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diff --git a/old/65114-0.txt b/old/65114-0.txt deleted file mode 100644 index b6ac861..0000000 --- a/old/65114-0.txt +++ /dev/null @@ -1,7073 +0,0 @@ -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. - - ------------------------------------------------------------------------- - - - - - 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. - - ------------------------------------------------------------------------- - - - - - 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. - - ------------------------------------------------------------------------- - - - - - 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. -] - - ------------------------------------------------------------------------- - - - - - 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. - - ------------------------------------------------------------------------- - - - - - 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. - - ------------------------------------------------------------------------- - - - - - 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. - - ------------------------------------------------------------------------- - - - - - 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. - - - -*** END OF THE PROJECT GUTENBERG EBOOK THE WONDERS OF OPTICS *** - -Updated editions will replace the previous one--the old editions will -be renamed. - -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the -United States without permission and without paying copyright -royalties. 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