their backs by the reflection seen in
-the corner of their glasses.</p>
-
-<p class='c021'>Such are the principal optical recreations founded on
-the reflecting and refracting properties of mirrors and
-lenses. We shall end this chapter by appending to it
-the description of a few additional optical amusements
-that are quite within the reach of the amateur.</p>
-
-<p class='c021'>If the reader is in possession of a concave mirror, it
-may be made the means of performing a number of
-amusing experiments. In front of it is placed a plaster
-head, a skull or any other object, mounted on wheels
-and running along a grooved platform, which is naturally
-kept perfectly concealed from the spectators. The
-mirror is slightly inclined, so as to reflect the image of
-the object at an angle to the observer’s eye. By running
-the cast backwards and forwards, it will have the
-appearance of advancing and retiring from the spectator
-in a very imposing manner. A dagger may be substituted
-for the cast, and by being made to work up and
-down on a pivot, will have the appearance of striking
-at the spectator. We have already seen that an experiment
-of this sort had such an effect on Louis XIV.
-that he drew his sword to defend himself from his
-imaginary aggressor. There is another way of performing
-this trick, by suddenly illuminating the skull
-or dagger by means of a dark-coloured box containing
-a light, which may be made to throw its reflections on
-the object, by sliding it along a couple of wires. In
-the case of the dagger, however, the hinged arrangement
-will be found more effective.</p>
-
-<div id='i213'  class='figcenter id021'>
-<img src='images/i213.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 61.</span>—Protection against ill-natured people.</p>
-</div>
-</div>
-
-<p class='c021'>One of Robertson’s tricks was called the “Magic
-Box,” and he astonished a numerous party of visitors
-who were staying at a country house to which he had
-been invited. One of the gentlemen who was always
-boasting of his freedom from superstitious feelings of
-any kind, had had several arguments with Robertson on
-<span class='pageno' id='Page_215'>215</span>the subject of apparitions, and the latter thought that
-he would at any rate surprise his strong-minded friend
-by an easy trick or two. He consequently chose as his
-confederate a lady to whom the gentleman had been
-paying great attention during the time of his visit.
-Robertson one evening mysteriously delivered a small
-box to him, which he was to place upon his toilet table,
-and unlock exactly at midnight. The gentleman did
-so, and what was his astonishment to see the face of the
-lady with whose charms he had been so deeply impressed
-suddenly spring out of the box. His look of terror and
-surprise was evidently too much for Robertson’s confederate,
-who burst into a merry peal of laughter, leaving
-her admirer in a very disconcerted state.</p>
-
-<p class='c021'>After all we have said on the subject of mirrors, it is
-not difficult to guess how this trick was performed. The
-box in question was painted black on the inside, and
-contained a concave mirror placed at an angle of 45°.
-The reflection of the lady, who was of course in the next
-room, was carried by means of several plane mirrors
-placed in boxes communicating with each other through
-the partition of the room, the head of the lady only being
-strongly illuminated, the rest of her figure not appearing
-by being kept quite dark.</p>
-
-<p class='c021'>The figures reflected from smoke are extremely surprising.
-To perform such experiments a phantasmagoria
-is necessary. The focus is so adjusted that the distant
-image falls just above a brasier containing lighted
-charcoal. Everything being ready, a few grains of olibanum
-or other gum are thrown on the coals, and the
-smoke that rises immediately affords a screen for the
-reflection of the images proceeding from the phantasmagoria.
-If the amateur is not the possessor of a magic
-lantern, a properly arranged concave mirror will answer
-almost the same purpose.</p>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div class='chapter'>
-  <span class='pageno' id='Page_216'>216</span>
-  <h2 id='ch3-04' class='c012'>CHAPTER IV.<br /><span class='c020'>THE PROPERTIES OF MIRRORS.</span></h2>
-</div>
-<p class='c014'><span class='sc'>Almost</span> every one in his younger days has possessed
-and broken that pretty instrument known as the kaleidoscope.
-His researches into its construction no doubt
-taught him that it consisted of a cylindrical tube in tin
-or cardboard, with a moveable cap at one end and a
-small hole at the other. In the interior of the tube
-were found three long glasses, blackened on the back,
-placed at an angle, and kept in position by pieces of
-cork. The moveable cap was provided with two circular
-pieces of glass, one ground and the other transparent,
-between which were placed a number of pieces
-of coloured glass. On holding the instrument up to
-the light and looking through the eye-hole, a beautifully
-coloured star was seen whose form and hue
-changed by simply shaking the tube.</p>
-
-<p class='c021'>The kaleidoscope was invented by Sir David Brewster,
-and is exceedingly simple in principle. We all know
-that if a luminous object, such as a taper, is placed before
-a mirror, it gives forth rays of light in all directions.
-Amongst these luminous rays, those that fall on
-the surface of the mirror are, of course, reflected in such
-a manner that the angle of reflection is equal to the
-angle of incidence. If another mirror be placed at right
-angles to the first, and an object be put in the angle,
-the image of it will be multiplied four times. If the
-angle be diminished to 60°, six reflections will be seen,
-<span class='pageno' id='Page_217'>217</span>and so on. A symmetrical figure is constantly obtained,
-forming in one case a cross composed of four similar
-portions; in the other a triple star, the halves of each
-ray being similar. It is the symmetry of the figure that
-gives the pleasing effect. In the ordinary kaleidoscope
-the angle made by the reflecting surfaces is thirty degrees,
-and a star of six rays is formed, the halves of
-each ray being alike. The figures formed in the kaleidoscope
-are simply endless; and if the space between
-the glasses in the moveable cap be filled with bits of
-opaque as well as transparent substances, the varieties
-of light and shade may be added to those of colour. It
-was at one time the fashion to copy the images
-formed in the kaleidoscope as patterns for room papers,
-muslins, curtains, shawls, and other similar fabrics, but
-thanks to the spread of artistic taste in this country the
-decorative designer now relies more on his own talent
-than any aid he may receive from optical instruments.</p>
-
-<p class='c021'>Plane mirrors, as we have seen, reflect objects upright
-and symmetrical, reversing only the sides. Concave
-mirrors reverse them, and if they are not placed
-exactly in the proper focus, distort them by making one
-portion appear smaller than the other; while convex
-mirrors reflect them in an upright position, but also
-similarly slightly distorted. But when the mirror is
-not a portion of a sphere, like those whose properties
-we have been considering, the distortion is increased to
-so great an extent as to deform the object so that it is
-difficult to recognise its nature from its reflection. We
-all know the distortion that our face undergoes when
-reflected from the shining surface of a teapot or spoon,
-and the cylindrical mirrors that hang in the shop windows
-of many opticians are the source of much amusement
-to the passers by, whose physiognomies are shown
-to them either lengthened to many times their natural
-size, or widened to an extent that is ludicrously hideous,
-<span class='pageno' id='Page_218'>218</span>according to the position in which the mirror is
-hung. Such distortions are known to opticians as <i>anamorphoses</i>,
-from two Greek words signifying the destruction
-of form; and distorted drawings used to be
-sold at one time which when reflected from the surface
-of the cylindrical mirror, became perfectly symmetrical.
-Anamorphic drawings may be also made, which when
-looked at in the ordinary manner appear distorted, but
-when viewed from a particular point have their symmetry
-restored to them. With a little knowledge of drawing,
-it is not difficult to produce these in great variety.</p>
-
-<p class='c021'>Suppose the portrait in <a href='#i218'>fig. 62</a> to be divided horizontally
-and vertically by equidistant lines comprehended
-within the square <span class='fss'>A B C D</span>.</p>
-
-<div id='i218'  class='figcenter id023'>
-<img src='images/i218.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p>Fig. 62.</p>
-</div>
-</div>
-
-<p class='c021'>Upon a second piece of paper draw the figure shown
-in <a href='#i220'>fig. 63</a> in the following manner. Draw the horizontal
-line <i>a b</i> equal to <span class='fss'>A B</span> (<a href='#i218'>fig. 62</a>), and divide it into the
-same number of parts. Through the centre draw a
-perpendicular line to <span class='fss'>V</span>, and cross it by a line <i>e d</i>
-parallel to <i>a b</i>. Lastly, draw <span class='fss'>V S</span> horizontal to <i>e d</i>.
-The length of the two lines <i>e</i> <span class='fss'>V</span> and <span class='fss'>S V</span> is quite arbitrary,
-but the longer you make the former in proportion to the
-<span class='pageno' id='Page_219'>219</span>latter the greater will be the distortion of the drawing.
-Now draw the lines <span class='fss'>V</span> 1, <span class='fss'>V</span> 2, <span class='fss'>V</span> 3, and <span class='fss'>V</span> 4, and join <span class='fss'>S</span>
-to <i>a</i>. Wherever <span class='fss'>S</span> <i>a</i> crosses the divisions 1, 2, <i>e</i>, 3, 4,
-and <i>b</i>, draw a horizontal line, parallel of course with
-<i>a b</i>. You will thus have a trapezium <i>a b c d</i> divided
-into as many spaces as the square <span class='fss'>A B C D</span> in <a href='#i218'>fig. 62</a>,
-and it now remains to fill them in with similar portions
-of the figure. Thus, for instance, the nose is in the
-fourth vertical division, starting from the left, and in
-the third and fourth counting from the top; in order,
-therefore, to make it occupy so lengthened a space it
-must be considerably distorted by the pencil. It will
-be readily seen also that the more numerous the spaces
-into which the square is divided, the easier it will be
-to draw the distorted picture. It is by this means that
-the <i>anamorphosis</i> shown in <a href='#i220'>fig. 63</a> has been drawn.</p>
-
-<p class='c021'>The next thing to do is to find the point of view
-from which we can see the figure in its natural proportions.
-This will be found to be at a distance above the
-point <span class='fss'>V</span> equal to the line <span class='fss'>V S</span>. In order to complete the
-experiment it is simply necessary to place the distorted
-picture in a horizontal position, and fix a piece of cardboard
-vertically at the point <span class='fss'>V</span>. If a hole be punched
-in it at a distance from <span class='fss'>V</span> equal to <span class='fss'>S</span>, and the drawing
-be looked at through it, the whole of the parts will fall
-into symmetry immediately.</p>
-
-<p class='c021'>The experiment may be tried first with <a href='#i225'>fig. 63</a>, the
-hole being made rather large, and the eye placed at a
-distance of from 3 to 4 inches.</p>
-
-<p class='c021'>It would be difficult, without having recourse to
-geometrical formulæ, to explain how it happens that
-by placing the eye at a particular point the distorted
-lines of the drawing become symmetrical; but perhaps
-a mechanical demonstration will help to make this
-difficult subject a little plainer.</p>
-
-<div id='i220'  class='figcenter id022'>
-<span class='pageno' id='Page_220'>220</span>
-<img src='images/i220.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 63.</span>—Anamorphosis.</p>
-</div>
-</div>
-
-<p class='c021'>Draw in outline any figure upon a piece of cardboard,
-and make a series of pin-holes along the most prominent
-lines of the drawing, taking care that they are pretty
-close together. Place the perforated card in a vertical
-position on a sheet of paper, so that the rays from a
-candle or lamp may fall, on the flat surface beneath. On
-looking at the luminous figure formed from the drawing,
-you will find that it is as much distorted as the lady’s
-<span class='pageno' id='Page_221'>221</span>head in <a href='#i220'>fig. 63</a>, and that the lower you place the candle
-the greater will be the deformity. You may if you
-please, trace the luminous figure on the paper, and the
-result will appear distorted when looked at in the ordinary
-manner, but symmetrical when viewed from the
-point at which the flame of the candle was placed.</p>
-
-<p class='c021'>In the foregoing experiments we have spoken of the
-anamorphic drawings as being placed in a horizontal
-position, but they may be looked at just as well vertically,
-the card with the hole being in this instance horizontal.
-It is also not necessary that the point of sight
-(<span class='fss'>V</span>, <a href='#i220'>fig. 63</a>) should be in the centre of the picture; it
-may be placed at one side or the other, care being
-taken to draw all the divisional lines so that they meet
-at this particular spot. A few experiments with a candle
-and a perforated figure will soon show the student
-how to accomplish this.</p>
-
-<p class='c021'>Anamorphoses by reflection may be prepared, if this
-principle is carried out, which appear a mass of confused
-lines until they are reflected in a cylindrical mirror.
-Formerly opticians were accustomed to construct
-anamorphoses which became symmetrical pictures when
-viewed in a conical mirror; but the fashion for such
-toys appears to have gone out. Such drawings were
-extremely difficult to make, and the mirrors, having to
-be ground and polished with great care, were very expensive.</p>
-
-<p class='c021'>Some experimentalists have carried the subject so
-far that, by looking at the drawing of an object in particular
-positions, it changed into quite a different subject.
-In the cloister of an abbey that once existed in
-Paris, there were two anamorphoses of this kind. They
-were the work of a certain Father Niceron, who has
-left behind him a treatise in Latin on optical wonders,
-entitled <i>Thaumaturgus Opticus</i>, which contains a long
-essay on anamorphoses. One of these pictures represented
-<span class='pageno' id='Page_222'>222</span>St. John the Evangelist writing his Gospel; the
-other Mary Magdalene. When looked at in the ordinary
-manner, they appeared to be landscapes; but
-when the observer placed himself in a particular position,
-they changed into the figures we have mentioned.</p>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div class='chapter'>
-  <span class='pageno' id='Page_223'>223</span>
-  <h2 id='ch3-05' class='c012'>CHAPTER V.<br /><span class='c020'>CHINESE SHADOWS.</span></h2>
-</div>
-<p class='c014'><span class='sc'>While</span> upon the subject of optical wonders, we
-should hardly be forgiven if we did not give a description
-of the amusement known as Chinese shadows, or
-Fantocini. In the winter time it is difficult to pass
-through any of the large thoroughfares of London after
-nightfall, without seeing a crowd admiring the popular
-fantocini farces of the “Broken Bridge,” or “Billy
-Button;” and although these dramatic exhibitions are
-not always free from vulgarity, they are received with
-vociferous applause by at least the younger portion of
-the audience.</p>
-
-<p class='c021'>The apparatus for the exhibition of the fantocini is
-generally very simple. The screen on which they are
-shown is generally made of calico rendered semi-transparent
-with copal varnish, and the figures are cut out
-of cardboard. Frames containing landscapes and scenes
-of different kinds are also provided, which are cut out
-in the same material. The <i>dramatis personæ</i> are generally
-made with moveable limbs, which they throw about
-in the most unanatomical manner, and the showman is
-often endowed with ventriloquial talents of no mean order.
-This amusement is to be found in all parts of the
-world, from the Strand and Tottenham Court Road
-London, to the streets of Algiers and Java. A graphic
-writer in the <i>Magasin Pittoresque</i> gives a pleasant description
-<span class='pageno' id='Page_224'>224</span>of the fantocini, as exhibited at the Arabs’
-theatre in the Mohammedan quarter of the city of Algiers.
-It was on the occasion of the feast of the Bairam,
-which immediately follows the termination of the
-Ramadan, or Mohammedan Lent. The theatre, which
-was the only one frequented by the Arab population,
-consisted simply of a long vaulted hall, without seats,
-boxes, or galleries; but the audience, who had already
-been there some time, did not seem to regard the omission
-as of any consequence, but had seated themselves
-on the ground with great coolness, chatting in whispers,
-and waiting patiently until the director should consider
-the place full enough to begin the performance. Half
-an hour elapsed, and the spectators still chatted on
-quite unconcernedly; an hour, and yet there was no
-hissing or stamping of feet from the grave and patient
-spectators. At last they reached the maximum, and a
-boy came forward and blew out the few lamps with
-which the theatre was lighted, leaving them to smoulder
-away with a perfume that was certainly not Oriental in
-its character. First came the legend of the Seven
-Sleepers; then Scheherazade relating her bewitching
-stories to the Sultan. These were followed by Aladdin
-and the Wonderful Lamp, a story that is as popular in
-Algiers as it is in London or Paris; the whole culminating
-in a kind of burlesque, in which a great deal of
-gross fun was mixed up with a number of rebellious allusions.
-The devil, for instance, who is of course one
-of the members of the troupe, is portrayed as a French
-soldier, bearing a cross on his breast like an ancient
-Crusader. After him came Carhageuse, who is the
-buffoon of the Eastern stage, and who makes violent
-but unsuccessful love to a charming young Jewess.
-There was a poor barber who was raised to the dignity
-of grand vizier, his successor’s head being cut off by the
-yataghan of the Oriental Jack Ketch, to the great delight
-<span class='pageno' id='Page_227'>227</span>of the people. Then a wretched Jew receives the
-bastinado, amidst vociferous applause, which increases
-still higher when the ears of an unhappy Giaour are cut
-off and thrown to the dogs. Throughout the piece, it
-is of course the Mussulman who always triumphs, like
-the French guards at the <i>Cirque Impériale</i>, or the British
-grenadiers at old Astley’s. The performance concluded
-with a grand naval battle between the Moorish
-and Spanish fleets. The drum as usual served for cannon,
-there was a great deal of smoke and confusion,
-and the Christian fleet gradually sank under the continuous
-fire of the Mussulmans amidst the plaudits and
-bravos of the crowd.</p>
-
-<div id='i225'  class='figcenter id021'>
-<img src='images/i225.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 64.</span>—Effect of cut paper-work.</p>
-</div>
-</div>
-
-<p class='c021'>In Java, the subjects of the fantocini are generally
-taken from the native mythology. The screen on which
-the shadows are exhibited is ten or twelve feet long,
-and five feet high, and the figures are cut of thick
-leather, their limbs being moved by thin pieces of nearly
-transparent horn.</p>
-
-<p class='c021'>In <a href='#i225'>fig. 64</a> we see another kind of Chinese shadows,
-in which the lights of the figure are cut out. These
-pictures are perfectly unrecognisable as being even the
-basest imitation of any known form; but when their
-shadows are thrown on the wall, the cut-out portions
-show us lights, whilst those that have been left form
-the shadows. On the Boulevard des Capucines, at
-Paris, there used to be a man who managed to pick up
-a good living by selling these candle shadows. Of course
-he used to carry on his trade of an evening, and with
-a strong lamp he would throw the shadows of his
-figures on the white walls of the houses, or the blind of
-a shop window, or even on the pavement. With a little
-care and ingenuity a number of these amusing cards
-may be easily designed. In showing them, care must
-be taken to choose the best distances between the light
-and the paper, and between this latter and the wall.
-<span class='pageno' id='Page_228'>228</span>If the card be placed too close to the wall, the resulting
-shadows will be too dark, and the outlines too sharp;
-if, on the contrary, the light is placed too far off, the
-outlines become confused, and the proper effect is
-lost.</p>
-
-<p class='c021'>Shadows have been applied before now to the propagation
-of seditious ideas. “In 1817,” says an esteemed
-French author, “one winter’s night we were all sitting
-round the table listening to my father, who was reading
-aloud an interesting book of the period, when a friend
-of our family, who had been formerly an officer of the
-Empire, entered the room. He was a serious, upright,
-soldierly man, and wore his coat buttoned up to his
-chin. He had hardly replied to our salutations, when
-he drew a chair to the table, and made a sign with his
-hands and eyes that plainly indicated silence and discretion.
-There was something in the expression of his
-countenance that seemed to show that he had something
-mysterious in store for us, and we fully expected to hear
-some extraordinary news, or to see him bring out a
-Bonapartist pamphlet of more than usual importance.
-Our surprise was consequently great when we saw him
-slowly unscrew the top of his cane, which was turned
-out of boxwood, and presented nothing very remarkable
-either in form or material. He, however, took up a
-copybook which was lying on the table, placed it at a
-certain distance from the lamp, and then laid upon it
-the little piece of turned boxwood. At first we noticed
-nothing at all extraordinary, and he smiled at our want
-of intelligence, until at last my youngest brother cried
-out suddenly, ‘Look! there’s the head of Napoleon!’
-and truly enough, we found, on looking more attentively
-at the shadows of the turned knob of the cane, that their
-profile was that of the great exile, most correctly and
-clearly portrayed. The old captain’s face lighted up at
-the sight, and the tears came into his eyes. ‘We shall
-<span class='pageno' id='Page_229'>229</span>see him again,’ he murmured in a low voice, and he
-hummed the burden of a Bonapartist song then in
-vogue. During the rest of the evening he was very
-lively, and proved to us most conclusively, that before
-six months the <i>Grande Armée</i> would be revenged for
-<span class='pageno' id='Page_230'>230</span>their defeat at Waterloo. Some weeks after, there was
-hardly a soldier in the town that did not possess a stick
-or a tobacco-pipe stopper, turned in this fashion, but
-one day a panic seized everybody, and the canes and
-pipe stoppers were all burnt.”</p>
-
-<div id='i229'  class='figcenter id021'>
-<img src='images/i229.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p>Seal.&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Cane.<br /><span class='sc'>Fig. 65.</span>—Seditious Toys.</p>
-</div>
-</div>
-
-<p class='c021'>Fig. 65 represents historic heads cut in this way.
-During the Shakespeare Tercentenary excitement, a
-London turner made quite a little fortune by making
-heads of the great poet on the same principle.</p>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div class='chapter'>
-  <span class='pageno' id='Page_231'>231</span>
-  <h2 id='ch3-06' class='c012'>CHAPTER VI.<br /><span class='c020'>POLYORAMA—DISSOLVING VIEWS—DIORAMA.</span></h2>
-</div>
-<p class='c014'><span class='sc'>The</span> description of the polyorama naturally follows
-that of the phantasmagoria, being a practical application
-of precisely the same principles. In the case of
-the polyorama, however, two or even more lanterns of
-the best construction, are used. There are therefore
-two sets of lenses identical in every particular, placed
-side by side, in the same line, the foci of both being
-adjusted for the same spot, so that the images refracted
-from each may superpose each other without difficulty.
-In each instrument there are the same pictures, but
-they differ in certain particulars, as we shall see presently.</p>
-
-<p class='c021'>In the phantascopes shown in <a href='#i184'>figs. 52</a> and <a href='#i194'>54</a> there
-are two sets of lenses; the first carries a glass bearing
-the image of a skeleton in a winding sheet, while on the
-glass belonging to the second a naked skeleton is portrayed.
-If, therefore, at a given instant the first lantern
-is shut off, the spectators see the winding sheet
-torn, as it were, suddenly from the spectre before them.
-The first lantern being turned on once more, the skeleton
-is instantly reclothed in its hideous garb.</p>
-
-<p class='c021'>It is of course not necessary always to choose such
-horrible subjects for representation, as it is possible
-to produce changes of a much more agreeable nature.
-For instance, a volcano may be depicted during its
-<span class='pageno' id='Page_232'>232</span>tranquillity, with the sun shining on its verdant sides,
-and surmounted with a gently rising wreath of smoke.
-Then it may be shown at night, with its crater vomiting
-flames and red-hot stones, while streams of lava are
-flowing beneath. By proper mechanism, one lantern
-may be gradually shut and the other as gradually
-opened, producing an effect that appears perfectly
-natural, from the gentle change which takes place.
-Daylight, twilight, and moonlight effects may be easily
-made to succeed each other in their proper order, and
-the most opposite scenes may be made to change each
-other by proper appliances. Those who have seen the
-dissolving views at the Polytechnic, know what effects
-are produced by this very simple means. A virgin
-forest changes to a crowded church, which in turn dissolves
-into a scene on the Alps.</p>
-
-<p class='c021'>The diorama, properly so called, invented by the illustrious
-Daguerre, differs completely in principle from
-the apparatus we have just been describing. As its
-etymology indicates, the pictures shown are seen
-<i>through</i>. As in the case of the polyorama, there are
-two different effects painted upon the cloth, which are
-brought out by a double system of illumination.</p>
-
-<div id='i234'  class='figcenter id021'>
-<img src='images/i234.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 66.</span>—Diorama.</p>
-</div>
-</div>
-
-<p class='c021'>Fig. 66 will show the way in which these changes are
-managed. The large picture, which is hanging vertically,
-is painted both in front and behind. The front
-is illuminated by reflection from a semi-transparent
-screen placed over it, which receives the light of the
-floor above. The back is lighted from the windows behind,
-which are provided with blinds to regulate the
-amount of light. The effects produced by the diorama
-were truly marvellous, and Daguerre had a special talent
-for this kind of painting. His famous <i>Midnight
-Mass</i>, which was exhibited at the Regent’s Park, was
-one of the most renowned of his works. The scene first
-represented a dark, empty church, feebly lighted by a
-<span class='pageno' id='Page_235'>235</span>small altar lamp, but gradually the lights appeared here
-and there, worshippers congregated in front of the altar,
-filling the nave and aisles. In Paris the same
-scene was exhibited, representing the interior of the
-Church of St. Germain l’Auxerrois with such perfect
-reality, that a countryman actually threw a halfpenny
-against the painted canvas, to see whether he were
-really in a church or not.</p>
-
-<p class='c021'>The next scene represented the destruction of the
-village of Goldau, near Lucerne, by a landslip. First
-there appeared a smiling fertile valley, its sides crowned
-with verdure; a storm gradually rose, the rain fell, the
-wind blew, the lightnings flashed, and the thunder rolled
-in the distance. Darkness at last closed in, and when
-the sun once more rose over the valley, nothing was to
-be seen but a mass of fallen rocks.</p>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div class='chapter'>
-  <span class='pageno' id='Page_236'>236</span>
-  <h2 id='ch3-07' class='c012'>CHAPTER VII.<br /><span class='c020'>THE STEREOSCOPE.</span></h2>
-</div>
-<p class='c014'><span class='sc'>Having</span> devoted so much space in the preceding
-chapters to optical amusements of a purely recreative
-character, it is only right that we should now say a few
-words on certain instruments of a less frivolous character
-than those we have lately been considering, and which
-deserve at our hands the most serious attention. We
-shall, therefore, in the present chapter, speak of an ingenious
-instrument which serves to show in relief the
-images of objects depicted on a flat surface. We have
-already seen, that although we have two eyes, provided
-with lenses and screens by means of which the images
-of things around us are formed, we only perceive
-a single object; and the student has no doubt long
-since wondered why nature has bestowed two eyes upon
-us, when only one would have apparently served the
-same purpose. This question was for a long time a
-complete puzzle to philosophers, and it was not until
-Professor Wheatstone made his experiments on binocular
-vision in 1838, that the matter received a satisfactory
-explanation. He showed that each eye receives a different
-impression of any object upon the retina, and that
-it is in consequence of the union of these slightly dissimilar
-images that the sensation of relief is experienced.
-A one-eyed man or a Cyclops would only partially
-perceive relief in the objects presented to his view, in
-<span class='pageno' id='Page_237'>237</span>consequence of a single image being sent to his brain.
-He would, no doubt, after examining the things he saw
-with his hands, know they were solid, and generally see
-them so; but if a new object were presented to his view
-he would have some difficulty in knowing whether it
-had a flat surface or not.</p>
-
-<div id='i237'  class='figcenter id025'>
-<img src='images/i237.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 67.</span></p>
-</div>
-</div>
-
-<p class='c021'>The principle of binocular vision may be explained
-as follows: If a playing die, such as is represented in
-fig. 67, be held out at arm’s length in the position indicated
-in the figure, and looked at first with the left eye
-and then with the right, we shall find that in the first
-case we see a little of the three dots on the left-hand
-side, and in the second we lose sight of the three dots
-and see a little of the single one on the right-hand side.
-The images seen by each eye are, therefore, slightly dissimilar,
-and it stands to reason that, if by any means we
-can combine two slightly dissimilar flat pictures of a
-solid object, we shall see it in relief. This was proved
-practically by Professor Wheatstone, who constructed
-an instrument capable of effecting the desired union,
-and which has since been called the stereoscope, from
-two Greek words signifying ‘to see solid.’ The instrument
-remained for a long time fallow, so to speak, from the
-difficulty of drawing two pictures that should be identical
-in size and details, although dissimilar in the
-arrangement of their perspective. It was, therefore, not
-until photography enabled us to do this with the greatest
-<span class='pageno' id='Page_238'>238</span>ease and exactitude that the stereoscope became common.
-The instrument first devised by Professor Wheatstone,
-was what is termed a reflecting stereoscope, and
-was expensive to make and cumbrous to use. It was
-modified by Sir David Brewster, by the substitution of
-prisms for reflectors, and was thus made cheaper and
-more portable. The refracting form of stereoscope is
-so familiar to most people, that it really needs no
-description. It will only be necessary to mention that
-the prisms used in the eye-pieces are made by cutting
-a double convex lens in two, and reversing the halves.
-They are so placed that the centre of each prism is just
-in the centre of each eye; but as the eyes of different
-people vary in distance, an arrangement is generally
-added so that the eye-pieces may slide from side to side.
-Being cut from lenses, the prisms have a magnifying
-power; consequently other means are provided for
-sliding them up and down to suit the length of focus
-in different eyes.</p>
-
-<div id='i238'  class='figcenter id025'>
-<img src='images/i238.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 68.</span>—Stereoscope.</p>
-</div>
-</div>
-
-<p class='c021'>In <a href='#i239'>fig. 69</a> we can follow the path of the rays proceeding
-from each picture, and reach the eyes apparently
-from a spot exactly between the two.</p>
-
-<p class='c021'>In the reflecting stereoscope two mirrors are joined
-<span class='pageno' id='Page_239'>239</span>together at right angles to each other, the two pictures
-being placed at each side, at a distance corresponding
-to their size. The reflecting instrument, although not
-so portable, is in some sort superior to the other,
-inasmuch as pictures of any size can be seen by it,
-whilst in the smaller instrument the size of the photograph
-is limited by the distance at which the eyes are
-placed.</p>
-
-<div id='i239'  class='figcenter id015'>
-<img src='images/i239.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 69.</span>—The Principle of the Refracting Stereoscope.</p>
-</div>
-</div>
-
-<p class='c021'>It should be mentioned, that no optical instrument of
-any kind is absolutely necessary to obtain a stereoscopic
-effect from two suitable drawings or photographs, as it
-is quite possible by a little management of the eyes to
-cause the two images to combine with each other. Referring
-again to <a href='#i237'>fig. 67</a>, it will be perceived that the
-two figures of the dice are about an inch and a half
-from each other. Holding the book at about ten inches
-from the eye, they are viewed by squinting strongly
-until the <i>right</i> eye looks at the <i>left</i> die, and the <i>left</i> eye
-at the right. This may be also done by converging the
-eyes on a point beyond the centre of the figure, which
-may be easily done by looking at a point midway between
-the two. In both cases the images at first appear doubled,
-and we see four dice, but a little practice will soon
-enable you to cause the two inside images to coalesce,
-<span class='pageno' id='Page_240'>240</span>and so give the effect of relief. It is true that even then
-three images are seen, but the eye soon grows accustomed
-to neglect them altogether. This habit is a very
-pleasant acquirement for the London <i>flâneur</i>, who can
-thus see in perfection the numberless stereoscopic views
-now shown in our shop-windows without the intervention
-of an instrument of any kind.</p>
-
-<p class='c021'>The method of photographing subjects for the stereoscope
-is very simple, and consists in taking two views
-of the object to be depicted, from two different points.
-According to the distance of these points from each
-other, so will the resulting pictures appear in greater or
-less relief. This is readily seen in some stereoscopic
-portraits which have been taken at a large angle, and
-consequently show such increased relief as to produce
-distortion. Theoretically, the interval of the two points
-of view ought to be two inches and a half, that being
-the average distance between the two eyes; but in
-practice it is better to increase it in the case of portraits
-or other near objects to about twelve inches, and in that
-of views to even several feet. Brewster’s original rule
-for taking stereoscopic photographs, was to place the
-cameras one foot apart for every twenty-five feet of distance.
-The beautiful stereoscopic pictures of the moon
-photographed by Mr. Warren de la Rue were taken at
-more than 1,000 miles’ distance, in order to obtain the
-necessary relief. The principle of the stereoscope has
-received many useful applications in the way of book
-illustrations, art teaching, and anatomical demonstration,
-and has thus gained a position among philosophical
-instruments that it did not at first possess.</p>
-
-<p class='c021'>A combination of the principles of the phenakistiscope
-(<a href='#i055'>fig. 4</a>) and stereoscope, has resulted in the
-invention of an instrument called the stereotrope. A
-number of binocular photographs of some object in
-motion—a steam-engine, for instance—are taken when
-<span class='pageno' id='Page_241'>241</span>the moving parts are in different positions, and mounted
-on two revolving discs, the images being combined by
-means of a pair of semi-lenses, as in the ordinary refracting
-stereoscope.</p>
-
-<p class='c021'>We cannot leave this subject without describing the
-pseudoscope, also the invention of Professor Wheatstone.
-If a stereoscopic pair of photographs of some
-solid body—a ball, for instance—are mounted the reverse
-way, that is to say, if the picture intended to be
-looked at by the right eye is placed on the left, the
-relief of the object will be reversed, and the ball will
-appear as a hollow hemisphere. If, therefore, we can
-by means of lenses or prisms cause the image of any
-natural object, as seen by the right eye, to be conveyed
-to the left, and <i>vice versâ</i>, we shall see the relief reversed.
-A conical cap will appear in relief as a cone, a globe
-will look like a hollow sphere, and the human face will
-take the semblance of the inside of a mask. The same
-deception may be effected by looking at a seal through
-a short-focused lens, so that the image shall seem reversed.
-In this case, the light coming apparently from
-the wrong side, and shining on the parts in relief, gives
-them the appearance of being hollow. An intaglio
-will, of course, appear in relief when so looked at.
-Photographs of gems and bas-reliefs will also present
-a pseudoscopic appearance, if looked at in a light
-coming from the opposite side to that in which they
-were taken. The same appearance may be seen sometimes
-in wall papers having patterns painted in strong
-relief.</p>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div class='chapter'>
-  <span class='pageno' id='Page_242'>242</span>
-  <h2 id='ch3-08' class='c012'>CHAPTER VIII.<br /><span class='c020'>THE CAMERA OBSCURA AND CAMERA LUCIDA.</span></h2>
-</div>
-<p class='c014'><span class='sc'>The</span> construction of the camera obscura is founded
-on the fact that the rays of light, when collected
-into a point either by being passed through a small
-hole or a converging lens, form an image of the objects
-from which they proceed at the point of meeting.
-This may be readily tried by piercing the shutter of a
-room with a small hole, and holding a piece of paper
-within a short distance of it. It will be noticed that
-the smaller the hole the more distant will be the
-image formed. The first person who observed this fact
-was John Baptist Porta, an Italian philosopher who
-lived in the latter part of the seventeenth century. He
-noticed that when a screen was placed opposite a small
-hole in the shutter of his room, the objects outside were
-depicted on it in a reversed position with moderate distinctness;
-but that when a biconvex lens was placed
-over the hole, the picture was rendered much more
-distinct. This was the first attempt at the formation of
-the camera obscura, an instrument that has since bestowed
-such incalculable benefits on humanity.</p>
-
-<div id='i243'  class='figcenter id021'>
-<img src='images/i243.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 70.</span>—The Camera Obscura.</p>
-</div>
-</div>
-
-<p class='c021'>The shape of the images so formed is independent of
-the shape of the opening, which, as long as it is sufficiently
-small, may be square, oval, or triangular. This
-may be easily seen when the sun shines through the
-intervals between the leaves of a shady avenue or bower
-<span class='pageno' id='Page_245'>245</span>of trees. The image of the sun as a circular patch of
-light is seen scattered over the surface of the ground,
-although the accidental intervals formed by the leaves
-above were of a thousand different shapes. These
-images at the time of an eclipse of the sun are very
-surprising, taking, as they do, the form of a crescent,
-more or less large according to the magnitude of the
-eclipse.</p>
-
-<p class='c021'>This property possessed by the rays of light, of depicting
-on a screen the forms and colours of the objects
-from which they proceed when passed through a small
-aperture or a lens, is taken advantage of in most places
-famous for their natural scenery. The apparatus employed
-for this purpose is comparatively simple, consisting
-merely of a dark wooden hut, with a whitened table
-in the centre, and a mirror and lens in the apex of the
-roof. In <a href='#i243'>fig. 70</a> we have a section of a camera obscura
-of this kind. The mirror and lens at the top of the
-apparatus are made to revolve, so as to bring every part
-of the landscape into view in turn. A camera obscura
-in a position commanding a view of moving objects,
-such as ships sailing to and fro, or the busy streets of
-a populous town, is an unending source of amusement,
-and may be easily and cheaply constructed.</p>
-
-<p class='c021'>The camera obscura has been much utilized for taking
-hasty but exact sketches of various places. For this
-purpose it is made very light, and mounted on three legs
-carrying at their junction a flat table, whereon is placed
-the paper to receive the drawing. The tripod is covered
-with a black curtain, which, falling over the artist,
-effectually excludes all the rays of light except those
-which pass through the lens and are reflected downwards
-by the mirror. In the better kind of apparatus the
-mirror is replaced by a prism, which throws a clearer
-image than a mirror upon the screen.</p>
-
-<p class='c021'>It is on these properties of the camera obscura that
-<span class='pageno' id='Page_246'>246</span>the art of photography was founded. Everybody who
-saw the beautiful images formed by this instrument was
-struck with the idea that by some means or other they
-could be fixed on paper. After numberless attempts the
-long-wished-for goal was at length arrived at; and now
-optics, aided by chemistry, is enabled to depict for us
-natural objects of every kind, from the distorted limb
-of the hospital patient to the beautiful forms of the
-queens and empresses of the world—from the tiniest
-animalcule to the great sun itself, who is compelled by
-the might of science to paint his own portrait for us
-with all his faults and imperfections.</p>
-
-<p class='c021'>The lenses used for photographic purposes have only
-reached their present state of perfection after ceaseless
-labours of the philosophers and opticians of all countries.
-At first only a single lens was used, but it was
-found that the rays which exercised a chemical action
-did not meet in the same point as the rays of light, for
-it must be remembered that it is not the light we see
-that acts upon the substances used in photography, but
-another influence, known as actinism. It was also
-found that a single lens would not give a flat picture
-when the whole of its aperture was used, the edges of
-the image being always blurred and indistinct. This
-latter defect was found to be partially obviated by decreasing
-the opening, but this remedy shut off the light
-and prolonged the process. Gradually these two defects
-were removed, and now every photographer, no
-matter how humble, is possessed of a lens capable of
-taking a clear picture, every detail of which is perfectly
-distinct and faithful.</p>
-
-<p class='c021'>The camera lucida bears a great analogy to the camera
-obscura in the purpose for which it is used, though
-not in the principle on which it is constructed. It is
-employed, like the preceding instrument, for obtaining
-faithful copies of a landscape, a building, or even of another
-<span class='pageno' id='Page_247'>247</span>drawing. It was invented by Dr. Wollaston, in
-1804, and consists of a little four-sided prism, of which
-fig. 71 is a section.</p>
-
-<div id='i247'  class='figcenter id021'>
-<img src='images/i247.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 71.</span>—Section of Camera Lucida.</p>
-</div>
-</div>
-
-<p class='c021'>The angle at <span class='fss'>A</span> is a right angle; the angle <span class='fss'>B</span> measures
-67½°, the angle <span class='fss'>C</span> 135°, and the angle <span class='fss'>D</span> is, of course,
-equal to <span class='fss'>B</span>. It is mounted on a sliding foot, so that it
-may be raised or lowered at will, or turned in a horizontal
-direction. The path of the rays in this case is
-easy to follow, the object to be copied being placed at
-<span class='fss'>L</span>, and the eye at <span class='fss'>I</span>. On looking downwards the image
-of the object to be drawn is seen on the paper; and if
-the eye is placed so that the edge of the prism will just
-cut the pupil in two, the paper and pencil will be seen
-at the same time. It will be seen from the diagram,
-that the rays proceeding from <span class='fss'>L</span> strike on the surface <span class='fss'>A
-B</span> at right angles, and, being then reflected from <span class='fss'>C B</span>,
-pass upwards again to point <span class='fss'>E</span>. The direction of the
-rays is in reality a little more complicated than this.
-In the case of distant objects it is impossible to see
-both the object and the pencil at the same time; a lens
-is sometimes introduced at <span class='fss'>I</span> to modify this defect. The
-original instrument has also been modified by the introduction
-<span class='pageno' id='Page_248'>248</span>of a triangular prism, in conjunction with plates
-of coloured glass, but the difficulty of rendering the
-image and the paper of the same strength is very great.
-The instrument is also hard to use, from the additional
-difficulty of always keeping the head in the same position,
-for the least movement from left or right is sufficient
-to throw the whole drawing out.</p>
-
-<p class='c021'>A simple camera lucida may be made out of a small
-piece of looking-glass, mounted at an angle of 45°, or
-half-way between the horizontal and the perpendicular.
-If this be turned towards the drawing or view to be
-copied, and the left eye applied to the mirror, the image
-of the object will be seen on the paper below, and the
-pencil may be guided with the right. The proper use
-of this simple little instrument depends in a great measure
-upon the focus of each eye being the same. The
-light falling on the paper, too, requires very careful
-adjusting, otherwise the brighter object will eclipse the
-other. It is a good plan, too, to whiten the pencil or
-pen used, so that it may not so easily be lost when drawing
-the brighter parts of the object. We have seen
-excellent drawings made from plants by means of a
-little instrument of this kind, which simply consisted
-of a piece of looking-glass inserted in a cork stuck in a
-glass bottle.</p>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div class='chapter'>
-  <span class='pageno' id='Page_249'>249</span>
-  <h2 id='ch3-09' class='c012'>CHAPTER IX.<br /><span class='c020'>THE SPECTROSCOPE.</span></h2>
-</div>
-<p class='c014'><span class='sc'>We</span> now come to speak of an instrument which may
-fairly rank, after the telescope and microscope, as one
-of the most wonderful discoveries of modern optical
-science. By its means we have not only discovered
-four new elementary bodies, which are found in certain
-minerals in inconceivably small quantities, but we have
-also determined the chemical composition of some of
-the remotest stars and nebulæ.</p>
-
-<p class='c021'>In 1701 Newton discovered that if an ordinary ray
-of white light was admitted through a small hole into a
-dark chamber, and thence passed through a triangular
-prism, it became decomposed into a coloured band,
-known as the solar spectrum. As we have already explained
-that this decomposition is caused by the different
-coloured rays that make up white light being bent unequally
-by the action of the prism, we trust the following
-explanations will be readily understood. In 1802
-Dr. Wollaston, an English philosopher, discovered that
-by using a narrow slit, instead of a round hole, the resulting
-spectrum was no longer continuous, but was
-divided at intervals by dark lines extending across it in
-a direction parallel to the edges of the prism. These
-lines attracted considerable attention at the time, but it
-was not until 1815, that Fraunhofer, an optician of
-Munich, investigated them with accuracy. He mapped
-<span class='pageno' id='Page_250'>250</span>and counted no less than six hundred of them, identifying
-eight of the most conspicuous by the first eight
-letters of the alphabet. Their positions are as follows:—</p>
-<div class='lg-container-l c028'>
-  <div class='linegroup'>
-    <div class='group'>
-      <div class='line'>A.  Beginning of red.</div>
-      <div class='line'>B.  Middle of red.</div>
-      <div class='line'>C.  Beginning of orange.</div>
-      <div class='line'>D.  Middle of yellow.</div>
-      <div class='line'>E.  Middle of green.</div>
-      <div class='line'>F.  Beginning of blue.</div>
-      <div class='line'>G.  Middle of indigo.</div>
-      <div class='line'>H.  Middle of violet.</div>
-    </div>
-  </div>
-</div>
-
-<p class='c024'>The designations of these lines have been retained to
-the present day, and they have been named after the
-Munich philosopher, being known as Fraunhofer’s lines.
-They are to be seen in all parts of the spectrum, and
-increase in number and fineness according as the width
-of the slit through which the light passes is diminished.
-It may be asked, how it happens that they increase in
-proportion to the narrowness of the aperture admitting
-the light? A little consideration will soon show the
-reason of this.</p>
-
-<p class='c021'>When a beam of light is passed through a hole of, let
-us say, the eighth of an inch in diameter and decomposed
-by a prism, the spectrum so produced is imperfect,
-inasmuch as an infinite number of spectra are thus superposed,
-and for this reason, that the rays of light
-entering on the right side of the aperture will give a
-spectrum falling in a different place to that formed by
-the rays entering on the left. In order, therefore, to
-diminish the confusion caused by the superposition of a
-number of spectra, the aperture ought to be reduced to
-a narrow slit. When the thin slice of light passing
-through the slit is decomposed by the prism, we find
-that not only is the purity of the colours greatly increased,
-but the lines in question make their appearance
-more or less in all parts of the coloured band.</p>
-
-<p class='c021'>These lines are very unequally distributed, some being
-crowded together in masses, while others are extremely
-faint, and are separated by large intervals. Their
-<span class='pageno' id='Page_251'>251</span>position is well marked and determined, no matter from
-what source we obtain our beam of sunlight. Whether
-the spectrum be produced from the sun itself, or from
-the reflected light proceeding from the moon or planets,
-they are still found in the same place; only that in the
-latter case they are not so numerous, on account of the
-light being much fainter. For many years the cause of
-these lines remained a complete mystery, and it was not
-until Bunsen and Kirchhoff undertook their investigation
-that a satisfactory explanation of their origin was arrived
-at. In order to explain this, we must consider
-briefly the properties of the spectra of flames, and other
-luminous bodies.</p>
-
-<p class='c021'>If, instead of the light of the sun, we examine prismatically
-the light given off by an incandescent body,
-such as a white-hot piece of platinum, we shall find that
-the lines seen in the solar spectrum are absent, and that
-we have a continuous band of coloured light quite uninterrupted
-by dark spaces or bands. The same absence
-of lines is seen in the spectra of the electric light and
-the flame of an ordinary candle, the light in each of
-these cases being produced by particles of carbon in a
-state of vivid incandescence. But if we examine the
-flame of incandescent gases, we shall find a spectrum of
-an entirely new kind. Thus if we examine an ordinary
-gaslight through a slit with a prism, we shall obtain a
-continuous spectrum, in consequence of the luminous
-portion of the flame consisting of solid carbon in a state
-of incandescence; but if we turn down the flame, so as
-to lessen the amount of carbon to be burned, we shall
-find the whole of that body is converted into feebly luminous
-gas, giving off a faint reddish blue light. If we
-now again examine it in the same manner, we shall find
-that the spectrum produced consists of black spaces,
-here and there crossed by a few faint coloured lines or
-bands. The reason of this is obvious: in the faint flame
-<span class='pageno' id='Page_252'>252</span>caused by the carbon and hydrogen in a state of luminous
-vapour, which only have a few of the colours of the
-spectrum, which, when passed through the prism, fall
-into their proper places. All substances with which we
-are acquainted are capable of being converted into luminous
-vapour by means of heat, and when thus burnt
-produce flames of more or less faint luminosity, generally
-characteristically coloured. A piece of soda
-inserted in the wick of a spirit-lamp gives a yellow tinge
-to the flame; a morsel of saltpetre (nitrate of potash)
-or nitrate of strontia will give a purple and crimson
-tint respectively. These hues are caused by the metals
-sodium, potassium, and strontium contained in these
-salts being converted into luminous vapour. On analyzing
-these coloured flames with a prism, as before, we
-should find in the case of the soda a single broad yellow
-line, situated just in the middle of the yellow portion of
-the spectrum, the rest of the space where the spectrum
-should be being perfectly dark. The reason of this is
-pretty simple. Sodium burns with a pure yellow flame,
-consequently when passed through a prism it cannot
-split into any other colours, but takes its place in the
-position belonging to yellow of that particular hue.
-Were it a little more orange or green in tint, it would
-take its place nearer to the red or violet end of the
-spectrum. The light from saltpetre, which contains
-potassium may next be examined. It will be found to
-tinge the flame with the spirit-lamp of a beautiful purple.
-We can almost guess what will happen when this flame
-is submitted to the action of the prism. We shall find
-that the purple light emitted will split into red and
-violet, which will immediately arrange themselves in
-their proper positions according to their hues. If in
-like manner we substitute nitrate of strontia for saltpetre,
-we shall get a splendid crimson flame which is
-decomposed by the prism into red, orange, or blue.</p>
-
-<p class='c021'><span class='pageno' id='Page_253'>253</span>On submitting the compounds of the other elements
-to the same tests, we shall find that each of them, when
-converted into luminous gas, is capable of producing
-coloured lines of various kinds when the light of their
-flames is passed through a prism. If, therefore, we had
-a number of salts of whose composition we were ignorant,
-all we need do is to burn them in a spirit-lamp,
-and by the number and position in the lines of their
-spectra we should be able to tell immediately of what
-they were composed.</p>
-
-<p class='c021'>The spectra of nearly all the elements capable of
-being connected with luminous gas have been determined
-with great accuracy. Perhaps the number and position
-of the lines of a few spectra will be interesting to the
-student.</p>
-
-<p class='c021'><i>Sodium.</i>—This is the metallic base of soda salts, and
-gives a double bright yellow line in the middle of the
-yellow.</p>
-
-<p class='c021'><i>Potassium.</i>—The base of the various salts of potash.
-It gives one line in the extreme red, one in the middle
-of the red, one in the violet, and a peculiar glow in the
-centre of the spectrum.</p>
-
-<p class='c021'><i>Strontium.</i>—The base of the strontia salts, of which
-the nitrate is used as the principal ingredient in the red
-fire of the theatres. It gives a group of lines in the
-red and orange, and a beautiful blue one in the middle
-of the blue.</p>
-
-<p class='c021'><i>Barium.</i>—The base of the baryta salts, one of which
-is used in making green fire. It gives several strong
-lines in the green, and a few in the red, orange, and
-yellow.</p>
-
-<p class='c021'>After the position of the spectral lines of most of the
-elements had been discovered, Messrs. Bunsen and
-Kirchhoff were one day examining the saline deposit of
-a spring which issues from the earth near Durkheim, in
-<span class='pageno' id='Page_254'>254</span>the Palatinate, and were surprised to find that a blue
-line belonging to no known metal made its appearance
-in addition to the potassium, sodium, and other lines
-produced by the saline ingredients of the water. These
-philosophers immediately concluded that the unknown
-line was caused by an unknown metal, and they at once
-set to work to obtain a larger quantity of the saline
-residue from the spring. They evaporated down no less
-than forty tons of water, and succeeded in isolating the
-new substance, which turned out to be a metal resembling
-potassium. While examining the residue more
-carefully, a new, dark <i>red</i> line, beyond that belonging
-to potassium, was discovered, pointing to the existence
-of a second new element, which was also afterwards obtained
-in the pure state. These two new metals, which
-closely resemble potassium in their properties, were
-named in accordance with the lines given by them when
-converted into luminous gas. The first was called cæsium,
-from <i>cœsius</i>, Lat. light blue; and the other, rubidium,
-from <i>rubidus</i>, Lat. dark red. Since the publication
-of MM. Bunsen and Kirchhoff’s experiments, these
-two elements have been found in comparatively large
-quantities in various minerals, and these properties have
-been closely studied.</p>
-
-<p class='c021'>Spectrum analysis has yielded us two more new
-metals since first these philosophers applied the prism
-to the determination of the chemical composition of
-various bodies. Mr. W. Crookes, F.R.S., an English
-chemist of eminence, while examining the flame of a
-deposit obtained during the manufacture of sulphuric
-acid from a certain sulphur mineral found in the Hartz
-mountains, perceived a brilliant green line with which
-he was previously unacquainted, which quickly flashed
-into view, and then disappeared. After numerous experiments
-on various other minerals (for the deposit he had
-first experimented upon only yielded him a few grains
-<span class='pageno' id='Page_255'>255</span>of the new body), Mr. Crookes succeeded in discovering
-a comparatively large quantity of it in a sulphur mineral
-found in Belgium. The new element was found to be
-a heavy metal, closely resembling lead in its properties.
-It was named by the discoverer, thallium, from the
-Greek word <i>thallos</i>, a green twig, from the brilliancy of
-the single green line that indicates its presence. In
-like manner, Messrs. Reich and Richter have discovered
-a fourth new metal, which has been named <i>indium</i>, from
-its principal lines being found in the centre of the <i>indigo</i>
-of the spectrum.</p>
-
-<p class='c021'>The delicacy of spectrum analysis may be imagined
-from the fact that a quantity of sodium amounting to
-less than the <i>two-millionth</i> of a grain can be detected by
-its means. Indeed, it has taught us that sodium in one
-form or other exists almost everywhere. This mode
-of analysis is only serviceable to indicate the composition
-of any salt or other substance, the quantities of the
-different elements found by its use having no influence
-on the appearances brought out by the prism. Thus,
-a substance which has only been contaminated with sodium
-from being handled by warm fingers, will show the yellow
-bands as strongly as if it contained a large proportion
-of that metal.</p>
-
-<p class='c021'>For ordinary experiments in spectrum analysis the
-apparatus used is very simple. It consists of a tube
-with a fine slit at one end, and a convex lens at the
-other, for concentrating the light from the coloured
-flame upon the centre of the prism. After the light
-passes through the prism, it is examined by a small
-telescope of low magnifying power. The lamp used
-may be either a spirit-lamp or a colourless gas flame
-into which the substance to be examined is introduced
-upon a platinum wire.</p>
-
-<p class='c021'>We now come to another very important discovery,
-made by means of our prism and narrow slit—the
-<span class='pageno' id='Page_256'>256</span>determination of the composition of the photosphere or
-mass of luminous vapour surrounding the body of the
-sun.</p>
-
-<p class='c021'>A simple experiment will show how this brilliant discovery
-was arrived at. The light of a candle or other
-flame containing incandescent <i>solid</i> matter is passed
-through the spectroscope, and is found to decompose
-into a continuous spectrum, uninterrupted by dark lines.
-Between the light and the slit a spirit-lamp is placed,
-but no difference in the appearance of the spectrum is
-perceived. Introduce, however, the smallest portion of
-a soda salt into the non-luminous flame of the second-lamp,
-and a broad black line is immediately seen, crossing
-the middle of the yellow portion of the band of colour.
-Remove the sodium flame and the band disappears;
-but do the same with the lamp producing the
-spectrum, and the spectrum of course disappears, and
-the dark band caused by the sodium flame is changed
-to the yellow line produced by that metal. The same
-experiments may be tried with potassium, strontium,
-and other metals; and we shall always find that when
-a coloured flame is introduced between an incandescent
-solid and its continuous spectrum, it produces a series
-of black lines corresponding to the substances by which
-it is coloured. Thallium, in like manner, would give a
-black band in the middle of the green, and indium a
-similar one in the indigo. (<a href='#i004a'>Fig. 6</a>, Frontispiece).</p>
-
-<p class='c021'>The exact position of the black band in the middle of
-the yellow is shown in the coloured figure of the spectrum
-so beautifully printed in the frontispiece of this
-book, and it has been found to correspond exactly with
-the dark line <span class='fss'>D</span> of the solar spectrum. The inference
-from this fact is obvious. The incandescent portion of
-the sun gives off light corresponding in its properties to
-that emitted by the solid matter contained in the candle
-flame, but the photosphere containing the vapour of sodium
-<span class='pageno' id='Page_257'>257</span>cuts off that portion corresponding to the sodium
-line. Accurate measurements prove that numberless
-other lines occurring in the solar spectrum are due to
-the vapours of other well known metals existing on the
-earth. Amongst these may be mentioned potassium,
-calcium (the base of lime), iron, nickel, chromium, and
-several others. This discovery with regard to the sun
-has resulted in the spectral examination of a large number
-of the fixed stars and nebulæ. For centuries the
-fixed stars refused to answer all questions put to them
-by mortals. The telescope showed them merely as bright
-points. Their nature and origin remained a beautiful
-mystery, until Dr. Miller, Mr. Huggins, Father Secchi,
-and a few other philosophers interrogated them in a
-manner that could not fail to draw forth an answer.
-They brought their light within range of their prisms,
-and forthwith they declared themselves to be suns like
-our own. It is true that before this they were looked
-on by most astronomers as bodies analogous to our
-own sun, but it was only reasoning from analogy, after
-all; but we are now able to assert with all the certainty
-that is compatible with human fallibility that many of
-these heavenly bodies are possessed of an incandescent
-centre, surrounded by a photosphere or envelope of
-gaseous matter in a luminous condition. It would be
-impossible to give a list of all the stars that have been
-examined up to the present time; the composition of
-the photospheres of a few must therefore suffice. It is
-singular that the elements hitherto discovered in the
-stars are those which are more or less abundant on the
-earth. Amongst them we may name hydrogen, nitrogen,
-sodium, magnesium, barium, iron, antimony, bismuth,
-tellurium, and mercury. The bright star in the
-constellation of Orion known as Betelgeux is one of the
-most singular in composition, the lines of its spectrum
-indicating the absence of hydrogen. If, as Messrs.
-<span class='pageno' id='Page_258'>258</span>Huggins and Miller suggest, the worlds revolving round
-this star are also deficient in this element, they would
-be without water, like our moon.</p>
-
-<p class='c021'>Upon a very clear night it may be noticed that the
-stars are not all of the same colour, but that many of
-them appear to be of a ruddy or yellowish tint. The
-cause of this is plainly seen when they are submitted to
-spectral analysis. Thus, Sirius, which is a brilliant
-white star, shows but three dark lines, while one of the
-stars in the constellation of Hercules shows several
-groups of bands in the red, blue, and green portions of
-its spectrum, fully accounting for its orange tint.</p>
-
-<p class='c021'>The double star <i>β</i> Cygni is a very beautiful example
-of the distribution of colour between two members of a
-stellar group. One star shows a strong spectrum with
-the blue and violet portions almost totally blotted out,
-while its companion is similarly circumstanced with respect
-to the yellow and orange portions of its spectrum.
-The colour of one is consequently orange, while the
-other is of a delicate blue. If these stars are the
-principal members of a system, the alternation of blue
-and orange days must be indeed a singular phenomenon
-to those who inhabit their planets.</p>
-
-<p class='c021'>In some of the stars lines have been discovered
-which do not possess any equivalent amongst those produced
-by terrestrial matter; they consequently contain
-elements of which we know nothing; at the same time,
-however, it has been found that terrestrial elements
-exist in some of the remote nebulæ, which are so distant
-that their light takes many thousands of years to
-reach our earth.</p>
-
-<p class='c021'>Spectrum analysis has decided the grand question of
-the physical composition of the nebulæ. Those bodies
-were supposed, with some reason, to be aggregations of
-stars, like our Milky Way, which only required telescopes
-of sufficient power to resolve them. That they
-<span class='pageno' id='Page_259'>259</span>partly consist of gaseous matter in a luminous condition
-is evidenced by their showing a series of bright lines in
-the spectroscope, exactly like those produced by terrestrial
-gases. Their light is therefore not emitted by a
-solid or liquid incandescent body, but by a glowing gas.
-The lines mentioned by Messrs. Huggins and Miller
-showed that the nebula in the sword-handle of Orion
-consists of hydrogen and nitrogen in a state of luminous
-incandescence. Not the slightest trace of a continuous
-spectrum can be detected in the light emanating from
-this body; consequently, according to present hypotheses,
-it contains no solid matter at all. A number of
-other nebulæ have given similar results.</p>
-
-<p class='c021'>There are numerous star clusters which, unlike the
-true nebulæ, give continuous spectra when their light
-is submitted to the action of the prism. Of these may
-be specially mentioned the great clusters in Andromeda
-and Hercules, which give continuous spectra, interrupted
-by dark bands on the red and orange. The
-light thrown by these experiments upon the nebular
-hypotheses of Sir William Herschel, who considered
-that true nebulæ consisted of the primordial gaseous
-matter out of which suns and stars have been elaborated,
-is very great, and will be appreciated even by
-those whose knowledge of astronomy is small.</p>
-
-<p class='c021'>Spectral analysis has also been the means of our witnessing
-a celestial conflagration, and understanding the
-cause of this marvellous event. It is well known to
-most people that from time to time stars have suddenly
-burst upon us, and have almost as suddenly disappeared.
-The theories advanced to account for these singular
-celestial visitors, have been more numerous than satisfactory.
-In May 1866, a star of the second magnitude
-suddenly burst forth in the Northern Crown, and
-was almost immediately noticed by Mr. Huggins who
-brought every power of prism and telescope to bear
-<span class='pageno' id='Page_260'>260</span>upon this extraordinary celestial phenomenon. He
-found the spectrum of the star to consist of two distinct
-spectra, one being formed by four bright lines, the
-other analogous to the spectra of the sun and stars.
-Consequently two kinds of light were given off by this
-star; one forming a series of bright lines indicative of
-luminous gas, the other consisting of a continuous
-spectrum, crossed by dark lines, showing the existence
-of a solid body in a state of incandescence, surrounded
-by a photosphere of luminous vapours. Two of the
-bright lines undoubtedly showed the presence of
-hydrogen in a state of illumination, the great brightness
-of the lines indicating that the burning gas was
-hotter than the photosphere. These facts taken in
-conjunction with the suddenness of the outburst in the
-star, and its immediate decline in brightness from the
-second down to the eighth magnitude in twelve days,
-suggest the startling speculation that the star had become
-suddenly wrapped in the flames of burning
-hydrogen, consequent possibly on some violent convulsion
-in the interior of the star having set free
-enormous quantities of this gas. As the free hydrogen
-became exhausted, the spectrum showing the bright
-lines gradually waned until the star decreased in brilliancy.
-It must not be forgotten that the event seen by
-Mr. Huggins occurred many years ago, and that the
-light emitted by this marvellous celestial convulsion has
-been travelling to us ever since.</p>
-
-<p class='c021'>Comets and meteors have been submitted to the test
-of spectral analysis. The former erratic visitors have
-been but few and small since stellar spectrum analysis
-has been perfected. In January 1866, Mr. Huggins
-brought his apparatus to bear upon a small comet,
-which gave a somewhat unexpected result. When the
-object was viewed in the spectroscope, two spectra were
-distinguishable—a very faint continuous spectrum of
-<span class='pageno' id='Page_261'>261</span>the tail, showing that it reflected solar light, and a
-bright space towards the centre of the spectrum, indicating
-that the nucleous was self-luminous and gaseous.</p>
-
-<p class='c021'>Mr. Alexander Herschel—the nephew and the grandson
-of Sir John and Sir William Herschel—has recently
-succeeded in obtaining indications of the composition
-of the meteors that people the heavens in the months
-of August and November. The principal result of his
-observations appears to be, that sodium in a state of
-luminous vapour is present in the trains left behind
-these singular bodies.</p>
-
-<p class='c021'>Lightning has also been similarly examined, and lines
-showing that hydrogen and nitrogen were rendered luminous
-during the electrical discharge, were seen with
-great distinctness. In fact, the applications of the
-prism to scientific discovery are almost endless, and
-in describing them it is difficult to tell where to draw
-the line.</p>
-
-<p class='c021'>Before quitting this subject, it will be as well to say
-a few words on the fluorescent rays of the spectrum, to
-which allusion has already been made towards the end
-of <a href='#ch2-04'>Chapter IV</a>., Part II. It was there said that the
-chemical power of the spectrum extends to some distance
-beyond the extreme violet, a fact that may be
-readily proved by exposing a piece of photographic
-paper to the action of the dark portion of the spectrum.
-Professor Stokes found that there were means of rendering
-these rays visible to the eye by altering their rate
-of vibration. This he found was possible by passing
-them through the solutions of certain substances, such
-as sulphate of quinine, horse-chestnut bark, &amp;c. We
-have already said, that light vibrating at the rate of
-from 458 to 727 billion times a second, was capable of
-exciting luminous sensations upon the optic nerve. The
-latter is the rate of vibration of the extreme violet ray,
-and it has been found that the eyes of many persons are
-<span class='pageno' id='Page_262'>262</span>not sufficiently sensitive to be influenced by it; it is,
-therefore, just probable that there are animals whose
-eyes are so much more sensitive than ours, that they
-can see rays that exist far beyond those seen by us.
-Now, as difference of colour is produced by difference
-in the rate of vibration, it follows that those whose eyes
-are sensitive enough to perceive the extreme violet rays,
-see tints of violet that are inappreciable by others.</p>
-
-<p class='c021'>The power of sulphate of quinine in reducing the
-luminous vibrations is easily seen by passing a tube
-filled with the solution successively through each of the
-colours of the spectrum formed by a quartz prism; the
-ordinary colours will pass through the liquid as if it
-were simply water, but on arriving near the violet
-extremity a gleam of pale blue light will shoot across
-the tube, and continue to increase. As it is moved
-onwards the light will gradually die away, until a point
-is reached nearly equal in length to the whole of the
-visible spectrum, when it will disappear altogether. It
-is somewhat singular that no substance has yet been
-found that will increase the refrangibility of the dark
-rays beyond the red end of the spectrum. There are
-many artificial flames which produce this dark light (if
-we may use such a paradoxical expression) in greater
-quantity than the sun, whose light is no doubt greatly
-deteriorated in this respect during its passage through the
-atmosphere. The substance of which the prism is made
-also greatly influences the length of the invisible portion
-of the spectrum. By using a quartz prism and
-lenses of the same material Professor Stokes, found that
-the spectrum of the electric light could be traced for a
-distance equal to six times that of the visible portion.</p>
-
-<p class='c021'>The action of certain substances in rendering the invisible
-rays of light perceptible may be easily shown
-by any one possessing a horse-chestnut tree. A weak
-decoction of the inner portion of the bark having been
-<span class='pageno' id='Page_263'>263</span>made and filtered through blotting-paper, or at any rate
-allowed to settle, the room is made quite dark and a
-piece of common brimstone is ignited. The pale blue
-light given off is comparatively feeble, but it is very
-rich in the ultra-violet rays; consequently, when the
-infusion of horse-chestnut bark is poured into a tall jar
-of water, beautiful waves of phosphorescent light are
-seen flashing backwards and forwards as the two liquids
-mingle. The tincture of stramonium is also possessed
-of this property, and characters traced on paper with
-it, although nearly invisible by ordinary daylight, appear
-distinctly when examined by the light of burning
-sulphur.</p>
-<div class='pbb'>
- <hr class='pb c003' />
-</div>
-<div class='chapter'>
-  <span class='pageno' id='Page_264'>264</span>
-  <h2 id='ch3-10' class='c012'>CHAPTER X.<br /><span class='c020'>SPECTRES—THE GHOST ILLUSION.</span></h2>
-</div>
-<p class='c014'><span class='sc'>We</span> close our account of the wonders of optics by a
-description of the ghost illusion, which has been exhibited
-with such great success by M. Robin, the well-known
-French conjurer, Mr. Pepper, the enterprising
-manager of the Royal Polytechnic Institution, and several
-others. Before doing so, however, we will say a few
-words on those unpleasant visitations known as spectres,
-to which some people are liable, either through an over-worked
-brain or some organic disease.</p>
-
-<p class='c021'>The peculiar appearances known as spectres in optics
-are certain illusions of vision in which an object is apparently
-presented to the view which does not really
-exist. In such cases either the brain, the retina, or the
-optic nerve are unnaturally excited, and made sensitive
-to an appearance that, physically speaking, does not
-exist. There is such a close connexion between the
-senses and the mind, that we continually, and without
-knowing it, transfer to the physical world that which
-belongs to the domain of thought. A picture which has
-struck us during the day will reappear to us at night
-during sleep, with every detail perfect, or possibly under
-a form modified by the capricious wanderings of our
-thoughts. A sudden fright may sometimes be the cause
-of optical illusions which will pursue us unceasingly.
-Fear, despair, passion, ambition, and other violent mental
-<span class='pageno' id='Page_265'>265</span>phases, are capable of evoking images closely connected
-with the state of our brain, appearances that we
-often take for realities, and whose truths we have to
-test by our faculty of reasoning, before we can set them
-down as positive illusions. “In the most insignificant
-phenomena,” says Sir David Brewster, “we find that
-the retina is so powerfully influenced by exterior impressions
-as to retain the images of visible objects for a
-long time after they have passed out of sight; besides,
-this portion of the eye is so strongly influenced by local
-impressions of which we know neither the nature nor
-the origin, that we see the shapeless forms of coloured
-light moving about in the dark. In fact we have, in
-the cases of Newton and many others, examples of the
-ease with which the imagination revivifies the images
-of luminous objects for months or even years, after
-these impressions took place. After the occurrence of
-such phenomena, the mind can readily comprehend how
-thin is the division that separates reality from those
-spectral illusions which during a particular state of
-health have afflicted the most intelligent men, not merely
-those belonging to the community at large, but also
-the most learned philosophers.”</p>
-
-<p class='c021'>Spectres may properly be divided into two classes,
-those which may be termed subjective, which result from
-some unnatural action of our minds or bodies, and
-which properly belong to the science of physiology, and
-those which may be called objective, which are caused
-by some peculiar illusion acting on us from without.
-We shall pass lightly over the first, illustrating them
-by a single example, while we shall pay more serious
-attention to those belonging to the second class.</p>
-
-<p class='c021'>Sir Walter Scott, in his <i>Letters on Demonology and
-Witchcraft</i>, mentions a remarkable instance of the first
-order of spectres. A doctor of eminence was called in
-to attend a gentleman who occupied a high place in a
-<span class='pageno' id='Page_266'>266</span>particular department connected with the administration
-of justice. Until the time that the physician’s services
-became necessary, he had shown strong common sense
-and extraordinary firmness and integrity in every case
-in which he had been called upon to arbitrate. But after
-a certain epoch his temper became saddened, although
-his mind preserved its habitual strength and calmness.
-At the same time, the feebleness of his pulse, the loss
-of appetite, and impaired digestion seemed to point out
-to his medical adviser the existence of some serious
-source of disturbance. At first the sick man seemed
-inclined to keep the cause of the change in his health a
-profound secret; but his melancholy bearing, confused
-answers, and the badly disguised constraint with which
-he sharply replied to the interrogations of the doctor,
-caused the latter to seek for information as to the cause
-of the disorder in other directions. He made minute
-inquiries of the various members of his unhappy patient’s
-family, but he could obtain no explanation of the
-mystery. Every one was lost in conjecture as to the
-reason of the alarming condition of the patient, which
-did not appear to be justified by any loss of fortune or
-beloved friends. His age rendered the idea of an unsuccessful
-love affair improbable, and his known integrity
-precluded the possibility of remorse. The doctor accordingly
-was compelled to return once more to the straight
-road, and he used the most serious arguments with his patient
-to induce him to conquer his obstinacy. At last
-the doctor’s efforts took effect; the patient allowed himself
-to be convinced, and manifested his desire to open
-his mind frankly to the doctor. They were accordingly
-left alone, all the doors were securely fastened, and the
-patient made the following singular avowal.</p>
-
-<p class='c021'>“You cannot be more firmly convinced, my dear
-friend, than I am myself, that I am on the eve of death,
-crushed by the fatal malady which has dried up the
-<span class='pageno' id='Page_267'>267</span>sources of my life. You remember, without doubt, the
-disease of which the Duke of Olivarez died in Spain?”</p>
-
-<p class='c021'>“From the idea,” replied the doctor, “that he was
-pursued by an apparition in whose existence he did not
-believe, and he died from the continual presence of this
-imaginary vision weighing down his strength, and breaking
-his heart.”</p>
-
-<p class='c021'>“Well, my dear doctor,” the patient went on, “I am
-in the same condition, and the presence of the vision
-that persecutes me is so painful and frightful, that my
-reason is totally helpless in controlling the effects of my
-imagination, and I feel that I am dying from the effects
-of an imaginary illness. My visions began two or three
-years since. At first I found myself embarrassed from
-time to time by the presence of a great cat, which appeared
-and disappeared I knew not how. But at last
-the truth flashed across my mind, and I was compelled
-to look upon the creature, not as an ordinary domestic
-animal, but as a vision which had its origin in some derangement
-of the organs of sight or in my imagination.
-I have no antipathy to cats, in fact I am rather fond of
-them, so I endured the presence of my imaginary companion
-so well that at last I treated the whole affair
-with indifference. But at the end of several months
-the cat disappeared, and was replaced by a spectre of
-greater importance, and whose exterior was, to say the
-least of it, very imposing. It was neither more nor less
-than one of the high officials of the House of Lords, in
-the full dress belonging to his dignity.</p>
-
-<p class='c021'>“This personage, who was in court dress, with a bag-wig
-on his head, and a sword by his side, his coat splendidly
-embroidered and his <i>chapeau bras</i> under his arm,
-glided along by my side like a shadow. Whether I was
-in my own house or elsewhere, he mounted the stairs
-before me, as if to announce my coming. Sometimes he
-seemed to mix with the company, although it was evident
-<span class='pageno' id='Page_268'>268</span>that no one remarked his presence, and I was the sole
-witness of the chimerical honours that this imaginary
-individual seemed to render to me. This phantasy of
-my brain did not make a very strong impression on me,
-although it made me conceive doubts as to the state of
-my health, and the effects it would produce upon my
-reason.</p>
-
-<p class='c021'>“This second phase of my malady, like the first, also
-came to an end. Some months after, the usher of the
-Upper House ceased showing himself, and he was replaced
-by an apparition that was at once wearing to the
-mind and terrible to the sight. It was a skeleton.
-Whether I was alone or in company this frightful image
-of death never quitted me; it dogged my footsteps and
-followed me everywhere, and seemed to be a shadow
-inseparable from myself. It was in vain that I repeated
-to myself a hundred times over that the vision was not
-real, and was only an illusion of my senses. The reasoning
-of philosophy and my religious principles, strong
-though they are, are powerless to triumph over the influence
-that besets me, and I feel that I shall die a
-victim to this cruel evil.”</p>
-
-<p class='c021'>“It seems then,” interrupted the doctor, “that this
-skeleton is always before your eyes?”</p>
-
-<p class='c021'>“It is my evil fate to see it continually before me.”</p>
-
-<p class='c021'>“In which case it is at this moment visible to your
-eyes?”</p>
-
-<p class='c021'>“It is at present.”</p>
-
-<p class='c021'>“And in what part of the room do you imagine that
-you see it now?” asked the doctor.</p>
-
-<p class='c021'>“At the foot of my bed,” replied the patient: “when
-the curtains are half open I can see it place itself in the
-empty space between them.”</p>
-
-<p class='c021'>“You say that you are convinced that it is only an illusion,”
-replied the doctor; “have you the firmness to convince
-yourself of it positively? Have you the necessary
-<span class='pageno' id='Page_269'>269</span>courage to get up and go and place yourself in the
-position which appears to be occupied by the spectre, in
-order to demonstrate to yourself positively that it is
-only a vision?”</p>
-
-<p class='c021'>The unfortunate man sighed and shook his head.</p>
-
-<p class='c021'>“Well,” went on the doctor, “let us try another
-plan.”</p>
-
-<p class='c021'>He quitted the chair on which he was sitting, at the
-head of his patient’s bed, and placing himself between
-the half opened curtains, in the place where the patient
-had pointed out the skeleton, he asked if the apparition
-was still visible.</p>
-
-<p class='c021'>“Not the whole of it,” answered the patient, “because
-you are standing between him and me; but I see
-his skull looking at me over your shoulder.”</p>
-
-<p class='c021'>In spite of his philosophy, the learned physician could
-not help starting to hear that the spectre was immediately
-behind him. He had recourse to other questions,
-and tried endless remedies, but without success. The
-prostration of the patient, however, increased, and he
-died in the same distress of mind in which he had passed
-the last months of his life. This example is a sad proof
-of the power of the imagination over the life of the body
-even when the terrors endured are powerless in destroying
-the judgment of the unfortunate sufferer. We will
-say more; men who have the strongest nerves are not
-free from similar illusions.</p>
-
-<p class='c021'>The second kind of spectres, in which the science of
-optics plays so important a part, is the result of the
-imagination being deceived by art with the assistance of
-science.</p>
-
-<p class='c021'>These spectres are displayed in the ghost trick which
-has been practised at various Parisian theatres for a
-number of years, with very great success, more especially
-at the <i>Théâtres du Châtelet</i> and <i>Dejazet</i>. The Adelphi,
-in London, also employed Mr. Pepper to heighten the
-<span class='pageno' id='Page_270'>270</span>effect of the excellent acting of Mr. Toole and Mrs. Alfred
-Mellon, in the dramatic version of Dickens’ “Haunted
-Man,” by the introduction of various spectral effects.
-And the same trick was also called into requisition with
-some success in several of the minor theatres in New
-York and other cities of the United States. At the
-Polytechnic, in London, very remarkable effects were
-produced, and few who ever saw them will forget the
-surprise they felt at seeing the first representation
-of an imponderable ghost endowed with motion, and
-even speech. Amongst the most successful productions
-in this way was the entertainment of M.
-Robin, one of the cleverest of the many successors of
-the great Robert Houdin, the prince of prestidigitators.
-M. Robin claims to be the inventor of the ghost illusion,
-and to have shown it frequently since 1847. Whether
-this be so or not it is not our business to decide, but we
-can testify that his exhibition in the Boulevard du
-Temple drew all Paris to see it. Evening after evening
-he not only “called spirits from the vasty deep,” but
-“made them come.” He pierced them with swords, he
-fired pistols through them, and he made them appear
-and disappear at his slightest wish. He showed the
-Zouave at Inkermann, lying dead amongst a heap of
-slain, who at the familiar sound of the drum, rose, pale
-and grave, and showed the bleeding wounds from which
-he died. Amongst other scenes shown by M. Robin was
-one of a spectre appearing to an armed man, who after
-trying in vain to shut out the vision from his sight fires
-a pistol at the intruder. <a href='#i272'>Fig. 72</a> shows the scene as seen
-by the audience, and <a href='#i273'>fig. 73</a>, the method by which the
-illusion is worked. The theatre is shown in section. On
-the left, at the end, are seen the spectators; on the right is
-the stage upon which the scene is represented. Beneath
-the stage is an actor clothed in white to personate a
-ghost, whose image is reflected by the glass above.</p>
-
-<div id='i272'  class='figcenter id021'>
-<span class='pageno' id='Page_272'>272</span>
-<img src='images/i272.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 72.</span>—The Spectre. An optical illusion.</p>
-</div>
-</div>
-
-<div id='i273'  class='figcenter id021'>
-<span class='pageno' id='Page_273'>273</span>
-<img src='images/i273.jpg' alt='' class='ig001' />
-<div class='ic001'>
-<p><span class='sc'>Fig. 73.</span>—How to produce Spectres.</p>
-</div>
-</div>
-
-<p class='c021'><span class='pageno' id='Page_275'>275</span>This glass is placed at an angle, and fills up the whole
-of the front of the stage, the edges being carefully concealed
-by curtains. The glass of course must be of a
-very large size, and should be of the very best quality,
-so that it cannot be seen by the audience. The actor
-must take care to place himself in such a position as to
-counteract the effect produced by the glass being placed
-at an angle. At first the cavalier is seen sitting at a
-table. After soliloquizing for a time in a very remorseful
-manner touching several murders that he has committed,
-the ghost of one of his victims gradually appears.
-This is effected by gently turning the electric light upon
-the concealed actor. The murderer and victim parley for
-a short time, when the former, being unable to withstand
-the reproaches of the ghost any longer, fires a pistol at
-him point-blank. The ball of course takes no effect, so
-the villain draws a sword, but before it has left its
-scabbard the spirit of the victim has vanished with a
-mocking laugh, or, in other words, the electric light is
-suddenly turned off. The management of the light is
-exceedingly difficult under these circumstances; the
-theatre, the stage, and the portion beneath ought to be
-lighted in a very careful manner, for if either is too
-bright or too dark it mars the whole effect. It must be
-remembered, too, that the person performing the part of
-the spectre and the real actor above cannot see each
-other, consequently all their action has to be carried
-on by guess-work. The actor below has to walk along
-an inclined plane, keeping himself exactly at right angles
-to it. Again, the movements of the latter are obliged to
-be reversed; for the cavalier already mentioned drew
-his sword with his left hand in order that the reflected
-figure should appear to use the right.</p>
-
-<p class='c021'>When well arranged, the ghost trick leaves far behind
-all the efforts of a similar nature that were obtained by
-the ancients in the way of magical illusions. It is also
-<span class='pageno' id='Page_276'>276</span>incontestably true, contrary to what some people have
-supposed, that they were unable to perform this illusion
-in the way we have described, for they were ignorant of
-the method of manufacturing and polishing glass plates
-of sufficient size and clearness for the purpose.</p>
-
-<p class='c021'>The production of living but impalpable spectres is
-thus a completely modern achievement, as we have
-already proved, and which has taken its place amongst
-the applications of science to stage art, to the total exclusion
-of all effects depending for their production on
-the old-fashioned phantasmagoria and magic lantern.</p>
-
-<div class='nf-center-c0'>
-<div class='nf-center c007'>
-    <div>THE END.</div>
-  </div>
-</div>
-
-<div class='pbb'>
- <hr class='pb c005' />
-</div>
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author	nfenwick <nfenwick@pglaf.org>	2025-01-23 03:14:30 -0800
committer	nfenwick <nfenwick@pglaf.org>	2025-01-23 03:14:30 -0800
commit	07808570186a1be434bbabacbaca6ba620c03112 (patch)
tree	845b5e08a2ed247b45b8667df92a89d9bff89a06
parent	858a493d0970377a744f0356a949bf0676824479 (diff)