73 files changed, 11896 insertions, 0 deletions

diff --git a/.gitattributes b/.gitattributes
new file mode 100644
index 0000000..6833f05
--- /dev/null
+++ b/.gitattributes
@@ -0,0 +1,3 @@
+* text=auto
+*.txt text
+*.md text
diff --git a/16767-8.txt b/16767-8.txt
new file mode 100644
index 0000000..bf5c767
--- /dev/null
+++ b/16767-8.txt
@@ -0,0 +1,3833 @@
+Project Gutenberg's Half-hours with the Telescope, by Richard A. Proctor
+
+This eBook is for the use of anyone anywhere 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
+
+
+Title: Half-hours with the Telescope
+       Being a Popular Guide to the Use of the Telescope as a
+       Means of Amusement and Instruction.
+
+Author: Richard A. Proctor
+
+Release Date: September 28, 2005 [EBook #16767]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK HALF-HOURS WITH THE TELESCOPE ***
+
+
+
+
+Produced by Jason Isbell and the Online Distributed
+Proofreading Team at https://www.pgdp.net
+
+
+
+
+
+[Illustration: PLATE I. Maps I.-IV.]
+
+
+
+HALF-HOURS
+
+WITH
+
+THE TELESCOPE;
+
+BEING A POPULAR GUIDE TO THE USE OF THE TELESCOPE
+AS A MEANS OF AMUSEMENT AND INSTRUCTION.
+
+BY
+
+RICHARD A. PROCTOR, B.A., F.R.A.S.,
+AUTHOR OF "SATURN AND ITS SYSTEM," ETC.
+
+WITH ILLUSTRATIONS ON STONE AND WOOD.
+
+
+       *       *       *       *       *
+
+    An undevout astronomer is mad:
+    True, all things speak a God; but, in the small
+    Men trace out Him: in great He seizes man.
+                                  YOUNG.
+
+       *       *       *       *       *
+
+NEW YORK:
+
+G.P. PUTNAM'S SONS.
+
+1873.
+
+LONDON:
+
+PRINTED BY WILLIAM CLOWES AND SONS, STAMFORD STREET
+AND CHARING CROSS.
+
+
+
+
+PREFACE.
+
+
+The object which the Author and Publisher of this little work have
+proposed to themselves, has been the production, at a moderate price, of
+a useful and reliable guide to the amateur telescopist.
+
+Among the celestial phenomena described or figured in this treatise, by
+far the larger number may be profitably examined with small telescopes,
+and there are none which are beyond the range of a good 3-inch
+achromatic.
+
+The work also treats of the construction of telescopes, the nature and
+use of star-maps, and other subjects connected with the requirements of
+amateur observers.
+
+R.A.P.
+
+_January_, 1868.
+
+
+
+
+CONTENTS.
+
+
+CHAPTER I.                                          PAGE
+A HALF-HOUR ON THE STRUCTURE OF THE TELESCOPE          1
+
+CHAPTER II.
+A HALF-HOUR WITH ORION, LEPUS, TAURUS, ETC.           33
+
+CHAPTER III.
+A HALF-HOUR WITH LYRA, HERCULES, CORVUS, CRATER, ETC. 47
+
+CHAPTER IV.
+A HALF-HOUR WITH BOOTES, SCORPIO, OPHIUCHUS, ETC.     56
+
+CHAPTER V.
+A HALF-HOUR WITH ANDROMEDA, CYGNUS, ETC.              66
+
+CHAPTER VI.
+HALF-HOURS WITH THE PLANETS                           74
+
+CHAPTER VII.
+HALF-HOURS WITH THE SUN AND MOON                      93
+
+
+
+
+DESCRIPTION OF PLATES.
+
+
+PLATE I.--_Frontispiece._
+
+This plate presents the aspect of the heavens at the four seasons, dealt
+with in Chapters II., III., IV., and V. In each map of this plate the
+central point represents the point vertically over the observer's head,
+and the circumference represents his horizon. The plan of each map is
+such that the direction of a star or constellation, as respects the
+compass-points, and its elevation, also, above the horizon, at the given
+season, can be at once determined. Two illustrations of the use of the
+maps will serve to explain their nature better than any detailed
+description. Suppose first, that--at one of the hours named under Map
+I.--the observer wishes to find Castor and Pollux:--Turning to Map I. he
+sees that these stars lie in the lower left-hand quadrant, and very
+nearly towards the point marked S.E.; that is, they are to be looked for
+on the sky towards the south-east. Also, it is seen that the two stars
+lie about one-fourth of the way from the centre towards the
+circumference. Hence, on the sky, the stars will be found about
+one-fourth of the way from the zenith towards the horizon: Castor will
+be seen immediately above Pollux. Next, suppose that at one of the hours
+named the observer wishes to learn what stars are visible towards the
+west and north-west:--Turning the map until the portion of the
+circumference marked W ... N.W. is lowermost, he sees that in the
+direction named the square of Pegasus lies not very high above the
+horizon, one diagonal of the square being vertical, the other nearly
+horizontal. Above the square is Andromeda, to the right of which lies
+Cassiopeia, the stars [beta] and [epsilon] of this constellation lying
+directly towards the north-west, while the star [alpha] lies almost
+exactly midway between the zenith and the horizon. Above Andromeda, a
+little towards the left, lies Perseus, Algol being almost exactly
+towards the west and one-third of the way from the zenith towards the
+horizon (because one-third of the way from the centre towards the
+circumference of the map). Almost exactly in the zenith is the star
+[delta] Aurigæ.
+
+The four maps are miniatures of Maps I., IV., VII., and X. of my
+'Constellation Seasons,' fourth-magnitude stars, however, being omitted.
+
+
+PLATES II., III., IV., and V., illustrating Chapters II., III., IV., and V.
+
+Plates II. and IV. contain four star-maps. They not only serve to
+indicate the configuration of certain important star-groups, but they
+illustrate the construction of maps, such as the observer should make
+for himself when he wishes to obtain an accurate knowledge of particular
+regions of the sky. They are all made to one scale, and on the conical
+projection--the simplest and best of all projections for maps of this
+sort. The way in which the meridians and parallels for this projection
+are laid down is described in my 'Handbook of the Stars.' With a little
+practice a few minutes will suffice for sweeping out the equidistant
+circular arcs which mark the parallels and ruling in the straight
+meridians.
+
+The dotted line across three of the maps represents a portion of the
+horizontal circle midway between the zenith and the horizon at the hour
+at which the map is supposed to be used. At other hours, of course, this
+line would be differently situated.
+
+Plates III. and V. represent fifty-two of the objects mentioned in the
+above-named chapters. As reference is made to these figures in the text,
+little comment is here required. It is to be remarked, however, that the
+circles, and especially the small circles, do not represent the whole
+of the telescope's field of view, only a small portion of it. The object
+of these figures is to enable the observer to know what to expect when
+he turns his telescope towards a difficult double star. Many of the
+objects depicted are very easy doubles: these are given as objects of
+reference. The observer having seen the correspondence between an easy
+double and its picture, as respects the relation between the line
+joining the components and the apparent path of the double across the
+telescope's field of view, will know how to interpret the picture of a
+difficult double in this respect. And as all the small figures are drawn
+to one scale, he will also know how far apart he may expect to find the
+components of a difficult double. Thus he will have an exact conception
+of the sort of duplicity he is to look for, and this is--_crede
+experto_--a great step towards the detection of the star's duplicity.
+
+
+PLATES VI. and VII., illustrating Chapters VI. and VII.
+
+The views of Mercury, Venus, and Mars in these plates (except the
+smaller view of Jupiter in Plate VII.) are supposed to be seen with the
+same "power."
+
+The observer must not expect to see the details presented in the views
+of Mars with anything like the distinctness I have here given to them.
+If he place the plate at a distance of six or seven yards he will see
+the views more nearly as Mars is likely to appear in a good three-inch
+aperture.
+
+The chart of Mars is a reduction of one I have constructed from views by
+Mr. Dawes. I believe that nearly all the features included in the chart
+are permanent, though not always visible. I take this opportunity of
+noting that the eighteen orthographic pictures of Mars presented with my
+shilling chart are to be looked on rather as maps than as representing
+telescopic views. They illustrate usefully the varying presentation of
+Mars towards the earth. The observer can obtain other such illustrations
+for himself by filling in outlines, traced from those given at the foot
+of Plate VI., with details from the chart. It is to be noted that Mars
+varies in presentation, not only as respects the greater or less opening
+out of his equator towards the north or south, but as respects the
+apparent slope of his polar axis to the right or left. The four
+projections as shown, or inverted, or seen from the back of the plate
+(held up to the light) give presentations of Mars towards the sun at
+twelve periods of the Martial year,--viz., at the autumnal and vernal
+equinoxes, at the two solstices, and at intermediate periods
+corresponding to our terrestrial months.
+
+In fact, by means of these projections one might readily form a series
+of sun-views of Mars resembling my 'Sun-views of the Earth.'
+
+In the first view of Jupiter it is to be remarked that the three
+satellites outside the disc are supposed to be moving in directions
+appreciably parallel to the belts on the disc--the upper satellites from
+right to left, the lower one from left to right. In general the
+satellites, when so near to the disc, are not seen in a straight line,
+as the three shown in the figure happen to be. Of the three spots on the
+disc, the faintest is a satellite, the neighbouring dark spot its
+shadow, the other dark spot the shadow of the satellite close to the
+planet's disc.
+
+
+
+
+HALF-HOURS WITH THE TELESCOPE.
+
+
+
+
+CHAPTER I.
+
+A HALF-HOUR ON THE STRUCTURE OF THE TELESCOPE.
+
+
+There are few instruments which yield more pleasure and instruction than
+the Telescope. Even a small telescope--only an inch and a half or two
+inches, perhaps, in aperture--will serve to supply profitable amusement
+to those who know how to apply its powers. I have often seen with
+pleasure the surprise with which the performance even of an opera-glass,
+well steadied, and directed towards certain parts of the heavens, has
+been witnessed by those who have supposed that nothing but an expensive
+and colossal telescope could afford any views of interest. But a
+well-constructed achromatic of two or three inches in aperture will not
+merely supply amusement and instruction,--it may be made to do useful
+work.
+
+The student of astronomy is often deterred from telescopic observation
+by the thought that in a field wherein so many have laboured, with
+abilities and means perhaps far surpassing those he may possess, he is
+little likely to reap results of any utility. He argues that, since the
+planets, stars, and nebulæ have been scanned by Herschel and Rosse, with
+their gigantic mirrors, and at Pulkova and Greenwich with refractors
+whose construction has taxed to the utmost the ingenuity of the
+optician and mechanic, it must be utterly useless for an unpractised
+observer to direct a telescope of moderate power to the examination of
+these objects.
+
+Now, passing over the consideration that a small telescope may afford
+its possessor much pleasure of an intellectual and elevated character,
+even if he is never able by its means to effect original discoveries,
+two arguments may be urged in favour of independent telescopic
+observation. In the first place, the student who wishes to appreciate
+the facts and theories of astronomy should familiarize himself with the
+nature of that instrument to which astronomers have been most largely
+indebted. In the second place, some of the most important discoveries in
+astronomy have been effected by means of telescopes of moderate power
+used skilfully and systematically. One instance may suffice to show what
+can be done in this way. The well-known telescopist Goldschmidt (who
+commenced astronomical observation at the age of forty-eight, in 1850)
+added fourteen asteroids to the solar system, not to speak of important
+discoveries of nebulæ and variable stars, by means of a telescope only
+five feet in focal length, mounted on a movable tripod stand.
+
+The feeling experienced by those who look through a telescope for the
+first time,--especially if it is directed upon a planet or nebula--is
+commonly one of disappointment. They have been told that such and such
+powers will exhibit Jupiter's belts, Saturn's rings, and the
+continent-outlines on Mars; yet, though perhaps a higher power is
+applied, they fail to detect these appearances, and can hardly believe
+that they are perfectly distinct to the practised eye.
+
+The expectations of the beginner are especially liable to
+disappointment in one particular. He forms an estimate of the view he is
+to obtain of a planet by multiplying the apparent diameter of the planet
+by the magnifying power of his telescope, and comparing the result with
+the apparent diameter of the sun or moon. Let us suppose, for instance,
+that on the day of observation Jupiter's apparent diameter is 45", and
+that the telescopic power applied is 40, then in the telescope Jupiter
+should appear to have a diameter of 1800", or half a degree, which is
+about the same as the moon's apparent diameter. But when the observer
+looks through the telescope he obtains a view--interesting, indeed, and
+instructive--but very different from what the above calculation would
+lead him to expect. He sees a disc apparently much smaller than the
+moon's, and not nearly so well-defined in outline; in a line with the
+disc's centre there appear three or four minute dots of light, the
+satellites of the planet; and, perhaps, if the weather is favourable and
+the observer watchful, he will be able to detect faint traces of belts
+across the planet's disc.
+
+Yet in such a case the telescope is not in fault. The planet really
+appears of the estimated size. In fact, it is often possible to prove
+this in a very simple manner. If the observer wait until the planet and
+the moon are pretty near together, he will find that it is possible to
+view the planet with one eye through the telescope and the moon with the
+unaided eye, in such a manner that the two discs may coincide, and thus
+their relative apparent dimensions be at once recognised. Nor should the
+indistinctness and incompleteness of the view be attributed to
+imperfection of the telescope; they are partly due to the nature of the
+observation and the low power employed, and partly to the inexperience
+of the beginner.
+
+It is to such a beginner that the following pages are specially
+addressed, with the hope of affording him aid and encouragement in the
+use of one of the most enchanting of scientific instruments,--an
+instrument that has created for astronomers a new sense, so to speak, by
+which, in the words of the ancient poet:
+
+    Subjecere oculis distantia sidera nostris,
+      Ætheraque ingenio supposuere suo.
+
+In the first place, it is necessary that the beginner should rightly
+know what is the nature of the instrument he is to use. And this is the
+more necessary because, while it is perfectly easy to obtain such
+knowledge without any profound acquaintance with the science of optics,
+yet in many popular works on this subject the really important points
+are omitted, and even in scientific works such points are too often left
+to be gathered from a formula. When the observer has learnt what it is
+that his instrument is actually to do for him, he will know how to
+estimate its performance, and how to vary the application of its
+powers--whether illuminating or magnifying--according to the nature of
+the object to be observed.
+
+Let us consider what it is that limits the range of _natural_ vision
+applied to distant objects. What causes an object to become invisible as
+its distance increases? Two things are necessary that an object should
+be visible. It must be _large_ enough to be appreciated by the eye, and
+it must _send light_ enough. Thus increase of distance may render an
+object invisible, either through diminution of its apparent size, or
+through diminution in the quantity of light it sends to the eye, or
+through both these causes combined. A telescope, therefore, or (as its
+name implies) an instrument to render distant objects visible, must be
+both a magnifying and an illuminating instrument.
+
+[Illustration: _Fig. 1._]
+
+Let EF, fig. 1, be an object, not near to AB as in the figure, but so
+far off that the bounding lines from A and B would meet at the point
+corresponding to the point P. Then if a large convex glass AB (called an
+_object-glass_) be interposed between the object and the eye, all those
+rays which, proceeding from P, fall on AB, will be caused to converge
+nearly to a point _p_. The same is true for every point of the object
+EMF, and thus a small image, _emf_, will be formed. This image will not
+lie exactly on a flat surface, but will be curved about the point midway
+between A and B as a centre. Now if the lens AB is removed, and an eye
+is placed at _m_ to view the distant object EMF, those rays only from
+each point of the object which fall on the pupil of the eye (whose
+diameter is about equal to _mp_ suppose) will serve to render the object
+visible. On the other hand, every point of the image _emf_ has received
+the whole of the light gathered up by the large glass AB. If then we can
+only make this light _available_, it is clear that we shall have
+acquired a large increase of _light_ from the distant object. Now it
+will be noticed that the light which has converged to _p_, diverges from
+_p_ so that an eye, placed that this diverging pencil of rays may fall
+upon it, would be too small to receive the whole of the pencil. Or, if
+it did receive the whole of this pencil, it clearly could not receive
+the whole of the pencils proceeding from other parts of the image _emf_.
+_Something_ would be gained, though, even in this case, since it is
+clear that an eye thus placed at a distance of ten inches from _emf_
+(which is about the average distance of distinct vision) would not only
+receive much more light from the image _emf_, than it would from the
+object EMF, but see the image much larger than the object. It is in this
+way that a simple object-glass forms a telescope, a circumstance we
+shall presently have to notice more at length. But we want to gain the
+full benefit of the light which has been gathered up for us by our
+object-glass. We therefore interpose a small convex glass _ab_ (called
+an eye-glass) between the image and the eye, at such a distance from the
+image that the divergent pencil of rays is converted into a pencil of
+parallel or nearly parallel rays. Call this an emergent pencil. Then all
+the emergent pencils now converge to a point on the axial line _m_M
+(produced beyond _m_), and an eye suitably placed can take in all of
+them at once. Thus the whole, or a large part, of the image is seen at
+once. But the image is seen inverted as shown. This is the Telescope, as
+it was first discovered, and such an arrangement would now be called a
+_simple astronomical Telescope_.
+
+Let us clearly understand what each part of the astronomical telescope
+does for us:--
+
+The object-glass AB gives us an illuminated image, the amount of
+illumination depending on the size of the object-glass. The eye-glass
+enables us to examine the image microscopically.
+
+We may apply eye-glasses of different focal length. It is clear that the
+shorter the focal length of _ab_, the nearer must _ab_ be placed to the
+image, and the smaller will the emergent pencils be, but the greater the
+magnifying power of the eye-glass. If the emergent pencils are severally
+larger than the pupil of the eye, light is wasted at the expense of
+magnifying power. Therefore the eye-glass should never be of greater
+focal length than that which makes the emergent pencils about equal in
+diameter to the pupil of the eye. On the other hand, the eye-glass must
+not be of such small focal length that the image appears indistinct and
+contorted, or dull for want of light.
+
+[Illustration: _Fig. 2._]
+
+Let us compare with the arrangement exhibited in fig. 1 that adopted by
+Galileo. Surprise is sometimes expressed that this instrument, which in
+the hands of the great Florentine astronomer effected so much, should
+now be known as the _non-astronomical Telescope_. I think this will be
+readily understood when we compare the two arrangements.
+
+In the Galilean Telescope a small concave eye-glass, _ab_ (fig. 2), is
+placed between the object-glass and the image. In fact, no image is
+allowed to be formed in this arrangement, but the convergent pencils are
+intercepted by the concave eye-glass, and converted into parallel
+emergent pencils. Now in fig. 2 the concave eye-glass is so placed as to
+receive only a part of the convergent pencil A _p_ B, and this is the
+arrangement usually adopted. By using a concave glass of shorter focus,
+which would therefore be placed nearer to _m p_, the whole of the
+convergent pencil might be received in this as in the former case. But
+then the axis of the emergent pencil, instead of returning (as we see it
+in fig. 1) _towards_ the axis of the telescope, would depart as much
+_from_ that axis. Thus there would be no point on the axis at which the
+eye could be so placed as to receive emergent pencils showing any
+considerable part of the object. The difference may be compared to that
+between looking through the small end of a cone-shaped roll of paper and
+looking through the large end; in the former case the eye sees at once
+all that is to be seen through the roll (supposed fixed in position), in
+the latter the eye may be moved about so as to command the same range of
+view, but _at any instant_ sees over a much smaller range.
+
+To return to the arrangement actually employed, which is illustrated by
+the common opera-glass. We see that the full illuminating power of the
+telescope is not brought into play. But this is not the only objection
+to the Galilean Telescope. It is obvious that if the part C D of the
+object-glass were covered, the point P would not be visible, whereas, in
+the astronomical arrangement no other effect is produced on the
+visibility of an object, by covering part of the object-glass, than a
+small loss of illumination. In other words, the dimensions of the field
+of view of a Galilean Telescope depend on the size of the object-glass,
+whereas in the astronomical Telescope the field of view is independent
+of the size of the object-glass. The difference may be readily tested.
+If we direct an opera-glass upon any object, we shall find that any
+covering placed over a part of the object-glass _becomes visible_ when
+we look through the instrument, interfering therefore _pro tanto_ with
+the range of view. A covering similarly placed on any part of the
+object-glass of an astronomical telescope does not become visible when
+we look through the instrument. The distinction has a very important
+bearing on the theory of telescopic vision.
+
+In considering the application of the telescope to practical
+observation, the circumstance that in the Galilean Telescope no real
+image is formed, is yet more important. A real image admits of
+measurement, linear or angular, while to a _virtual_ image (such an
+image, for instance, as is formed by a common looking-glass) no such
+process can be applied. In simple observation the only noticeable effect
+of this difference is that, whereas in the astronomical Telescope a
+_stop_ or diaphragm can be inserted in the tube so as to cut off what is
+called the _ragged edge_ of the field of view (which includes all the
+part not reached by _full pencils of light_ from the object-glass),
+there is no means of remedying the corresponding defect in the Galilean
+Telescope. It would be a very annoying defect in a telescope intended
+for astronomical observation, since in general the edge of the field of
+view is not perceptible at night. The unpleasant nature of the defect
+may be seen by looking through an opera-glass, and noticing the gradual
+fading away of light round the circumference of the field of view.
+
+The properties of reflection as well as of refraction have been enlisted
+into the service of the astronomical observer. The formation of an image
+by means of a concave mirror is exhibited in fig. 3. As the observer's
+head would be placed between the object and the mirror, if the image,
+formed as in fig. 3, were to be microscopically examined, various
+devices are employed in the construction of reflecting telescopes to
+avoid the loss of light which would result--a loss which would be
+important even with the largest mirrors yet constructed. Thus, in
+Gregory's Telescope, a small mirror, having its concavity towards the
+great one, is placed in the axis of the tube and forms an image which is
+viewed through an aperture in the middle of the great mirror. A similar
+plan is adopted in Cassegrain's Telescope, a small convex mirror
+replacing the concave one. In Newton's Telescope a small inclined-plane
+reflector is used, which sends the pencil of light off at right-angles
+to the axis of the tube. In Herschel's Telescope the great mirror is
+inclined so that the image is formed at a slight distance from the axis
+of the telescope. In the two first cases the object is viewed in the
+usual or direct way, the image being erect in Gregory's and inverted in
+Cassegrain's. In the third the observer looks through the side of the
+telescope, seeing an inverted image of the object. In the last the
+observer sees the object inverted, but not altered as respects right and
+left. The last-mentioned method of viewing objects is the only one in
+which the observer's back is turned towards the object, yet this method
+is called the _front view_--apparently _quasi lucus a non lucendo_.
+
+[Illustration: _Fig. 3._]
+
+It appears, then, that in all astronomical Telescopes, reflecting or
+refracting, a _real image_ of an object is submitted to microscopical
+examination.
+
+Of this fact the possessor of a telescope may easily assure himself;
+for if the eye-glass be removed, and a small screen be placed at the
+focus of the object-glass, there will appear upon the screen a small
+picture of any object towards which the tube is turned. But the image
+may be viewed in another way which requires to be noticed. If the eye,
+placed at a distance of five or six inches from the image, be directed
+down the tube, the image will be seen as before; in fact, just as a
+single convex lens of short focus is the simplest microscope, so a
+simple convex lens of long focus is the simplest telescope.[1] But a
+singular circumstance will immediately attract the observer's notice. A
+real picture, or the image formed on the screen as in the former case,
+can be viewed at varying distances; but when we view the image directly,
+it will be found that for distinct vision the eye must be placed almost
+exactly at a fixed distance from the image. This peculiarity is more
+important than it might be thought at first sight. In fact, it is
+essential that the observer who would rightly apply the powers of his
+telescope, or fairly test its performance, should understand in what
+respect an image formed by an object-glass or object-mirror differs from
+a real object.
+
+The peculiarities to be noted are the _curvature_, _indistinctness_, and
+_false colouring_ of the image.
+
+The curvature of the image is the least important of the three defects
+named--a fortunate circumstance, since this defect admits neither of
+remedy nor modification. The image of a distant object, instead of lying
+in a plane, that is, forming what is technically called a _flat field_,
+forms part of a spherical surface whose centre is at the centre of the
+object-glass. Hence the centre of the field of view is somewhat nearer
+to the eye than are the outer parts of the field. The amount of
+curvature clearly depends on the extent of the field of view, and
+therefore is not great in powerful telescopes. Thus, if we suppose that
+the angular extent of the field is about half a degree (a large or
+low-power field), the centre is nearer than the boundary of the field by
+about 1-320th part only of the field's diameter.
+
+The indistinctness of the image is partly due to the obliquity of the
+pencils which form parts of the image, and partly to what is termed
+_spherical aberration_. The first cause cannot be modified by the
+optician's skill, and is not important when the field of view is small.
+Spherical aberration causes those parts of a pencil which fall near the
+boundary of a convex lens to converge to a nearer (_i.e._ shorter) focus
+than those which fall near the centre. This may be corrected by a proper
+selection of the forms of the two lenses which replace, in all modern
+telescopes, the single lens hitherto considered.
+
+The false colouring of the image is due to _chromatic aberration_. The
+pencil of light proceeding from a point, converges, not to one point,
+but to a short line of varying colour. Thus a series of coloured images
+is formed, at different distances from the object-glass. So that, if a
+screen were placed to receive the mean image _in focus_, a coloured
+fringe due to the other images (_out of focus, and therefore too large_)
+would surround the mean image.
+
+Newton supposed that it was impossible to get rid of this defect, and
+therefore turned his attention to the construction of reflectors. But
+the discovery that the _dispersive_ powers of different glasses are not
+proportional to their reflective powers, supplied opticians with the
+means of remedying the defect. Let us clearly understand what is the
+discovery referred to. If with a glass prism of a certain form we
+produce a spectrum of the sun, this spectrum will be thrown a certain
+distance away from the point on which the sun's rays would fall if not
+interfered with. This distance depends on the _refractive_ power of the
+glass. The spectrum will have a certain length, depending on the
+_dispersive_ power of the glass. Now, if we change our prism for another
+of exactly the same shape, but made of a different kind of glass, we
+shall find the spectrum thrown to a different spot. If it appeared that
+the length of the new spectrum was increased or diminished in exactly
+the same proportion as its distance from the line of the sun's direct
+light, it would have been hopeless to attempt to remedy chromatic
+aberration. Newton took it for granted that this was so. But the
+experiments of Hall and the Dollonds showed that there is no such strict
+proportionality between the dispersive and refractive powers of
+different kinds of glass. It accordingly becomes possible to correct the
+chromatic aberration of one glass by superadding that of another.
+
+[Illustration: _Fig. 4._]
+
+This is effected by combining, as shown in fig. 4, a convex lens of
+_crown_ glass with a concave lens of _flint_ glass, the convex lens
+being placed nearest to the object. A little colour still remains, but
+not enough to interfere seriously with the distinctness of the image.
+
+But even if the image formed by the object-glass were perfect, yet this
+image, viewed through a single convex lens of short focus placed as in
+fig. 1, would appear curved, indistinct, coloured, and also _distorted_,
+because viewed by pencils of light which do not pass through the centre
+of the eye-glass. These effects can be diminished (but not entirely
+removed _together_) by using an _eye-piece_ consisting of two lenses
+instead of a single eye-glass. The two forms of eye-piece most commonly
+employed are exhibited in figs. 5 and 6. Fig. 5 is Huyghens' eye-piece,
+called also the _negative_ eye-piece, because a real image is formed
+_behind_ the _field-glass_ (the lens which lies nearest to the
+object-glass). Fig. 6 represents Ramsden's eye-piece, called also the
+_positive_ eye-piece, because the real image formed by the object-glass
+lies _in front of_ the field-glass.
+
+[Illustration: _Fig. 5._]
+
+[Illustration: _Fig. 6._]
+
+The course of a slightly oblique pencil through either eye-piece is
+exhibited in the figures. The lenses are usually plano-convex, the
+convexities being turned towards the object-glass in the negative
+eye-piece, and towards each other in the positive eye-piece. Coddington
+has shown, however, that the best forms for the lenses of the negative
+eye-piece are those shown in fig. 5.
+
+The negative eye-piece, being achromatic, is commonly employed in all
+observations requiring distinct vision only. But as it is clearly unfit
+for observations requiring micrometrical measurement, or reference to
+fixed lines at the focus of the object-glass, the positive eye-piece is
+used for these purposes.
+
+For observing objects at great elevations the diagonal eye-tube is
+often convenient. Its construction is shown in fig. 7. ABC is a totally
+reflecting prism of glass. The rays from the object-glass fall on the
+face AB, are totally reflected on the face BC, and emerge through the
+face AC. In using this eye-piece, it must be remembered that it
+lengthens the sliding eye-tube, which must therefore be thrust further
+in, or the object will not be seen in focus. There is an arrangement by
+which the change of direction is made to take place between the two
+glasses of the eye-piece. With this arrangement (known as the _diagonal
+eye-piece_) no adjustment of the eye-tube is required. However, for
+amateurs' telescopes the more convenient arrangement is the diagonal
+eye-tube, since it enables the observer to apply any eye-piece he
+chooses, just as with the simple sliding eye-tube.
+
+[Illustration: _Fig. 7._]
+
+We come next to the important question of the _mounting_ of our
+telescope.
+
+The best known, and, in some respects, the simplest method of
+mounting a telescope for general observation is that known as the
+_altitude-and-azimuth_ mounting. In this method the telescope is
+pointed towards an object by two motions,--one giving the tube the
+required _altitude_ (or elevation), the other giving it the required
+_azimuth_ (or direction as respects the compass points).
+
+For small alt-azimuths the ordinary pillar-and-claw stand is
+sufficiently steady. For larger instruments other arrangements are
+needed, both to give the telescope steadiness, and to supply slow
+movements in altitude and azimuth. The student will find no difficulty
+in understanding the arrangement of sliding-tubes and rack-work commonly
+adopted. This arrangement seems to me to be in many respects defective,
+however. The slow movement in altitude is not uniform, but varies in
+effect according to the elevation of the object observed. It is also
+limited in range; and quite a little series of operations has to be gone
+through when it is required to direct the telescope towards a new
+quarter of the heavens. However expert the observer may become by
+practice in effecting these operations, they necessarily take up some
+time (performed as they must be in the dark, or by the light of a small
+lantern), and during this time it often happens that a favourable
+opportunity for observation is lost.
+
+These disadvantages are obviated when the telescope is mounted in such a
+manner as is exhibited in fig. 8, which represents a telescope of my own
+construction. The slow movement in altitude is given by rotating the rod
+_he_, the endless screw in which turns the small wheel at _b_, whose
+axle in turn bears a pinion-wheel working in the teeth of the quadrant
+_a_. The slow movement in azimuth is given in like manner by rotating
+the rod _h'e'_, the lantern-wheel at the end of which turns a
+crown-wheel on whose axle is a pinion-wheel working in the teeth of the
+circle _c_. The casings at _e_ and _e'_, in which the rods _he_ and
+_h'e'_ respectively work, are so fastened by elastic cords that an
+upward pressure on the handle _h_, or a downward pressure on the handle
+_h'_, at once releases the endless screw or the crown-wheel
+respectively, so that the telescope can be swept at once through any
+desired angle in altitude or azimuth. This method of mounting has other
+advantages; the handles are conveniently situated and constant in
+position; also, as they do not work directly on the telescope, they can
+be turned without setting the tube in vibration.
+
+[Illustration: _Fig. 8._]
+
+I do not recommend the mounting to be exactly as shown in fig. 8. That
+method is much too expensive for an alt-azimuth. But a simple
+arrangement of belted wheels in place of the toothed wheels _a_ and _c_
+might very readily be prepared by the ingenious amateur telescopist; and
+I feel certain that the comfort and convenience of the arrangement would
+amply repay him for the labour it would cost him. My own
+telescope--though the large toothed-wheel and the quadrant were made
+inconveniently heavy (through a mistake of the workman who constructed
+the instrument)--worked as easily and almost as conveniently as an
+equatorial.
+
+Still, it is well for the observer who wishes systematically to survey
+the heavens--and who can afford the expense--to obtain a well-mounted
+_equatorial_. In this method of mounting, the main axis is directed to
+the pole of the heavens; the other axis, at right angles to the first,
+carries the telescope-tube. One of the many methods adopted for mounting
+equatorials is that exhibited--with the omission of some minor
+details--in fig. 9. _a_ is the polar axis, _b_ is the axis (called the
+declination axis) which bears the telescope. The circles _c_ and _d_
+serve to indicate, by means of verniers revolving with the axes, the
+motion of the telescope in right ascension and declination,
+respectively. The weight _w_ serves to counterpoise the telescope, and
+the screws _s_, _s_, _s_, _s_, serve to adjust the instrument so that
+the polar axis shall be in its proper position. The advantage gained by
+the equatorial method of mounting is that only one motion is required to
+follow a star. Owing to the diurnal rotation of the earth, the stars
+appear to move uniformly in circles parallel to the celestial equator;
+and it is clear that a star so moving will be kept in the field of
+view, if the telescope, once directed to the star, be made to revolve
+uniformly and at a proper rate round the polar axis.
+
+[Illustration: _Fig. 9._]
+
+The equatorial can be directed by means of the circles _c_ and _d_ to
+any celestial object whose right ascension and declination are known. On
+the other hand, to bring an object into the field of view of an
+alt-azimuth, it is necessary, either that the object itself should be
+visible to the naked eye, or else that the position of the object should
+be pretty accurately learned from star-maps, so that it may be picked up
+by the alt-azimuth after a little searching. A small telescope called a
+_finder_ is usually attached to all powerful telescopes intended for
+general observation. The finder has a large field of view, and is
+adjusted so as to have its axis parallel to that of the large telescope.
+Thus a star brought to the centre of the large field of the finder
+(indicated by the intersection of two lines placed at the focus of the
+eye-glass) is at, or very near, the centre of the small field of the
+large telescope.
+
+If a telescope has no finder, it will be easy for the student to
+construct one for himself, and will be a useful exercise in optics. Two
+convex lenses not very different in size from those shown in fig. 1, and
+placed as there shown--the distance between them being the sum of the
+focal lengths of the two glasses--in a small tube of card, wood, or tin,
+will serve the purpose of a finder for a small telescope. It can be
+attached by wires to the telescope-tube, and adjusted each night before
+commencing observation. The adjustment is thus managed:--a low power
+being applied to the telescope, the tube is turned towards a bright
+star; this is easily effected with a low power; then the finder is to be
+fixed, by means of its wires, in such a position that the star shall be
+in the centre of the field of the finder when also in the centre of the
+telescope's field. When this has been done, the finder will greatly help
+the observations of the evening; since with high powers much time would
+be wasted in bringing an object into the field of view of the telescope
+without the aid of a finder. Yet more time would be wasted in the case
+of an object not visible to the naked eye, but whose position with
+reference to several visible stars is known; since, while it is easy to
+bring the point required to the centre of the _finder's_ field, in which
+the guiding stars are visible, it is very difficult to direct the
+_telescope's_ tube on a point of this sort. A card tube with wire
+fastenings, such as we have described, may appear a very insignificant
+contrivance to the regular observer, with his well-mounted equatorial
+and carefully-adjusted finder. But to the first attempts of the amateur
+observer it affords no insignificant assistance, as I can aver from my
+own experience. Without it--a superior finder being wanting--our
+"half-hours" would soon be wasted away in that most wearisome and
+annoying of all employments, trying to "pick up" celestial objects.
+
+It behoves me at this point to speak of star-maps. Such maps are of many
+different kinds. There are the Observatory maps, in which the places of
+thousands of stars are recorded with an amazing accuracy. Our beginner
+is not likely to make use of, or to want, such maps as these. Then there
+are maps merely intended to give a good general idea of the appearance
+of the heavens at different hours and seasons. Plate I. presents four
+maps of this sort; but a more complete series of eight maps has been
+published by Messrs. Walton and Maberly in an octavo work; and my own
+'Constellation-Seasons' give, at the same price, twelve quarto maps (of
+four of which those in Plate I. are miniatures), showing the appearance
+of the sky at any hour from month to month, or on any night, at
+successive intervals of two hours. But maps intermediate in character to
+these and to Observatory maps are required by the amateur observer.
+Such are the Society's six gnomonic maps, the set of six gnomonic maps
+in Johnstone's 'Atlas of Astronomy,' and my own set of twelve gnomonic
+maps. The Society's maps are a remarkably good set, containing on the
+scale of a ten-inch globe all the stars in the Catalogue of the
+Astronomical Society (down to the fifth magnitude). The distortion,
+however, is necessarily enormous when the celestial sphere is presented
+in only six gnomonic maps. In my maps all the stars of the British
+Association Catalogue down to the fifth magnitude are included on the
+scale of a six-inch globe. The distortion is scarcely a fourth of that
+in the Society's maps. The maps are so arranged that the relative
+positions of all the stars in each hemisphere can be readily gathered
+from a single view; and black duplicate-maps serve to show the
+appearance of the constellations.
+
+It is often convenient to make small maps of a part of the heavens we
+may wish to study closely. My 'Handbook of the Stars' has been prepared
+to aid the student in the construction of such maps.
+
+In selecting maps it is well to be able to recognise the amount of
+distortion and scale-variation. This may be done by examining the spaces
+included between successive parallels and meridians, near the edges and
+angles of the maps, and comparing these either with those in the centre
+of the map, or with the known figures and dimensions of the
+corresponding spaces on a globe.
+
+We may now proceed to discuss the different tests which the intending
+purchaser of a telescope should apply to the instrument.
+
+The excellence of an object-glass can be satisfactorily determined only
+by testing the performance of the telescope in the manner presently to
+be described. But it is well to examine the quality of the glass as
+respects transparency and uniformity of texture. Bubbles, scratches, and
+other such defects, are not very important, since they do not affect the
+distinctness of the field as they would in a Galilean Telescope,--a
+little light is lost, and that is all. The same remark applies to dust
+upon the glass. The glass should be kept as free as possible from dirt,
+damp, or dust, but it is not advisable to remove every speck which,
+despite such precaution, may accidentally fall upon the object-glass.
+When it becomes necessary to clean the glass, it is to be noted that the
+substance used should be soft, perfectly dry, and free from dust. Silk
+is often recommended, but some silk is exceedingly objectionable in
+texture,--old silk, perfectly soft to the touch, is perhaps as good as
+anything. If the dust which has fallen on the glass is at all gritty,
+the glass will suffer by the method of cleaning commonly adopted, in
+which the dust is _gathered up_ by pressure. The proper method is to
+clean a small space near the edge of the glass, and to _sweep_ from that
+space as centre. In this way the dust is _pushed before_ the silk or
+wash-leather, and does not cut the glass. It is well always to suspect
+the presence of gritty dust, and adopt this cautious method of cleaning.
+
+The two glasses should on no account be separated.
+
+In examining an eye-piece, the quality of the glass should be noted, and
+care taken that both glasses (but especially the field-glass) are free
+from the least speck, scratch, or blemish of any kind, for these defects
+will be exhibited in a magnified state in the field of view. Hence the
+eye-pieces require to be as carefully preserved from damp and dust as
+the object-glass, and to be more frequently cleaned.
+
+The tube of the telescope should be light, but strong, and free from
+vibration. Its quality in the last respect can be tested by lightly
+striking it when mounted; the sound given out should be dead or
+non-resonant. The inside of the tube must absorb extraneous light, and
+should therefore be coloured a dull black; and stops of varying radius
+should be placed along its length with the same object. Sliding tubes,
+rack-work, etc., should work closely, yet easily.
+
+The telescope should be well balanced for vision with the small
+astronomical eye-pieces. But as there is often occasion to use
+appliances which disturb the balance, it is well to have the means of at
+once restoring equilibrium. A cord ring running round the tube (pretty
+tightly, so as to rest still when the tube is inclined), and bearing a
+small weight, will be all that is required for this purpose; it must be
+slipped along the tube until the tube is found to be perfectly balanced.
+Nothing is more annoying than, after getting a star well in the field,
+to see the tube shift its position through defective balance, and thus
+to have to search again for the star. Even with such an arrangement as
+is shown in fig. 8, though the tube cannot readily shift its position,
+it is better to have it well balanced.
+
+The quality of the stand has a very important influence on the
+performance of a telescope. In fact, a moderately good telescope,
+mounted on a steady stand, working easily and conveniently, will not
+only enable the observer to pass his time much more pleasantly, but will
+absolutely exhibit more difficult objects than a finer instrument on a
+rickety, ill-arranged stand. A good observing-chair is also a matter of
+some importance, the least constraint or awkwardness of position
+detracting considerably from the power of distinct vision. Such, at
+least, is my own experience.
+
+But the mere examination of the glasses, tube, mounting, &c., is only
+the first step in the series of tests which should be applied to a
+telescope, since the excellence of the instrument depends, not on its
+size, the beauty of its mounting, or any extraneous circumstances, but
+on its performance.
+
+The observer should first determine whether the chromatic aberration is
+corrected. To ascertain this the telescope should be directed to the
+moon, or (better) to Jupiter, and accurately focussed for distinct
+vision. If, then, on moving the eye-piece towards the object-glass, a
+ring of purple appears round the margin of the object, and on moving the
+eye-glass in the contrary direction a ring of green, the chromatic
+aberration is corrected, since these are the colours of the secondary
+spectrum.
+
+To determine whether the spherical aberration is corrected, the
+telescope should be directed towards a star of the third or fourth
+magnitude, and focussed for distinct vision. A cap with an aperture of
+about one-half its diameter should then be placed over the object-glass.
+If no new adjustment is required for distinct vision, the spherical
+aberration is corrected, since the mean focal length and the focal
+length of the central rays are equal. If, when the cap is on, the
+eye-piece has to be pulled out for distinct vision, the spherical
+aberration has not been fully corrected; if the eye-piece has to be
+pushed in, the aberration has been over-corrected. As a further test, we
+may cut off the central rays, by means of a circular card covering the
+middle of the object-glass, and compare the focal length for distinct
+vision with the focal length when the cap is applied. The extent of the
+spherical aberration may be thus determined; but if the first experiment
+gives a satisfactory result, no other is required.
+
+A star of the first magnitude should next be brought into the field of
+view. If an irradiation from one side is perceived, part of the
+object-glass has not the same refractive power as the rest; and the
+part which is defective can be determined by applying in different
+positions a cap which hides half the object-glass. If the irradiation is
+double, it will probably be found that the object-glass has been too
+tightly screwed, and the defect will disappear when the glass is freed
+from such undue pressure.
+
+If the object-glass is not quite at right angles to the axis of the
+tube, or if the eye-tube is at all inclined, a like irradiation will
+appear when a bright star is in the field. The former defect is not
+easily detected or remedied; nor is it commonly met with in the work of
+a careful optician. The latter defect may be detected by cutting out
+three circular cards of suitable size with a small aperture at the
+centre of each, and inserting one at each end of the eye-tube, and one
+over the object-glass. If the tube is rightly placed the apertures will
+of course lie in a right line, so that it will be possible to look
+through all three at once. If not, it will be easy to determine towards
+what part of the object-glass the eye-tube is directed, and to correct
+the position of the tube accordingly.
+
+The best tests for determining the defining power of a telescope are
+close double or multiple stars, the components of which are not very
+unequal. The illuminating power should be tested by directing the
+telescope towards double or multiple stars having one or more minute
+components. Many of the nebulæ serve as tests both for illumination and
+defining power. As we proceed we shall meet with proper objects for
+testing different telescopes. For the present, let the following list
+suffice. It is selected from Admiral Smyth's tests, obtained by
+diminishing the aperture of a 6-in. telescope having a focal length of
+8-1/2 feet:
+
+A two-inch aperture, with powers of from 60 to 100, should exhibit
+
+[alpha] Piscium (3"·5).   | [delta] Cassiopeiæ (9"·5),
+                          |    mag. (4 and 7-1/2)
+[gamma] Leonis  (3"·2).   | Polaris (18"·6), mag. (2-1/2
+                          |    and 9-1/2)
+
+A four-inch, powers 80 to 120, should exhibit
+
+[xi] Ursæ Majoris (2"·4). | [sigma] Cassiopeiæ (3"·1),
+                          |    mag. (6 and 8).
+[gamma] Ceti (2"·6).      | [delta] Geminorum (7"·1),
+                          |    mag. (4 and 9).
+
+The tests in the first column are for definition, those in the second
+for illumination. It will be noticed that, though in the case of Polaris
+the smaller aperture may be expected to show the small star of less than
+the 9th magnitude, a larger aperture is required to show the 8th
+magnitude component of [sigma] Cassiopeiæ, on account of the greater
+closeness of this double.
+
+In favourable weather the following is a good general test of the
+performance of a telescope:--A star of the 3rd or 4th magnitude at a
+considerable elevation above the horizon should exhibit a small well
+defined disc, surrounded by two or three fine rings of light.
+
+A telescope should not be mounted within doors, if it can be
+conveniently erected on solid ground, as every movement in the house
+will cause the instrument to vibrate unpleasantly. Further, if the
+telescope is placed in a warm room, currents of cold air from without
+will render observed objects hazy and indistinct. In fact, Sir W.
+Herschel considered that a telescope should not even be erected near a
+house or elevation of any kind round which currents of air are likely to
+be produced. If a telescope is used in a room, the temperature of the
+room should be made as nearly equal as possible to that of the outer
+air.
+
+When a telescope is used out of doors a 'dew-cap,' that is, a tube of
+tin or pasteboard, some ten or twelve inches long, should be placed on
+the end of the instrument, so as to project beyond the object-glass. For
+glass is a good radiator of heat, so that dew falls heavily upon it,
+unless the radiation is in some way checked. The dew-cap does this
+effectually. It should be blackened within, especially if made of metal.
+"After use," says old Kitchener, "the telescope should be kept in a warm
+place long enough for any moisture on the object-glass to evaporate." If
+damp gets between the glasses it produces a fog (which opticians call a
+sweat) or even a seaweed-like vegetation, by which a valuable glass may
+be completely ruined.
+
+The observer should not leave to the precious hours of the night the
+study of the bearing and position of the objects he proposes to examine.
+This should be done by day--an arrangement which has a twofold
+advantage,--the time available for observation is lengthened, and the
+eyes are spared sudden changes from darkness to light, and _vice versâ_.
+Besides, the eye is ill-fitted to examine difficult objects, after
+searching by candle-light amongst the minute details recorded in maps or
+globes. Of the effect of rest to the eye we have an instance in Sir J.
+Herschel's rediscovery of the satellites of Uranus, which he effected
+after keeping his eyes in darkness for a quarter of an hour. Kitchener,
+indeed, goes so far as to recommend (with a _crede experto_) an
+_interval of sleep_ in the darkness of the observing-room before
+commencing operations. I have never tried the experiment, but I should
+expect it to have a bad rather than a good effect on the eyesight, as
+one commonly sees the eyes of a person who has been sleeping in his
+day-clothes look heavy and bloodshot.
+
+The object or the part of an object to be observed should be brought as
+nearly as possible to the centre of the field of view. When there is no
+apparatus for keeping the telescope pointed upon an object, the best
+plan is so to direct the telescope by means of the finder, that the
+object shall be just out of the field of view, and be brought (by the
+earth's motion) across the centre of the field. Thus the vibrations
+which always follow the adjustment of the tube will have subsided before
+the object appears. The object should then be intently watched during
+the whole interval of its passage across the field of view.
+
+It is important that the student should recognise the fact that the
+highest powers do not necessarily give the best views of celestial
+objects. High powers in all cases increase the difficulty of
+observation, since they diminish the field of view and the illumination
+of the object, increase the motion with which (owing to the earth's
+motion) the image moves across the field, and magnify all defects due to
+instability of the stand, imperfection of the object-glass, or
+undulation of the atmosphere. A good object-glass of three inches
+aperture will in very favourable weather bear a power of about 300, when
+applied to the observation of close double or multiple stars, but for
+all other observations much lower powers should be used. Nothing but
+failure and annoyance can follow the attempt to employ the highest
+powers on unsuitable objects or in unfavourable weather.
+
+The greatest care should be taken in focussing the telescope. When high
+powers are used this is a matter of some delicacy. It would be well if
+the eye-pieces intended for a telescope were so constructed that when
+the telescope is focussed for one, this might be replaced by any other
+without necessitating any use of the focussing rack-work. This could be
+readily effected by suitably placing the shoulder which limits the
+insertion of the eye-piece.
+
+It will be found that, even in the worst weather for observation, there
+are instants of distinct vision (with moderate powers) during which the
+careful observer may catch sight of important details; and, similarly,
+in the best observing weather, there are moments of unusually distinct
+vision well worth patient waiting for, since in such weather alone the
+full powers of the telescope can be employed.
+
+The telescopist should not be deterred from observation by the presence
+of fog or haze, since with a hazy sky definition is often singularly
+good.
+
+The observer must not expect distinct vision of objects near the
+horizon. Objects near the eastern horizon during the time of morning
+twilight are especially confused by atmospheric undulations; in fact,
+early morning is a very unfavourable time for the observation of all
+objects.
+
+The same rules which we have been applying to refractors, serve for
+reflectors. The performance of a reflector will be found to differ in
+some respects, however, from that of a refractor. Mr. Dawes is, we
+believe, now engaged in testing reflectors, and his unequalled
+experience of refractors will enable him to pronounce decisively on the
+relative merits of the two classes of telescopes.
+
+We have little to say respecting the construction of telescopes. Whether
+it is advisable or not for an amateur observer to attempt the
+construction of his own telescope is a question depending entirely on
+his mechanical ability and ingenuity. My own experience of telescope
+construction is confined to the conversion of a 3-feet into a 5-1/2-feet
+telescope. This operation involved some difficulties, since the aperture
+had to be increased by about an inch. I found a tubing made of alternate
+layers of card and calico well pasted together, to be both light and
+strong. But for the full length of tube I think a core of metal is
+wanted. A learned and ingenious friend, Mr. Sharp, Fellow of St. John's
+College, informs me that a tube of tin, covered with layers of brown
+paper, well pasted and thicker near the middle of the tube, forms a
+light and strong telescope-tube, almost wholly free from vibration.
+
+Suffer no inexperienced person to deal with your object-glass. I knew a
+valuable glass ruined by the proceedings of a workman who had been told
+to attach three pieces of brass round the cell of the double lens. What
+he had done remained unknown, but ever after a wretched glare of light
+surrounded all objects of any brilliancy.
+
+One word about the inversion of objects by the astronomical telescope.
+It is singular that any difficulty should be felt about so simple a
+matter, yet I have seen in the writings of more than one distinguished
+astronomer, wholly incorrect views as to the nature of the inversion.
+One tells us that to obtain the correct presentation from a picture
+taken with a telescope, the view should be inverted, held up to the
+light, and looked at from the back of the paper. Another tells us to
+invert the picture and hold it opposite a looking-glass. Neither method
+is correct. The simple correction wanted is to hold the picture upside
+down--the same change which brings the top to the bottom brings the
+right to the left, _i.e._, fully corrects the inversion.
+
+In the case, however, of a picture taken by an Herschelian reflector,
+the inversion not being complete, a different method must be adopted. In
+fact, either of the above-named processes, incorrect for the ordinary
+astronomical, would be correct for the Herschelian Telescope. The latter
+inverts but does not reverse right and left; therefore after inverting
+our picture we must interchange right and left because they have been
+reversed by the inversion. This is effected either by looking at the
+picture from behind, or by holding it up to a mirror.
+
+[Illustration: PLATE II.]
+
+
+
+
+CHAPTER II.
+
+A HALF-HOUR WITH ORION, LEPUS TAURUS, ETC.
+
+
+Any of the half-hours here assigned to the constellation-seasons may be
+taken first, and the rest in seasonal or cyclic order. The following
+introductory remarks are applicable to each:--
+
+If we stand on an open space, on any clear night, we see above us the
+celestial dome spangled with stars, apparently fixed in position. But
+after a little time it becomes clear that these orbs are slowly shifting
+their position. Those near the eastern horizon are rising, those near
+the western setting. Careful and continuous observation would show that
+the stars are all moving in the same way, precisely, as they would if
+they were fixed to the concave surface of a vast hollow sphere, and this
+sphere rotated about an axis. This axis, in our latitude, is inclined
+about 51-1/2° to the horizon. Of course only one end of this imaginary
+axis can be above our horizon. This end lies very near a star which it
+will be well for us to become acquainted with at the beginning of our
+operations. It lies almost exactly towards the north, and is raised from
+50° to 53° (according to the season and hour) above the horizon. There
+is an easy method of finding it.
+
+We must first find the Greater Bear. It will be seen from Plate 1, that
+on a spring evening the seven conspicuous stars of this constellation
+are to be looked for towards the north-east, about half way between the
+horizon and the point overhead (or _zenith_), the length of the set of
+stars being vertical. On a summer's evening the Great Bear is nearly
+overhead. On an autumn evening he is towards the north-west, the length
+of the set of seven being somewhat inclined to the horizon. Finally, on
+a winter's evening, he is low down towards the north, the length of the
+set of seven stars being nearly in a horizontal direction.
+
+Having found the seven stars, we make use of the pointers [alpha] and
+[beta] (shown in Plate 1) to indicate the place of the Pole-star, whose
+distance from the pointer [alpha] is rather more than three times the
+distance of [alpha] from [beta].
+
+Now stand facing the Pole-star. Then all the stars are travelling round
+that star _in a direction contrary to that in which the hands of a watch
+move_. Thus the stars below the pole are moving _towards the right_,
+those above the pole _towards the left_, those to the right of the pole
+_upwards_, those to the left of the pole _downwards_.
+
+Next face the south. Then all the stars on our left, that is, towards
+the east, are rising slantingly towards the south; those due south are
+moving horizontally to the right, that is, towards the west; and those
+on our right are passing slantingly downwards towards the west.
+
+It is important to familiarise ourselves with these motions, because it
+is through them that objects pass out of the field of view of the
+telescope, and by moving the tube in a proper direction we can easily
+pick up an object that has thus passed away, whereas if we are not
+familiar with the varying motions in different parts of the celestial
+sphere, we may fail in the attempt to immediately recover an object, and
+waste time in the search for it.
+
+The consideration of the celestial motions shows how advantageous it is,
+when using an alt-azimuth, to observe objects as nearly as possible due
+south. Of course in many cases this is impracticable, because a
+phenomenon we wish to watch may occur when an object is not situated
+near the meridian. But in examining double stars there is in general no
+reason for selecting objects inconveniently situated. We can wait till
+they come round to the meridian, and then observe them more comfortably.
+Besides, most objects are higher, and therefore better seen, when due
+south.
+
+Northern objects, and especially those within the circle of perpetual
+apparition, often culminate (that is, cross the meridian, or north and
+south line) at too great a height for comfortable vision. In this case
+we should observe them towards the east or west, and remember that in
+the first case they are rising, and in the latter they are setting, and
+that in both cases they have also a motion from left to right.
+
+If we allow an object to pass right across the field of view (the
+telescope being fixed), the apparent direction of its motion is the
+exact reverse of the true direction of the star's motion. This will
+serve as a guide in shifting the alt-azimuth after a star has passed out
+of the field of view.
+
+The following technical terms must be explained. That part of the field
+of view towards which the star appears to move is called the _preceding_
+part of the field, the opposite being termed the _following_ part. The
+motion for all stars, except those lying in an oval space extending from
+the zenith to the pole of the heavens, is more or less from right to
+left (in the inverted field). Now, if we suppose a star to move along a
+diameter of the field so as to divide the field into two semicircles,
+then in all cases in which this motion takes places from right to left,
+that semicircle which contains the lowest point (apparently) of the
+field is the _northern_ half, the other is the _southern_ half. Over the
+oval space just mentioned the reverse holds.
+
+Thus the field is divided into four quadrants, and these are termed
+_north following_ (_n.f._) and _south following_ (_s.f._); _north
+preceding_ (_n.p._), and _south preceding_ (_s.p._). The student can
+have no difficulty in interpreting these terms, since he knows which is
+the following and which the preceding _semicircle_, which the northern
+and which the southern. In the figures of plates 3 and 5, the letters
+_n.f._, _n.p._, &c., are affixed to the proper quadrants. It is to be
+remembered that the quadrants thus indicated are measured either way
+from the point and feather of the diametral arrows.
+
+Next, of the apparent annual motion of the stars. This takes place in
+exactly the same manner as the daily motion. If we view the sky at eight
+o'clock on any day, and again at the same hour one month later, we shall
+find that at the latter observation (as compared with the former) the
+heavens appear to have rotated by the _twelfth part_ of a complete
+circumference, and the appearance presented is precisely the same as we
+should have observed had we waited for two hours (the _twelfth part_ of
+a day) on the day of the first observation.
+
+       *       *       *       *       *
+
+Our survey of the heavens is supposed to be commenced during the first
+quarter of the year, at ten o'clock on the 20th of January, or at nine
+on the 5th of February, or at eight on the 19th of February, or at seven
+on the 6th of March, or at hours intermediate to these on intermediate
+days.
+
+We look first for the Great Bear towards the north-east, as already
+described, and thence find the Pole-star; turning towards which we see,
+towards the right and downwards, the two guardians of the pole ([beta]
+and [gamma] Ursæ Minoris). Immediately under the Pole-star is the
+Dragon's Head, a conspicuous diamond of stars. Just on the horizon is
+Vega, scintillating brilliantly. Overhead is the brilliant Capella, near
+which the Milky Way is seen passing down to the horizon on either side
+towards the quarters S.S.E. and N.N.W.
+
+For the present our business is with the southern heavens, however.
+
+Facing the south, we see a brilliant array of stars, Sirius
+unmistakeably overshining the rest. Orion is shining in full glory, his
+leading brilliant, Betelgeuse[2] being almost exactly on the meridian,
+and also almost exactly half way between the horizon and the zenith. In
+Plate 2 is given a map of this constellation and its neighbourhood.
+
+Let us first turn the tube on Sirius. It is easy to get him in the field
+without the aid of a finder. The search will serve to illustrate a
+method often useful when a telescope has no finder. Having taking out
+the eye-piece--a low-power one, suppose--direct the tube nearly towards
+Sirius. On looking through it, a glare of light will be seen within the
+tube. Now, if the tube be slightly moved about, the light will be seen
+to wax and wane, according as the tube is more or less accurately
+directed. Following these indications, it will be found easy to direct
+the tube, so that the object-glass shall appear _full of light_. When
+this is done, insert the eye-piece, and the star will be seen in the
+field.
+
+But the telescope is out of focus, therefore we must turn the small
+focussing screw. Observe the charming chromatic changes--green, and
+red, and blue light, purer than the hues of the rainbow, scintillating
+and coruscating with wonderful brilliancy. As we get the focus, the
+excursions of these light flashes diminish until--if the weather is
+favourable--the star is seen, still scintillating, and much brighter
+than to the naked eye, but reduced to a small disc of light, surrounded
+(in the case of so bright a star as Sirius) with a slight glare. If
+after obtaining the focus the focussing rack work be still turned, we
+see a coruscating image as before. In the case of a very brilliant star
+these coruscations are so charming that we may be excused for calling
+the observer's attention to them. The subject is not without interest
+and difficulty as an optical one. But the astronomer's object is to get
+rid of all these flames and sprays of coloured light, so that he has
+very little sympathy with the admiration which Wordsworth is said to
+have expressed for out-of-focus views of the stars.
+
+We pass to more legitimate observations, noticing in passing that Sirius
+is a double star, the companion being of the tenth magnitude, and
+distant about ten seconds from the primary. But our beginner is not
+likely to see the companion, which is a very difficult object, vowing to
+the overpowering brilliancy of the primary.
+
+Orion affords the observer a splendid field of research. It is a
+constellation rich in double and multiple stars, clusters, and nebulæ.
+We will begin with an easy object.
+
+The star [delta] (Plate 3), or _Mintaka_, the uppermost of the three
+stars forming the belt, is a wide double. The primary is of the second
+magnitude, the secondary of the seventh, both being white.
+
+The star [alpha] (_Betelgeuse_) is an interesting object, on account of
+its colour and brilliance, and as one of the most remarkable variables
+in the heavens. It was first observed to be variable by Sir John
+Herschel in 1836. At this period its variations were "most marked and
+striking." This continued until 1840, when the changes became "much less
+conspicuous. In January, 1849, they had recommenced, and on December
+5th, 1852, Mr. Fletcher observed [alpha] Orionis brighter than Capella,
+and actually the largest star in the northern hemisphere." That a star
+so conspicuous, and presumably so large, should present such remarkable
+variations, is a circumstance which adds an additional interest to the
+results which have rewarded the spectrum-analysis of this star by Mr.
+Huggins and Professor Miller. It appears that there is decisive evidence
+of the presence in this luminary of many elements known to exist in our
+own sun; amongst others are found sodium, magnesium, calcium, iron, and
+bismuth. Hydrogen appears to be absent, or, more correctly, there are no
+lines in the star's spectrum corresponding to those of hydrogen in the
+solar spectrum. Secchi considers that there is evidence of an actual
+change in the spectrum of the star, an opinion in which Mr. Huggins does
+not coincide. In the telescope Betelgeuse appears as "a rich and
+brilliant gem," says Lassell, "a rich topaz, in hue and brilliancy
+differing from any that I have seen."
+
+Turn next to [beta] (Rigel), the brightest star below the belt. This is
+a very noted double, and will severely test our observer's telescope, if
+small. The components are well separated (see Plate 3), compared with
+many easier doubles; the secondary is also of the seventh magnitude, so
+that neither as respects closeness nor smallness of the secondary, is
+Rigel a difficult object. It is the combination of the two features
+which makes it a test-object. Kitchener says a 1-3/4-inch object-glass
+should show Rigel double; in earlier editions of his work he gave
+2-3/4-inches as the necessary aperture. Smyth mentions Rigel as a test
+for a 4-inch aperture, with powers of from 80 to 120. A 3-inch aperture,
+however, will certainly show the companion. Rigel is an orange star, the
+companion blue.
+
+Turn next to [lambda] the northernmost of the set of three stars in the
+head of Orion. This is a triple star, though an aperture of 3 inches
+will show it only as a double. The components are 5" apart, the colours
+pale white and violet. With the full powers of a 3-1/2-inch glass a
+faint companion may be seen above [lambda].
+
+The star [zeta], the lowest in the belt, may be tried with a 3-1/2-inch
+glass. It is a close double, the components being nearly equal, and
+about 2-1/2" apart (see Plate 3).
+
+For a change we will now try our telescope on a nebula, selecting the
+great nebula in the Sword. The place of this object is indicated in
+Plate 2. There can be no difficulty in finding it since it is clearly
+visible to the naked eye on a moonless night--the only sort of night on
+which an observer would care to look at nebulæ. A low power should be
+employed.
+
+The nebula is shown in Plate 3 as I have seen it with a 3-inch aperture.
+We see nothing of those complex streams of light which are portrayed in
+the drawings of Herschel, Bond, and Lassell, but enough to excite our
+interest and wonder. What is this marvellous light-cloud? One could
+almost imagine that there was a strange prophetic meaning in the words
+which have been translated "Canst thou loose the bands of Orion?"
+Telescope after telescope had been turned on this wonderful object with
+the hope of resolving its light into stars. But it proved intractable to
+Herschel's great reflector, to Lassell's 2-feet reflector, to Lord
+Rosse's 3-feet reflector, and even partially to the great 6-feet
+reflector. Then we hear of its supposed resolution into stars, Lord
+Rosse himself writing to Professor Nichol, in 1846, "I may safely say
+there can be little, if any, doubt as to the resolvability of the
+nebula;--all about the trapezium is a mass of stars, the rest of the
+nebula also abounding with stars, and exhibiting the characteristics of
+resolvability strongly marked."
+
+It was decided, therefore, that assuredly the great nebula is a
+congeries of stars, and not a mass of nebulous matter as had been
+surmised by Sir W. Herschel. And therefore astronomers were not a little
+surprised when it was proved by Mr. Huggins' spectrum-analysis that the
+nebula consists of gaseous matter. How widely extended this gaseous
+universe may be we cannot say. The general opinion is that the nebulæ
+are removed far beyond the fixed stars. If this were so, the dimensions
+of the Orion nebula would be indeed enormous, far larger probably than
+those of the whole system whereof our sun is a member. I believe this
+view is founded on insufficient evidence, but this would not be the
+place to discuss the subject. I shall merely point out that the nebula
+occurs in a region rich in stars, and if it is not, like the great
+nebula in Argo, clustered around a remarkable star, it is found
+associated in a manner which I cannot look upon as accidental with a set
+of small-magnitude stars, and notably with the trapezium which surrounds
+that very remarkable black gap within the nebula. The fact that the
+nebula shares the proper motion of the trapezium appears inexplicable if
+the nebula is really far out in space beyond the trapezium. A very small
+proper motion of the trapezium (alone) would long since have destroyed
+the remarkable agreement in the position of the dark gap and the
+trapezium which has been noticed for so many years.
+
+But whether belonging to our system or far beyond it, the great nebula
+must have enormous dimensions. A vast gaseous system it is, sustained by
+what arrangements or forces we cannot tell, nor can we know what
+purposes it subserves. Mr. Huggins' discovery that comets have gaseous
+nuclei, (so far as the two he has yet examined show) may suggest the
+speculation that in the Orion nebula we see a vast system of comets
+travelling in extensive orbits around nuclear stars, and so slowly as to
+exhibit for long intervals of time an unchanged figure. "But of such
+speculations" we may say with Sir J. Herschel "there is no end."
+
+To return to our telescopic observations:--The trapezium affords a
+useful test for the light-gathering power of the telescope. Large
+instruments exhibit nine stars. But our observer may be well satisfied
+with his instrument and his eye-sight if he can see five with a
+3-1/2-inch aperture.[3] A good 3-inch glass shows four distinctly. But
+with smaller apertures only three are visible.
+
+The whole neighbourhood of the great nebula will well repay research.
+The observer may sweep over it carefully on any dark night with profit.
+Above the nebula is the star-cluster 362 H. The star [iota] (double as
+shown in Plate 3) below the nebula is involved in a strong nebulosity.
+And in searching over this region we meet with delicate double, triple,
+and multiple stars, which make the survey interesting with almost any
+power that may be applied.
+
+Above the nebula is the star [sigma], a multiple star. To an observer
+with a good 3-1/2-inch glass [sigma] appears as an octuple star. It is
+well seen, however, as a fine multiple star with a smaller aperture.
+Some of the stars of this group appear to be variable.
+
+The star [rho] Orionis is an unequal, easy double, the components being
+separated by nearly seven seconds. The primary is orange, the smaller
+star smalt-blue (see Plate 3).
+
+The middle star of the belt ([epsilon]) has a distant blue companion.
+This star, like [iota], is nebulous. In fact, the whole region within
+the triangle formed by stars [gamma], [kappa] and [beta] is full of
+nebulous double and multiple stars, whose aggregation in this region I
+do not consider wholly accidental.
+
+We have not explored half the wealth of Orion, but leave much for future
+observation. We must turn, however, to other constellations.
+
+Below Orion is Lepus, the Hare, a small constellation containing some
+remarkable doubles. Among these we may note [xi], a white star with a
+scarlet companion; [gamma], a yellow and garnet double; and [iota], a
+double star, white and pale violet, with a distant red companion. The
+star [kappa] Leporis is a rather close double, white with a small green
+companion. The intensely red star R Leporis (a variable) will be found
+in the position indicated in the map.
+
+Still keeping within the boundary of our map, we may next turn to the
+fine cluster 2 H (vii.) in Monoceros. This cluster is visible to the
+naked eye, and will be easily found. The nebula 2 H (iv.) is a
+remarkable one with a powerful telescope.
+
+The star 11 Monocerotis is a fine triple star described by the elder
+Herschel as one of the finest sights in the heavens. Our observer,
+however, will see it as a double (see Plate 3). [delta] Monocerotis is
+an easy double, yellow and lavender.
+
+We may now leave the region covered by the map and take a survey of the
+heavens for some objects well seen at this season.
+
+Towards the south-east, high up above the horizon, we see the twin-stars
+Castor and Pollux. The upper is Castor, the finest double star visible
+in the northern heavens. The components are nearly equal and rather more
+than 5" apart (see Plate 3). Both are white according to the best
+observers, but the smaller is thought by some to be slightly greenish.
+
+Pollux is a coarse but fine triple star (in large instruments multiple).
+The components orange, grey, and lilac.
+
+There are many other fine objects in Gemini, but we pass to Cancer.
+
+The fine cluster Præsepe in Cancer may easily be found as it is
+distinctly visible to the naked eye in the position shown in Plate 1,
+Map I. In the telescope it is seen as shown in Plate 3.
+
+The star [iota] Cancri is a wide double, the colours orange and blue.
+
+Procyon, the first-magnitude star between Præsepe and Sirius, is finely
+coloured--yellow with a distant orange companion, which appears to be
+variable.
+
+Below the Twins, almost in a line with them, is the star [alpha] Hydræ,
+called Al Fard, or "the Solitary One." It is a 2nd magnitude variable. I
+mention it, however, not on its own account, but as a guide to the fine
+double [epsilon] Hydræ. This star is the middle one of a group of three,
+lying between Pollux and Al Fard rather nearer the latter. The
+components of [epsilon] Hydræ are separated by about 3-1/2" (see Plate
+3). The primary is of the fourth, the companion of the eighth magnitude;
+the former is yellow, the latter a ruddy purple. The period of [epsilon]
+Hydræ is about 450 years.
+
+The constellation Leo Minor, now due east and about midway between the
+horizon and the zenith, is well worth sweeping over. It contains several
+fine fields.
+
+Let us next turn to the western heavens. Here there are some noteworthy
+objects.
+
+To begin with, there are the Pleiades, showing to the naked eye only six
+or seven stars. In the telescope the Pleiades appear as shown in Plate
+3.
+
+The Hyades also show some fine fields with low powers.
+
+Aldebaran, the principal star of the Hyades, as also of the
+constellation Taurus, is a noted red star. It is chiefly remarkable for
+the close spectroscopic analysis to which it has been subjected by
+Messrs. Huggins and Miller. Unlike Betelgeuse, the spectrum of Aldebaran
+exhibits the lines corresponding to hydrogen, and no less than eight
+metals--sodium, magnesium, calcium, iron, bismuth, tellurium, antimony,
+and mercury, are proved to exist in the constitution of this brilliant
+red star.
+
+On the right of Aldebaran, in the position indicated in Plate 1, Map I.,
+are the stars [zeta] and [beta] Tauri. If with a low power the observer
+sweep from [zeta] towards [beta], he will soon find--not far from [zeta]
+(at a distance of about one-sixth of the distance separating [beta] from
+[zeta]), the celebrated Crab nebula, known as 1 M. This was the first
+nebula discovered by Messier, and its discovery led to the formation of
+his catalogue of 103 nebulæ. In a small telescope this object appears as
+a nebulous light of oval form, no traces being seen of the wisps and
+sprays of light presented in Lord Rosse's well known picture of the
+nebula.
+
+Here I shall conclude the labours of our first half-hour among the
+stars, noticing that the examination of Plate 1 will show what other
+constellations besides those here considered are well situated for
+observation at this season. It will be remarked that many constellations
+well seen in the third half-hour (Chapter IV.) are favourably seen in
+the first also, and _vice versâ_. For instance, the constellation Ursa
+Major well-placed towards the north-east in the first quarter of the
+year, is equally well-placed towards the north-west in the third, and
+similarly of the constellation Cassiopeia. The same relation connects
+the second and fourth quarters of the year.
+
+[Illustration: PLATE III.]
+
+
+
+
+CHAPTER III.
+
+A HALF-HOUR WITH LYRA, HERCULES, CORVUS, CRATER, ETC.
+
+
+The observations now to be commenced are supposed to take place during
+the second quarter of the year,--at ten o'clock on the 20th of April, or
+at nine on the 5th of May, or at eight on the 21st of May, or at seven
+on the 5th of June, or at hours intermediate to these on intermediate
+days.
+
+We again look first for the Great Bear, now near the zenith, and thence
+find the Pole-star. Turning towards the north, we see Cassiopeia between
+the Pole-star and the horizon. Towards the north-west is the brilliant
+Capella, and towards the north-east the equally brilliant Vega, beneath
+which, and somewhat northerly, is the cross in Cygnus. The Milky Way
+passes from the eastern horizon towards the north (low down), and so
+round to the western horizon.
+
+In selecting a region for special observation, we shall adopt a
+different plan from that used in the preceding "half-hour." The region
+on the equator and towards the south is indeed particularly interesting,
+since it includes the nebular region in Virgo. Within this space nebulæ
+are clustered more closely than over any corresponding space in the
+heavens, save only the greater Magellanic cloud. But to the observer
+with telescopes of moderate power these nebulæ present few features of
+special interest; and there are regions of the sky now well situated for
+observation, which, at most other epochs are either low down towards
+the horizon or inconveniently near to the zenith. We shall therefore
+select one of these, the region included in the second map of Plate 2,
+and the neighbouring part of the celestial sphere.
+
+At any of the hours above named, the constellation Hercules lies towards
+the east. A quadrant taken from the zenith to the eastern horizon passes
+close to the last star ([eta]) of the Great Bear's tail, through [beta],
+a star in Bootes' head, near [beta] Herculis, between the two "Alphas"
+which mark the heads of Hercules and Ophiuchus, and so past [beta]
+Ophiuchi, a third-magnitude star near the horizon. And here we may turn
+aside for a moment to notice the remarkable vertical row of six
+conspicuous stars towards the east-south-east; these are, counting them
+in order from the horizon, [zeta], [epsilon], and [delta] Ophiuchi,
+[epsilon], [alpha], and [delta] Serpentis.
+
+Let the telescope first be directed towards Vega. This orb presents a
+brilliant appearance in the telescope. Its colour is a bluish-white. In
+an ordinary telescope Vega appears as a single star, but with a large
+object-glass two distant small companions are seen. A nine-inch glass
+shows also two small companions within a few seconds of Vega. In the
+great Harvard refractor Vega is seen with no less than thirty-five
+companions. I imagine that all these stars, and others which can be seen
+in neighbouring fields, indicate the association of Vega with the
+neighbouring stream of the Milky Way.
+
+Let our observer now direct his telescope to the star [epsilon] Lyræ. Or
+rather, let him first closely examine this star with the naked eye. The
+star is easily identified, since it lies to the left of Vega, forming
+with [zeta] a small equilateral triangle. A careful scrutiny suffices to
+indicate a peculiarity in this star. If our observer possesses very
+good eye-sight, he will distinctly recognise it as a "naked-eye double";
+but more probably he will only notice that it appears lengthened in a
+north and south direction.[4] In the finder the star is easily divided.
+Applying a low power to the telescope itself, we see [epsilon] Lyræ as a
+wide double, the line joining the components lying nearly north and
+south. The southernmost component (the upper in the figure) is called
+[epsilon]^{1}, the other [epsilon]^{2}. Seen as a double, both
+components appear white.
+
+Now, if the observer's telescope is sufficiently powerful, each of the
+components may be seen to be itself double. First try [epsilon]^{1}, the
+northern pair. The line joining the components is directed as shown in
+Plate 3. The distance between them is 3"·2, their magnitudes 5 and
+6-1/2, and their colours yellow and ruddy. If the observer succeeds in
+seeing [epsilon]^{1} fairly divided, he will probably not fail in
+detecting the duplicity of [epsilon]^{2}, though this is a rather closer
+pair, the distance between the components being only 2"·6. The
+magnitudes are 5 and 5-1/2, both being white. Between [epsilon]^{1} and
+[epsilon]^{2} are three faint stars, possibly forming with the quadruple
+a single system.
+
+Let us next turn to the third star of the equilateral triangle mentioned
+above--viz. to the star [zeta] Lyræ. This is a splendid but easy double.
+It is figured in Plate 3, but it must be noticed that the figure of
+[zeta] and the other nine small figures are not drawn on the same scale
+as [epsilon] Lyræ. The actual distance between the components of [zeta]
+Lyra is 44", or about one-fourth of the distance separating
+[epsilon]^{1} from [epsilon]^{2}. The components of [zeta] are very
+nearly equal in magnitude, in colour topaz and green, the topaz
+component being estimated as of the fifth magnitude, the green component
+intermediate between the fifth and sixth magnitudes.
+
+We may now turn to a star not figured in the map, but readily found. It
+will be noticed that the stars [epsilon], [alpha], [beta], and [gamma]
+form, with two small stars towards the left, a somewhat regular
+hexagonal figure--a hexagon, in fact, having three equal long sides and
+three nearly equal short sides alternating with the others. The star
+[eta] Lyræ forms the angle next to [epsilon]. It is a wide and unequal
+double, as figured in Plate 3. The larger component is azure blue; the
+smaller is violet, and, being only of the ninth magnitude, is somewhat
+difficult to catch with apertures under 3 inches.
+
+The star [delta]^{2} Lyræ is orange, [delta]^{1} blue. In the same field
+with these are seen many other stars.
+
+The stars [gamma]^{1} and [gamma]^{2} may also be seen in a single
+field, the distance between them being about half the moon's mean
+diameter. The former is quadruple, the components being yellow, bluish,
+pale blue, and blue.
+
+Turn next to the stars [beta] and [gamma] Lyræ, the former a multiple,
+the latter an unequal double star. It is not, however, in these respects
+that these stars are chiefly interesting, but for their variability. The
+variability of [gamma] has not indeed been fully established, though it
+is certain that, having once been less bright, [gamma] is now
+considerably brighter than [beta]. The change, however, may be due to
+the variation of [beta] alone. This star is one of the most remarkable
+variables known. Its period is 12d. 21h. 53m. 10s. In this time it
+passes from a maximum brilliancy--that of a star of the 3·4
+magnitude--to a minimum lustre equal to that of a star of the 4·3
+magnitude, thence to the same maximum brilliancy as before, thence to
+another minimum of lustre--that of a star of the 4·5 magnitude--and so
+to its maximum lustre again, when the cycle of changes recommences.
+These remarkable changes seem to point to the existence of two unequal
+dark satellites, whose dimensions bear a much greater proportion to
+those of the bright components of [beta] Lyræ than the dimensions of the
+members of the Solar System bear to those of the sun. In this case, at
+any rate, the conjecture hazarded about Algol, that the star revolves
+around a dark central orb, would be insufficient to account for the
+observed variation.
+
+Nearly midway between [beta] and [gamma] lies the wonderful ring-nebula
+57 M, of which an imperfect idea will be conveyed by the last figure of
+Plate 3. This nebula was discovered in 1772, by Darquier, at Toulouse.
+It is seen as a ring of light with very moderate telescopic power. In a
+good 3-1/2-inch telescope the nebula exhibits a mottled appearance and a
+sparkling light. Larger instruments exhibit a faint light within the
+ring; and in Lord Rosse's great Telescope "wisps of stars" are seen
+within, and faint streaks of light stream from the outer border of the
+ring. This nebula has been subjected to spectrum-analysis by Mr.
+Huggins. It turns out to be a gaseous nebula! In fact, ring-nebulæ--of
+which only seven have been detected--seem to belong to the same class as
+the planetary nebulæ, all of which exhibit the line-spectrum indicative
+of gaseity. The brightest of the three lines seen in the spectrum of the
+ring-nebula in Lyra presents a rather peculiar appearance, "since it
+consists," says Mr. Huggins, "of two bright dots, corresponding to
+sections of the ring, and between these there is not darkness, but an
+excessively faint line joining them. This observation makes it probable
+that the faint nebulous matter occupying the central portion is similar
+in constitution to that of the ring."
+
+The constellation Hercules also contains many very interesting objects.
+Let us first inspect a nebula presenting a remarkable contrast with that
+just described. I refer to the nebula 13 M, known as Halley's nebula
+(Plate 3). This nebula is visible to the naked eye, and in a good
+telescope it is a most wonderful object: "perhaps no one ever saw it for
+the first time without uttering a shout of wonder." It requires a very
+powerful telescope completely to resolve this fine nebula, but the
+outlying streamers may be resolved with a good 3-inch telescope. Sir W.
+Herschel considered that the number of the stars composing this
+wonderful object was at least 14,000. The accepted views respecting
+nebulæ would place this and other clusters far beyond the limits of our
+sidereal system, and would make the component stars not very unequal (on
+the average) to our own sun. It seems to me far more probable, on the
+contrary, that the cluster belongs to our own system, and that its
+components are very much smaller than the average of separate stars.
+Perhaps the whole mass of the cluster does not exceed that of an average
+first-magnitude star.
+
+The nebulæ 92 M and 50 H may be found, after a little searching, from
+the positions indicated in the map. They are both well worthy of study,
+the former being a very bright globular cluster, the latter a bright and
+large round nebula. The spectra of these, as of the great cluster,
+resemble the solar spectrum, being continuous, though, of course, very
+much fainter.
+
+The star [delta] Herculis (seen at the bottom of the map) is a wide and
+easy double--a beautiful object. The components, situated as shown in
+Plate 3, are of the fourth and eighth magnitude, and coloured
+respectively greenish-white and grape-red.
+
+The star [kappa] Herculis is not shown in the map, but may be very
+readily found, lying between the two gammas, [gamma] Herculis and
+[gamma] Serpentis (_see_ Frontispiece, Map 2), rather nearer the latter.
+It is a wide double, the components of fifth and seventh magnitude, the
+larger yellowish-white, the smaller ruddy yellow.[5]
+
+Ras Algethi, or [alpha] Herculis, is also beyond the limits of the map,
+but may be easily found by means of Map 2, Frontispiece. It is, properly
+speaking, a multiple star. Considered as a double, the arrangement of
+the components is that shown in Plate 3. The larger is of magnitude
+3-1/2, the smaller of magnitude 5-1/2; the former orange, the latter
+emerald. The companion stars are small, and require a good telescope to
+be well seen. Ras Algethi is a variable, changing from magnitude 3 to
+magnitude 3-1/2 in a period of 66-1/3 days.
+
+The star [rho] Herculis is a closer double. The components are 3"·7
+apart, and situated as shown in Plate 3. The larger is of magnitude 4,
+the smaller 5-1/2; the former bluish-white, the latter pale emerald.
+
+There are other objects within the range of our map which are well
+worthy of study. Such are [mu] Draconis, a beautiful miniature of
+Castor; [gamma]^{1} and [gamma]^{2} Draconis, a wide double, the
+distance between the components being nearly 62" (both grey); and
+[gamma]^{1} and [gamma]^{2} Coronæ, a naked-eye double, the components
+being 6' apart, and each double with a good 3-inch telescope.
+
+We turn, however, to another region of the sky. Low down, towards the
+south is seen the small constellation Corvus, recognised by its
+irregular quadrilateral of stars. Of the two upper stars, the left-hand
+one is Algorab, a wide double, the components placed as in Plate 3,
+23"·5 apart, the larger of magnitude 3, the smaller 8-1/2, the colours
+pale yellow and purple.
+
+There is a red star in this neighbourhood which is well worth looking
+for. To the right of Corvus is the constellation Crater, easily
+recognised as forming a tolerably well-marked small group. The star
+Alkes, or [alpha] Crateris, must first be found. It is far from being
+the brightest star in the constellation, and may be assumed to have
+diminished considerably in brilliancy since it was entitled [alpha] by
+Bayer. It will easily be found, however, by means of the observer's star
+maps. If now the telescope be directed to Alkes, there will be found,
+following him at a distance of 42·5 s, and about one minute southerly, a
+small red star, R. Crateris. Like most red stars, this one is a
+variable. A somewhat smaller blue star may be seen in the same field.
+
+There is another red star which may be found pretty easily at this
+season. First find the stars [eta] and [omicron] Leonis, the former
+forming with Regulus (now lying towards the south-west, and almost
+exactly midway between the zenith and the horizon) the handle of the
+Sickle in Leo, the other farther off from Regulus towards the right, but
+lower down. Now sweep from [omicron] towards [eta] with a low power.[6]
+There will be found a sixth-magnitude star about one-fourth of the way
+from [omicron] to [eta]. South, following this, will be found a group of
+four stars, of which one is crimson. This is the star R Leonis. Like R
+Crateris and R Leporis it is variable.
+
+Next, let the observer turn towards the south again. Above Corvus, in
+the position shown in the Frontispiece, there are to be seen five stars,
+forming a sort of wide V with somewhat bowed legs. At the angle is the
+star [gamma] Virginis, a noted double. In 1756 the components were 6-1/2
+seconds apart. They gradually approached till, in 1836, they could not
+be separated by the largest telescopes. Since then they have been
+separating, and they are now 4-1/2 seconds apart, situated as shown in
+Plate 3. They are nearly equal in magnitude (4), and both pale yellow.
+
+The star [gamma] Leonis is a closer and more beautiful double. It will
+be found above Regulus, and is the brightest star on the blade of the
+Sickle. The components are separated by about 3-1/5 seconds, the larger
+of the second, the smaller of the fourth magnitude; the former
+yellow-orange, the latter greenish-yellow.
+
+Lastly, the star [iota] Leonis may be tried. It will be a pretty severe
+test for our observer's telescope, the components being only 2"·4 apart,
+and the smaller scarcely exceeding the eighth magnitude. The brighter
+(fourth magnitude) is pale yellow, the other light blue.
+
+
+
+
+CHAPTER IV.
+
+A HALF-HOUR WITH BOOTES, SCORPIO, OPHIUCHUS, ETC.
+
+
+We now commence a series of observations suited to the third quarter of
+the year, and to the following hours:--Ten o'clock on the 22nd of July;
+nine on the 8th of August; eight on the 23rd of August; seven on the 8th
+of October; and intermediate hours on days intermediate to these.
+
+We look first for the Great Bear towards the north-west, and thence find
+the Pole-star. Turning towards the north we see Capella and [beta]
+Aurigæ low down and slightly towards the left of the exact north point.
+The Milky Way crosses the horizon towards the north-north-east and
+passes to the opposite point of the compass, attaining its highest point
+above the horizon towards east-south-east. This part of the Milky Way is
+well worth observing, being marked by singular variations of brilliancy.
+Near Arided (the principal star of Cygnus, and now lying due east--some
+twenty-five degrees from the zenith) there is seen a straight dark rift,
+and near this space is another larger cavity, which has been termed the
+northern Coal-sack. The space between [gamma], [delta], and [beta] Cygni
+is covered by a large oval mass, exceedingly rich and brilliant. The
+neighbouring branch, extending from [epsilon] Cygni, is far less
+conspicuous here, but near Sagitta becomes brighter than the other,
+which in this neighbourhood suddenly loses its brilliancy and fading
+gradually beyond this point becomes invisible near [beta] Ophiuchi.
+The continuous stream becomes patchy--in parts very brilliant--where it
+crosses Aquila and Clypeus. In this neighbourhood the other stream
+reappears, passing over a region very rich in stars. We see now the
+greatest extent of the Milky Way, towards this part of its length, ever
+visible in our latitudes--just as in spring we see its greatest extent
+towards Monoceros and Argo.
+
+[Illustration: PLATE IV.]
+
+I may note here in passing that Sir John Herschel's delineation of the
+northern portion of the Milky Way, though a great improvement on the
+views given in former works, seems to require revision, and especially
+as respects the very remarkable patches and streaks which characterise
+the portion extending over Cepheus and Cygnus. It seems to me, also,
+that the evidence on which it has been urged that the stars composing
+the Milky Way are (on an average) comparable in magnitude to our own
+sun, or to stars of the leading magnitudes, is imperfect. I believe, for
+instance, that the brilliant oval of milky light in Cygnus comes from
+stars intimately associated with the leading stars in that
+constellation, and not far removed in space (proportionately) beyond
+them. Of course, if this be the case, the stars, whose combined light
+forms the patch of milky light, must be far smaller than the leading
+brilliants of Cygnus. However, this is not the place to enter on
+speculations of this sort; I return therefore to the business we have
+more immediately in hand.
+
+Towards the east is the square of Pegasus low down towards the horizon.
+Towards the south is Scorpio, distinguished by the red and brilliant
+Antares, and by a train of conspicuous stars. Towards the west is
+Bootes, his leading brilliant--the ruddy Arcturus--lying somewhat nearer
+the horizon than the zenith, and slightly south of west. Bootes as a
+constellation is easily found if we remember that he is delineated as
+chasing away the Greater Bear. Thus at present he is seen in a slightly
+inclined position, his head (marked by the third-magnitude star [beta])
+lying due west, some thirty degrees from the zenith. It has always
+appeared to me, by the way, that Bootes originally had nobler
+proportions than astronomers now assign to him. It is known that Canes
+Venatici now occupy the place of an upraised arm of Bootes, and I
+imagine that Corona Borealis, though undoubtedly a very ancient
+constellation, occupies the place of his other arm. Giving to the
+constellation the extent thus implied, it exhibits (better than most
+constellations) the character assigned to it. One can readily picture to
+oneself the figure of a Herdsman with upraised arms driving Ursa Major
+before him. This view is confirmed, I think, by the fact that the Arabs
+called this constellation the Vociferator.
+
+Bootes contains many beautiful objects. Partly on this account, and
+partly because this is a constellation with which the observer should be
+specially familiar, a map of it is given in Plate 4.
+
+Arcturus has a distant pale lilac companion, and is in other respects a
+remarkable and interesting object. It is of a ruddy yellow colour.
+Schmidt, indeed, considers that the star has changed colour of late
+years, and that whereas it was once very red it is now a yellow star.
+This opinion does not seem well grounded, however. The star _may_ have
+been more ruddy once than now, though no other observer has noticed such
+a peculiarity; but it is certainly not a pure yellow star at present (at
+any rate as seen in our latitude). Owing probably to the difference of
+colour between Vega, Capella and Arcturus, photometricians have not been
+perfectly agreed as to the relative brilliancy of these objects. Some
+consider Vega the most brilliant star in the northern heavens, while
+others assign the superiority to Capella. The majority, however,
+consider Arcturus the leading northern brilliant, and in the whole
+heavens place three only before him, viz., Sirius, Canopus, and [alpha]
+Centauri. Arcturus is remarkable in other respects. His proper motion is
+very considerable, so great in fact that since the time of Ptolemy the
+southerly motion (alone) of Arcturus has carried him over a space nearly
+half as great again as the moon's apparent diameter. One might expect
+that so brilliant a star, apparently travelling at a rate so great
+compared with the average proper motions of the stars, must be
+comparatively near to us. This, however, has not been found to be the
+case. Arcturus is, indeed, one of the stars whose distance it has been
+found possible to estimate roughly. But he is found to be some three
+times as far from us as the small star 61 Cygni, and more than seven
+times as far from us as [alpha] Centauri.
+
+The star [delta] Bootis is a wide and unequal double, the smaller
+component being only of the ninth magnitude.
+
+Above Alkaid the last star in the tail of the Greater Bear, there will
+be noticed three small stars. These are [theta], [iota], and [kappa]
+Bootis, and are usually placed in star-maps near the upraised hand of
+the Herdsman. The two which lie next to Alkaid, [iota] and [kappa], are
+interesting doubles. The former is a wide double (see Plate 5), the
+magnitudes of components 4 and 8, their colours yellow and white. The
+larger star of this pair is itself double. The star [kappa] Bootis is
+not so wide a double (see Plate 5), the magnitudes of the components 5
+and 8, their colours white and faint blue--a beautiful object.
+
+The star [xi] Bootis is an exceedingly interesting object. It is
+double, the colours of the components being orange-yellow and ruddy
+purple, their magnitudes 3-1/2 and 6-1/2. When this star was first
+observed by Herschel in 1780 the position of the components was quite
+different from that presented in Plate 5. They were also much closer,
+being separated by a distance of less than 3-1/2 seconds. Since that
+time the smaller component has traversed nearly a full quadrant, its
+distance from its primary first increasing, till in 1831 the stars were
+nearly 7-1/2 seconds apart, and thence slowly diminishing, so that at
+present the stars are less than 5 seconds apart. The period usually
+assigned to the revolution of this binary system is 117 years, and the
+period of peri-astral passage is said to be 1779. It appears to me,
+however, that the period should be about 108 years, the epoch of last
+peri-astral passage 1777 and of next peri-astral passage, therefore,
+1885. The angular motion of the secondary round the primary is now
+rapidly increasing, and the distance between the components is rapidly
+diminishing, so that in a few years a powerful telescope will be
+required to separate the pair.
+
+Not far from [xi] is [pi] Bootis, represented in Plate 5 as a somewhat
+closer double, but in reality--now at any rate--a slightly wider pair,
+since the distance between the components of [xi] has greatly diminished
+of late. Both the components of [pi] are white, and their magnitudes are
+3-1/2 and 6.
+
+We shall next turn to an exceedingly beautiful and delicate object, the
+double star [epsilon] Bootis, known also as Mirac and, on account of its
+extreme beauty, called Pulcherrima by Admiral Smyth. The components of
+this beautiful double are of the third and seventh magnitude, the
+primary orange, the secondary sea-green. The distance separating the
+components is about 3 seconds, perhaps more; it appears to have been
+slowly increasing during the past ten or twelve years. Smyth assigns to
+this system a period of revolution of 980 years, but there can be little
+doubt that the true period is largely in excess of this estimate.
+Observers in southern latitudes consider that the colours of the
+components are yellow and blue, not orange and green as most of our
+northern observers have estimated them.
+
+A little beyond the lower left-hand corner of the map is the star
+[delta] Serpentis, in the position shown in the Frontispiece, Map 3.
+This is the star which at the hour and season depicted in Map 2 formed
+the uppermost of a vertical row of stars, which has now assumed an
+almost horizontal position. The components of [delta] Serpentis are
+about 3-1/2 seconds apart, their magnitudes 3 and 5, both white.
+
+The stars [theta]^{1} and [theta]^{2} Serpentis form a wide double, the
+distance between the components being 21-1/2 seconds. They are nearly
+equal in magnitude, the primary being 4-1/2, the secondary 5. Both are
+yellow, the primary being of a paler yellow colour than the smaller
+star. But the observer may not know where to look for [theta] Serpentis,
+since it falls in a part of the constellation quite separated from that
+part in which [delta] Serpentis lies. In fact [theta] lies on the
+extreme easterly verge of the eastern half of the constellation. It is
+to be looked for at about the same elevation as the brilliant Altair,
+and (as to azimuth) about midway between Altair and the south.
+
+The stars [alpha]^{1} and [alpha]^{2} Libræ form a wide double, perhaps
+just separable by the naked eye in very favourable weather. The larger
+component is of the third, the smaller of the sixth magnitude, the
+former yellow the latter light grey.
+
+The star [beta] Libræ is a beautiful light-green star to the naked eye;
+in the telescope a wide double, pale emerald and light blue.
+
+In Scorpio there are several very beautiful objects:--
+
+The star Antares or Cor Scorpionis is one of the most beautiful of the
+red stars. It has been termed the Sirius of red stars, a term better
+merited perhaps by Aldebaran, save for this that, in our latitude,
+Antares is, like Sirius, always seen as a brilliant "scintillator"
+(because always low down), whereas Aldebaran rises high above the
+horizon. Antares is a double star, its companion being a minute green
+star. In southern latitudes the companion of Antares may be seen with a
+good 4-inch, but in our latitudes a larger opening is wanted. Mr. Dawes
+once saw the companion of Antares shining alone for seven seconds, the
+primary being hidden by the moon. He found that the colour of the
+secondary is not merely the effect of contrast, but that this small star
+is really a green sun.
+
+The star [beta] Scorpionis is a fine double, the components 13"·1 apart,
+their magnitudes 2 and 5-1/2, colours white and lilac. It has been
+supposed that this pair is only an optical double, but a long time must
+elapse before a decisive opinion can be pronounced on such a point.
+
+The star [sigma] Scorpionis is a wider but much more difficult double,
+the smaller component being below the 9th magnitude. The colour of the
+primary (4) is white, that of the secondary maroon.
+
+The star [xi] Scorpionis is a neat double, the components 7"·2 apart,
+their magnitudes 4-1/2 and 7-1/2, their colours white and grey. This
+star is really triple, a fifth-magnitude star lying close to the
+primary.
+
+In Ophiuchus, a constellation covering a wide space immediately above
+Scorpio, there are several fine doubles. Among others--
+
+39 Ophiuchi, distance between components 12"·1, their magnitudes 5-1/2
+and 7-1/2, their colours orange and blue.
+
+The star 70 Ophiuchi, a fourth-magnitude star on the right shoulder of
+Ophiuchus, is a noted double. The distance between the components about
+5-1/2", their magnitudes 4-1/2 and 7, the colours yellow and red. The
+pair form a system whose period of revolution is about 95 years.
+
+36 Ophiuchi (variable), distance 5"·2, magnitudes 4-1/2 and 6-1/2,
+colours red and yellow.
+
+[rho] Opiuchi, distance 4", colours yellow and blue, magnitudes 5 and 7.
+
+Between [alpha] and [beta] Scorpionis the fine nebula 80 M may be looked
+for. (Or more closely thus:--below [beta] is the wide Double [omega]^{1}
+and [omega]^{2} Scorpionis; about as far to the right of Antares is the
+star [sigma] Scorpionis, and immediately above this is the
+fifth-magnitude star 19.) The nebula we seek lies between 19 and
+[omega], nearer to 19 (about two-fifths of the way towards [omega]).
+This nebula is described by Sir W. Herschel as "the richest and most
+condensed mass of stars which the firmament offers to the contemplation
+of astronomers."
+
+There are two other objects conveniently situated for observation, which
+the observer may now turn to. The first is the great cluster in the
+sword-hand of Perseus (see Plate 4), now lying about 28° above the
+horizon between N.E. and N.N.E. The stars [gamma] and [delta] Cassiopeiæ
+(see Map 3 of Frontispiece) point towards this cluster, which is rather
+farther from [delta] than [delta] from [gamma], and a little south of
+the produced line from these stars. The cluster is well seen with the
+naked eye, even in nearly full moonlight. In a telescope of moderate
+power this cluster is a magnificent object, and no telescope has yet
+revealed its full glory. The view in Plate 5 gives but the faintest
+conception of the glories of [chi] Persei. Sir W. Herschel tried in
+vain to gauge the depths of this cluster with his most powerful
+telescope. He spoke of the most distant parts as sending light to us
+which must have started 4000 or 5000 years ago. But it appears
+improbable that the cluster has in reality so enormous a longitudinal
+extension compared with its transverse section as this view would imply.
+On the contrary, I think we may gather from the appearance of this
+cluster, that stars are far less uniform in size than has been commonly
+supposed, and that the mere irresolvability of a cluster is no proof of
+excessive distance. It is unlikely that the faintest component of the
+cluster is farther off than the brightest (a seventh-magnitude star) in
+the proportion of more than about 20 to 19, while the ordinary estimate
+of star magnitudes, applied by Herschel, gave a proportion of 20 or 30
+to 1 at least. I can no more believe that the components of this cluster
+are stars greatly varying in distance, but accidentally seen in nearly
+the same direction, (or that they form an _enormously long system_
+turned by accident directly towards the earth), than I could look on the
+association of several thousand persons in the form of a procession as a
+fortuitous arrangement.
+
+Next there is the great nebula in Andromeda--known as "the
+transcendantly beautiful queen of the nebulæ." It will not be difficult
+to find this object. The stars [epsilon] and [delta] Cassiopeiæ (Map 3,
+Frontispiece) point to the star [beta] Andromedæ. Almost in a vertical
+line above this star are two fourth-magnitude stars [mu] and [gamma],
+and close above [nu], a little to the right, is the object we
+seek--visible to the naked eye as a faint misty spot. To tell the truth,
+the transcendantly beautiful queen of the nebulæ is rather a
+disappointing object in an ordinary telescope. There is seen a long
+oval or lenticular spot of light, very bright near the centre,
+especially with low powers. But there is a want of the interest
+attaching to the strange figure of the Great Orion nebula. The Andromeda
+nebula has been partially resolved by Lord Rosse's great reflector, and
+(it is said) more satisfactorily by the great refractor of Harvard
+College. In the spectroscope, Mr. Huggins informs us, the spectrum is
+peculiar. Continuous from the blue to the orange, the light there
+"appears to cease very abruptly;" there is no indication of gaseity.
+
+Lastly, the observer may turn to the pair Mizar and Alcor, the former
+the middle star in the Great Bear's tail, the latter 15' off. It seems
+quite clear, by the way, that Alcor has increased in brilliancy of late,
+since among the Arabians it was considered an evidence of very good
+eyesight to detect Alcor, whereas this star may now be easily seen even
+in nearly full moonlight. Mizar is a double star, and a fourth star is
+seen in the same field of view with the others (see Plate 5). The
+distance between Mizar and its companion is 14"·4; the magnitude of
+Mizar 3, of the companion 5; their colours white and pale green,
+respectively.
+
+
+
+
+CHAPTER V.
+
+A HALF-HOUR WITH ANDROMEDA, CYGNUS, ETC.
+
+
+Our last half-hour with the double stars, &c., must be a short one, as
+we have already nearly filled the space allotted to these objects. The
+observations now to be made are supposed to take place during the fourth
+quarter of the year,--at ten o'clock on October 23rd; or at nine on
+November 7th; or at eight on November 22nd; or at seven on December 6th;
+or at hours intermediate to these on intermediate days.
+
+We look first, as in former cases, for the Great Bear, now lying low
+down towards the north. Towards the north-east, a few degrees easterly,
+are the twin-stars Castor and Pollux, in a vertical position, Castor
+uppermost. Above these, a little towards the right, we see the brilliant
+Capella; and between Capella and the zenith is seen the festoon of
+Perseus. Cassiopeia lies near the zenith, towards the north, and the
+Milky Way extends from the eastern horizon across the zenith to the
+western horizon. Low down in the east is Orion, half risen above
+horizon. Turning to the south, we see high up above the horizon the
+square of Pegasus. Low down towards the south-south-west is Fomalhaut,
+pointed to by [beta] and [alpha] Pegasi. Towards the west, about
+half-way between the zenith and the horizon, is the noble cross in
+Cygnus; below which, towards the left, we see Altair, and his companions
+[beta] and [gamma] Aquilæ: while towards the right we see the brilliant
+Vega.
+
+During this half-hour we shall not confine ourselves to any particular
+region of the heavens, but sweep the most conveniently situated
+constellations.
+
+[Illustration: PLATE V.]
+
+First, however, we should recommend the observer to try and get a good
+view of the great nebula in Andromeda, which is _not_ conveniently
+situated for observation, but is so high that after a little trouble the
+observer may expect a more distinct view than in the previous quarter.
+He will see [beta] Andromedæ towards the south-east, about 18° from the
+zenith, [mu] and [nu] nearly in a line towards the zenith, and the
+nebula about half-way between [beta] and the zenith.
+
+With a similar object it will be well to take another view of the great
+cluster in Perseus, about 18° from the zenith towards the
+east-north-east (_see_ the pointers [gamma] and [delta] Cassiopeiæ in
+Map 4, Frontispiece), the cluster being between [delta] Cassiopeiæ and
+[alpha] Persei.
+
+Not very far off is the wonderful variable Algol, now due east, and
+about 58° above the horizon. The variability of this celebrated object
+was doubtless discovered in very ancient times, since the name Al-gol,
+or "the Demon" seems to point to a knowledge of the peculiarity of this
+"slowly winking eye." To Goodricke, however, is due the rediscovery of
+Algol's variability. The period of variation is 2d. 20h. 48m.; during
+2h. 14m. Algol appears of the second magnitude; the remaining 6-3/4
+hours are occupied by the gradual decline of the star to the fourth
+magnitude, and its equally gradual return to the second. It will be
+found easy to watch the variations of this singular object, though, of
+course, many of the minima are attained in the daytime. The following
+may help the observer:--
+
+On October 8th, 1867, at about half-past eleven in the evening, I
+noticed that Algol had reached its minimum of brilliancy. Hence the next
+minimum was attained at about a quarter-past eight on the evening of
+October 11th; the next at about five on the evening of October 14th,
+and so on. Now, if this process be carried on, it will be found that the
+next evening minimum occurred at about 10h. (_circiter_) on the evening
+of October 31st, the next at about 11h. 30m. on the evening of November
+20th. Thus at whatever hour any minimum occurs, another occurs _six
+weeks and a day later_, at about the same hour. This would be exact
+enough if the period of variation were _exactly_ 2d. 20m. 48s., but the
+period is nearly a minute greater, and as there are fifteen periods in
+six weeks and a day, it results that there is a difference of about 13m.
+in the time at which the successive recurrences at nearly the same hour
+take place. Hence we are able to draw up the two following Tables, which
+will suffice to give all the minima conveniently observable during the
+next two years. Starting from a minimum at about 11h. 45m. on November
+20th, 1867, and noticing that the next 43-day period (with the 13m.
+added) gives us an observation at midnight on January 2nd, 1868, and
+that successive periods would make the hour later yet, we take the
+minimum next after that of January 2nd, viz. that of January 5th, 1868,
+8h. 48m., and taking 43-day periods (with 13m. added to each), we get
+the series--
+
+                 h.  m.
+Jan.   5, 1868,   8  45 P.M.
+Feb.  17, ----,   8  58 ----
+Mar.  31, ----,   9  11 ----
+May   13, ----,   9  24 ----
+June  25, ----,   9  37 ----
+Aug.   7, ----,   9  50 ----
+Sept. 19, ----,  10   3 ----
+Nov.   1  ----,  10  16 ----
+Dec.  14, ----,  10  29 ----
+Jan.  26, 1869,  10  42 ----
+Mar.  10, ----,  10  25 ----
+Mar.  13, ----,   7  43 ----[7]
+Apr.  25, ----,   7  56 ----
+June   7, ----,   8   9 ----
+July  20, ----,   8  22 ----
+Sept.  1, ----,   8  35 ----
+Oct.  14, ----,   8  48 ----
+Nov.  26, ----,   9   1 ----
+Jan.   8, 1870,   9  14 ----
+Feb.  20, ----,   9  27 ----
+
+From the minimum at about 10 P.M. on October 31st, 1867, we get in like
+manner the series--
+
+                 h.  m.
+Dec.  13, 1867,  10  13 P.M.
+Jan.  25, 1868,  10  26 ----
+Mar.   8, ----,  10  39 ----
+Apr.  20, ----,  10  52 ----
+June   2, ----,  11   5 ----
+June   5, ----,   7  53 ----[8]
+July  18, ----,   8   6 ----
+Aug.  30, ----,   8  19 ----
+Oct.  12, ----,   8  32 ----
+Nov.  24, ----,   8  45 ----
+Jan.   6, 1869,   8  58 ----
+Feb.  18, ----,   9  11 ----
+Apr.   2, ----,   9  24 ----
+May   15, ----,   9  37 ----
+June  27, ----,   9  50 ----
+Aug.   9, ----,  10   3 ----
+Sept. 21, ----,  10  16 ----
+Nov.   3, ----,  10  29 ----
+Dec.  16, ----,  10  42 ----
+Jan.  28, 1870,  10  55 ----
+
+From one or other of these tables every observable minimum can be
+obtained. Thus, suppose the observer wants to look for a minimum during
+the last fortnight in August, 1868. The first table gives him no
+information, the latter gives him a minimum at 8h. 19m. P.M. on August
+30; hence of course there is a minimum at 11h. 31m. P.M. on August 27;
+and there are no other conveniently observable minima during the
+fortnight in question.
+
+The cause of the remarkable variation in this star's brilliancy has been
+assigned by some astronomers to the presence of an opaque secondary,
+which transits Algol at regular intervals; others have adopted the view
+that Algol is a luminous secondary, revolving around an opaque primary.
+Of these views the former seems the most natural and satisfactory. It
+points to a secondary whose mass bears a far greater proportion to that
+of the primary, than the mass even of Jupiter bears to the sun; the
+shortness of the period is also remarkable. It may be noticed that
+observation points to a gradual diminution in the period of Algol's
+variation, and the diminution seems to be proceeding more and more
+rapidly. Hence (assuming the existence of a dark secondary) we must
+suppose that either it travels in a resisting medium which is gradually
+destroying its motion, or that there are other dependent orbs whose
+attractions affect the period of this secondary. In the latter case the
+decrease in the period will attain a limit and be followed by an
+increase.
+
+However, interesting as the subject may be, it is a digression from
+telescopic work, to which we now return.
+
+Within the confines of the second map in Plate 4 is seen the fine star
+[gamma] Andromedæ. At the hour of our observations it lies high up
+towards E.S.E. It is seen as a double star with very moderate telescopic
+power, the distance between the components being upwards of 10"; their
+magnitudes 3 and 5-1/2, their colours orange and green. Perhaps there is
+no more interesting double visible with low powers. The smaller star is
+again double in first-class telescopes, the components being yellow and
+blue according to some observers, but according to others, both green.
+
+Below [gamma] Andromedæ lie the stars [beta] and [gamma] Triangulorum,
+[gamma] a fine naked-eye triple (the companions being [delta] and [eta]
+Triangulorum), a fine object with a very low power. To the right is
+[alpha] Triangulorum, certainly less brilliant than [beta]. Below
+[alpha] are the three stars [alpha], [beta], and [gamma] Arietis, the
+first an unequal and difficult double, the companion being purple, and
+only just visible (under favourable circumstances) with a good 3-inch
+telescope; the last an easy double, interesting as being the first ever
+discovered (by Hook, in 1664), the colours of components white and grey.
+
+Immediately below [alpha] Arietis is the star [alpha] Ceti, towards the
+right of which (a little lower) is Mira, a wonderful variable. This star
+has a period of 331-1/3 days; during a fortnight it appears as a star of
+the 2nd magnitude,--on each side of this fortnight there is a period of
+three months during one of which the star is increasing, while during
+the other it is diminishing in brightness: during the remaining five
+months of the period the star is invisible to the naked eye. There are
+many peculiarities and changes in the variation of this star, into which
+space will not permit me to enter.
+
+Immediately above Mira is the star [alpha] Piscium at the knot of the
+Fishes' connecting band. This is a fine double, the distance between the
+components being about 3-1/2", their magnitudes 5 and 6, their colours
+pale green and blue (see Plate 5).
+
+Close to [gamma] Aquarii (see Frontispiece, Map 4), above and to the
+left of it, is the interesting double [zeta] Aquarii; the distance
+between the components is about 3-1/2", their magnitudes 4 and 4-1/2,
+both whitish yellow. The period of this binary seems to be about 750
+years.
+
+Turning next towards the south-west we see the second-magnitude star
+[epsilon] Pegasi, some 40° above the horizon. This star is a wide but
+not easy double, the secondary being only of the ninth magnitude; its
+colour is lilac, that of the primary being yellow.
+
+Towards the right of [epsilon] Pegasi and lower down are seen the three
+fourth-magnitude stars which mark the constellation Equuleus. Of these
+the lowest is [alpha], to the right of which lies [epsilon] Equulei, a
+fifth-magnitude star, really triple, but seen as a double star with
+ordinary telescopes (Plate 5). The distance between the components is
+nearly 11", their colours white and blue, their magnitudes 5-1/2 and
+7-1/2. The primary is a very close double, which appears, however, to be
+opening out rather rapidly.
+
+Immediately below Equuleus are the stars [alpha]^{1} and [alpha]^2
+Capricorni, seen as a naked-eye double to the right of and above [beta].
+Both [alpha]^1 and [alpha]^2 are yellow; [alpha]^2 is of the 3rd,
+[alpha]^1 of the 4th magnitude; in a good telescope five stars are seen,
+the other three being blue, ash-coloured, and lilac. The star [beta]
+Capricorni is also a wide double, the components yellow and blue, with
+many telescopic companions.
+
+To the right of Equuleus, towards the west-south-west is the
+constellation Delphinus. The upper left-hand star of the rhombus of
+stars forming the head of the Delphinus is the star [gamma] Delphini, a
+rather easy double (see Plate 5), the components being nearly 12" apart,
+their magnitudes 4 and 7, their colours golden yellow and flushed grey.
+
+Turn we next to the charming double Albireo, on the beak of Cygnus,
+about 36° above the horizon towards the west. The components are 34-1/2"
+apart, their magnitudes 3 and 6, their colours orange-yellow, and blue.
+It has been supposed (perhaps on insufficient evidence) that this star
+is merely an optical double. It must always be remembered that a certain
+proportion of stars (amongst those separated by so considerable a
+distance) _must_ be optically combined only.
+
+The star [chi] Cygni is a wide double (variable) star. The components
+are separated by nearly 26", their magnitudes 5 and 9, their colours
+yellow and light blue. [chi] may be found by noticing that there is a
+cluster of small stars in the middle of the triangle formed by the stars
+[gamma], [delta], and [beta] Cygni (see Map 4, Frontispiece), and that
+[chi] is the nearest star _of the cluster_ to [beta]. The star [phi]
+Cygni, which is just above and very close to [beta] (Albireo), does not
+belong to the cluster. [chi] is about half as far again from [phi] as
+[phi] from Albireo. But as [chi] descends to the 11th magnitude at its
+minimum the observer must not always expect to find it very easily. It
+has been known to be invisible at the epoch when it should have been
+most conspicuous. The period of this variable is 406 days.
+
+The star 61 Cygni is an interesting one. So far as observation has yet
+extended, it would seem to be the nearest to us of all stars visible in
+the northern hemisphere. It is a fine double, the components nearly
+equal (5-1/2 and 6), both yellow, and nearly 19" apart. The period of
+this binary appears to be about 540 years. To find 61 Cygni note that
+[epsilon] and [delta] Cygni form the diameter of a semicircle divided
+into two quadrants by [alpha] Cygni (Arided). On this semicircle, on
+either side of [alpha], lie the stars [nu] and [alpha] Cygni, [nu]
+towards [epsilon]. Now a line from [alpha] to [nu] produced passes very
+near to 61 Cygni at a distance from [nu] somewhat greater than half the
+distance of [nu] from [alpha].
+
+The star [mu] Cygni lies in a corner of the constellation, rather
+farther from [zeta] than [zeta] from [epsilon] Cygni. A line from
+[epsilon] to [zeta] produced meets [kappa] Pegasi, a fourth-magnitude
+star; and [mu] Cygni, a fifth-magnitude star, lies close above [kappa]
+Pegasi. The distance between the components is about 5-1/2", their
+magnitudes 5 and 6, their colours white and pale blue.
+
+The star [psi] Cygni may next be looked for, but for this a good map of
+Cygnus will be wanted, as [psi] is not pointed to by any well-marked
+stars. A line from [alpha], parallel to the line joining [gamma] and
+[delta], and about one-third longer than that line, would about mark the
+position of [psi] Cygni. The distance between the components of this
+double is about 3-1/2", their magnitudes 5-1/2 and 8, their colours
+white and lilac.
+
+Lastly, the observer may turn to the stars [gamma]_{1} and [gamma]_{2}
+Draconis towards the north-west about 40° above the horizon (they are
+included in the second map of Plate 2). They form a wide double, having
+equal (fifth-magnitude) components, both grey. (See Plate 5.)
+
+
+
+
+CHAPTER VI.
+
+HALF-HOURS WITH THE PLANETS.
+
+
+In observing the stars, we can select a part of the heavens which may be
+conveniently observed; and in this way in the course of a year we can
+observe every part of the heavens visible in our northern hemisphere.
+But with the planets the case is not quite so simple. They come into
+view at no fixed season of the year: some of them can never be seen _by
+night_ on the meridian; and they all shift their place among the stars,
+so that we require some method of determining where to look for them on
+any particular night, and of recognising them from neighbouring fixed
+stars.
+
+The regular observer will of course make use of the 'Nautical Almanac';
+but 'Dietrichsen and Hannay's Almanac' will serve every purpose of the
+amateur telescopist. I will briefly describe those parts of the almanac
+which are useful to the observer.
+
+It will be found that three pages are assigned to each month, each page
+giving different information. If we call these pages I. II. III., then
+in order that page I. for each month may fall to the left of the open
+double page, and also that I. and II. may be open together, the pages
+are arranged in the following order: I. II. III.; III. I. II.; I. II.
+III.; and so on.
+
+Now page III. for any month does not concern the amateur observer. It
+gives information concerning the moon's motions, which is valuable to
+the sailor, and interesting to the student of astronomy, but not
+applicable to amateur observation.
+
+[Illustration: PLATE VI.]
+
+We have then only pages I. and II. to consider:--
+
+Across the top of both pages the right ascension and declination of the
+planets Venus, Jupiter, Mars, Saturn, Mercury, and Uranus are given,
+accompanied by those of two conspicuous stars. This information is very
+valuable to the telescopist. In the first place, as we shall presently
+see, it shows him what planets are well situated for observation, and
+secondly it enables him to map down the path of any planet from day to
+day among the fixed stars. This is a very useful exercise, by the way,
+and also a very instructive one. The student may either make use of the
+regular maps and mark down the planet's path in pencil, taking a light
+curve through the points given by the data in his almanac, or he may lay
+down a set of meridians suited to the part of the heavens traversed by
+the planet, and then proceed to mark in the planet's path and the stars,
+taking the latter either from his maps or from a convenient list of
+stars.[9] My 'Handbook of the Stars' has been constructed to aid the
+student in these processes. It must be noticed that old maps are not
+suited for the work, because, through precession, the stars are all out
+of place as respects R.A. and Dec. Even the Society's maps, constructed
+so as to be right for 1830, are beginning to be out of date. But a
+matter of 20 or 30 years either way is not important.[10] My Maps,
+Handbook and Zodiac-chart have been constructed for the year 1880, so as
+to be serviceable for the next fifty years or so.
+
+Next, below the table of the planets, we have a set of vertical
+columns. These are, in order, the days of the month, the calendar--in
+which are included some astronomical notices, amongst others the
+diameter of Saturn on different dates, the hours at which the sun rises
+and sets, the sun's right ascension, declination, diameter, and
+longitude; then eight columns which do not concern the observer; after
+which come the hours at which the moon rises and sets, the moon's age;
+and lastly (so far as the observer is concerned) an important column
+about Jupiter's system of satellites.
+
+Next, we have, at the foot of the first page, the hours at which the
+planets rise, south, and set; and at the foot of the second page we have
+the dates of conjunctions, oppositions, and of other phenomena, the
+diameters of Venus, Jupiter, Mars, and Mercury, and finally a few words
+respecting the visibility of these four planets.
+
+After the thirty-six pages assigned to the months follow four (pp.
+42-46) in which much important astronomical information is contained;
+but the points which most concern our observer are (i.) a small table
+showing the appearance of Saturn's rings, and (ii.) a table giving the
+hours at which Jupiter's satellites are occulted or eclipsed, re-appear,
+&c.
+
+We will now take the planets in the order of their distance from the
+sun: we shall see that the information given by the almanac is very
+important to the observer.
+
+Mercury is so close to the sun as to be rarely seen with the naked eye,
+since he never sets much more than two hours and a few minutes after the
+sun, or rises by more than that interval before the sun. It must not be
+supposed that at each successive epoch of most favourable appearance
+Mercury sets so long after the sun or rises so long before him. It would
+occupy too much of our space to enter into the circumstances which
+affect the length of these intervals. The question, in fact, is not a
+very simple one. All the necessary information is given in the almanac.
+We merely notice that the planet is most favourably seen as an evening
+star in spring, and as a morning star in autumn.[11]
+
+The observer with an equatorial has of course no difficulty in finding
+Mercury, since he can at once direct his telescope to the proper point
+of the heavens. But the observer with an alt-azimuth might fail for
+years together in obtaining a sight of this interesting planet, if he
+trusted to unaided naked-eye observations in looking for him. Copernicus
+never saw Mercury, though he often looked for him; and Mr. Hind tells me
+he has seen the planet but once with the naked eye--though this perhaps
+is not a very remarkable circumstance, since the systematic worker in an
+observatory seldom has occasion to observe objects with the unaided eye.
+
+By the following method the observer can easily pick up the planet.
+
+Across two uprights (Fig. 10) nail a straight rod, so that when looked
+at from some fixed point of view the rod may correspond to the sun's
+path near the time of observation. The rod should be at right-angles to
+the line of sight to its centre. Fasten another rod at right angles to
+the first. From the point at which the rods cross measure off and mark
+on both rods spaces each subtending a degree as seen from the point of
+view. Thus, if the point of view is 9-1/2 feet off, these spaces must
+each be 2 inches long, and they must be proportionately less or greater
+as the eye is nearer or farther.
+
+[Illustration: _Fig. 10._]
+
+Now suppose the observer wishes to view Mercury on some day, whereon
+Mercury is an evening star. Take, for instance, June 9th, 1868. We find
+from 'Dietrichsen' that on this day (at noon) Mercury's R.A. is 6h. 53m.
+23s.: and the sun's 5h. 11m. 31s. We need not trouble ourselves about
+the odd hours after noon, and thus we have Mercury's R.A. greater than
+the sun's by 1h. 41m. 52s. Now we will suppose that the observer has so
+fixed his uprights and the two rods, that the sun, seen from the fixed
+point of view, appears to pass the point of crossing of the rods at
+half-past seven, then Mercury will pass the cross-rod at 11m. 52s. past
+nine. But where? To learn this we must take out Mercury's declination,
+which is 24° 43' 18" N., and the sun's, which is 22° 59' 10" N. The
+difference, 1° 44' 8" N. gives us Mercury's place, which it appears is
+rather less than 1-3/4 degree north of the sun. Thus, about 1h. 42m.
+after the sun has passed the cross-rod, Mercury will pass it between the
+first and second divisions above the point of fastening. The sun will
+have set about an hour, and Mercury will be easily found when the
+telescope is directed towards the place indicated.
+
+It will be noticed that this method does not require the time to be
+exactly known. All we have to do is to note the moment at which the sun
+passes the point of fastening of the two rods, and to take our 1h. 42m.
+from that moment.
+
+This method, it may be noticed in passing, may be applied to give
+naked-eye observations of Mercury at proper seasons (given in the
+almanac). By a little ingenuity it may be applied as well to morning as
+to evening observations, the sun's passage of the cross-rod being taken
+on one morning and Mercury's on the next, so many minutes _before_ the
+hour of the first observation. In this way several views of Mercury may
+be obtained during the year.
+
+Such methods may appear very insignificant to the systematic observer
+with the equatorial, but that they are effective I can assert from my
+own experience. Similar methods may be applied to determine from the
+position of a known object, that of any neighbouring unknown object even
+at night. The cross-rod must be shifted (or else two cross-rods used)
+when the unknown _precedes_ the known object. If two cross-rods are
+used, account must be taken of the gradual diminution in the length of a
+degree of right ascension as we leave the equator.
+
+Even simpler methods carefully applied may serve to give a view of
+Mercury. To show this, I may describe how I obtained my first view of
+this planet. On June 1st, 1863, I noticed, that at five minutes past
+seven the sun, as seen from my study window, appeared from behind the
+gable-end of Mr. St. Aubyn's house at Stoke, Devon. I estimated the
+effect of Mercury's northerly declination (different of course for a
+vertical wall, than for the cross-rod in fig. 8, which, in fact, agrees
+with a declination-circle), and found that he would pass out opposite a
+particular point of the wall a certain time after the sun. I then turned
+the telescope towards that point, and focussed for distinct vision of
+distant objects, so that the outline of the house was seen out of focus.
+As the calculated time of apparition approached, I moved the telescope
+up and down so that the field swept the neighbourhood of the estimated
+point of apparition. I need hardly say that Mercury did not appear
+exactly at the assigned point, nor did I see him make his first
+appearance; but I picked him up so soon after emergence that the outline
+of the house was in the field of view with him. He appeared as a
+half-disc. I followed him with the telescope until the sun had set, and
+soon after I was able to see him very distinctly with the naked eye. He
+shone with a peculiar brilliance on the still bright sky; but although
+perfectly distinct to the view when his place was indicated, he escaped
+detection by the undirected eye.[12]
+
+Mercury does not present any features of great interest in ordinary
+telescopes; though he usually appears better defined than Venus, at
+least as the latter is seen on a dark sky. The phases are pleasingly
+seen (as shown in Plate 6) with a telescope of moderate power. For their
+proper observation, however, the planet must be looked for with the
+telescope in the manner above indicated, as he always shows a nearly
+semi-circular disc when he is visible to the naked eye.
+
+We come next to Venus, the most splendid of all the planets to the eye.
+In the telescope Venus disappoints the observer, however. Her intense
+lustre brings out every defect of the instrument, and especially the
+chromatic aberration. A dark glass often improves the view, but not
+always. Besides, an interposed glass has an unpleasant effect on the
+field of view.
+
+Perhaps the best method of observing Venus is to search for her when she
+is still high above the horizon, and when therefore the background of
+the sky is bright enough to take off the planet's glare. The method I
+have described for the observation of Mercury will prove very useful in
+the search for Venus when the sun is above the horizon or but just set.
+Of course, when an object is to be looked for high above the horizon,
+the two rods which support the cross-rods must not be upright, but
+square to the line of view to that part of the sky.
+
+But the observer must not expect to see much during his observation of
+Venus. In fact, he can scarcely do more than note her varying phases
+(see Plate 6) and the somewhat uneven boundary of the terminator. Our
+leading observers have done so little with this fascinating but
+disappointing planet, that amateurs must not be surprised at their own
+failure.
+
+I suppose the question whether Venus has a satellite, or at any rate
+whether the object supposed to have been seen by Cassini and other old
+observers were a satellite, must be considered as decided in the
+negative. That Cassini should have seen an object which Dawes and Webb
+have failed to see must be considered utterly improbable.
+
+Leaving the inferior planets, we come to a series of important and
+interesting objects.
+
+First we have the planet Mars, nearly the last in the scale of planetary
+magnitude, but far from being the least interesting of the planets. It
+is in fact quite certain that we obtain a better view of Mars than of
+any object in the heavens, save the Moon alone. He may present a less
+distinguished appearance than Jupiter or Saturn, but we see his surface
+on a larger scale than that of either of those giant orbs, even if we
+assume that we ever obtain a fair view of their real surface.
+
+Nor need the moderately armed observer despair of obtaining interesting
+views of Mars. The telescope with which Beer and Mädler made their
+celebrated series of views was only a 4-inch one, so that with a 3-inch
+or even a 2-inch aperture the attentive observer may expect interesting
+views. In fact, more depends on the observer than on the instrument. A
+patient and attentive scrutiny will reveal features which at the first
+view wholly escape notice.
+
+In Plate 6 I have given a series of views of Mars much more distinct
+than an observer may expect to obtain with moderate powers. I add a
+chart of Mars, a miniature of one I have prepared from a charming
+series of tracings supplied me by Mr. Dawes. The views taken by this
+celebrated observer in 1852, 1856, 1860, 1862, and 1864, are far better
+than any others I have seen. The views by Beer and Mädler are good, as
+are some of Secchi's (though they appear badly drawn), Nasmyth's and
+Phillips'; Delarue's two views are also admirable; and Lockyer has given
+a better set of views than any of the others. But there is an amount of
+detail in Mr. Dawes' views which renders them superior to any yet taken.
+I must confess I failed at a first view to see the full value of Mr.
+Dawes' tracings. Faint marks appeared, which I supposed to be merely
+intended to represent shadings scarcely seen. A more careful study
+shewed me that every mark is to be taken as the representative of what
+Mr. Dawes actually saw. The consistency of the views is perfectly
+wonderful, when compared with the vagueness and inconsistency observable
+in nearly all other views. And this consistency is not shown by mere
+resemblance, which might have been an effect rather of memory
+(unconsciously exerted) than observation. The same feature changes so
+much in figure, as it appears on different parts of the disc, that it
+was sometimes only on a careful projection of different views that I
+could determine what certain features near the limb represented. But
+when this had been done, and the distortion through the effect of
+foreshortening corrected, the feature was found to be as true in shape
+as if it had been seen in the centre of the planet's disc.
+
+In examining Mr. Dawes' drawings it was necessary that the position of
+Mars' axis should be known. The data for determining this were taken
+from Dr. Oudemann's determinations given in a valuable paper on Mars
+issued from Mr. Bishop's observatory. But instead of calculating Mars'
+presentation by the formulæ there given, I found it convenient rather to
+make use of geometrical constructions applied to my 'Charts of the
+Terrestrial Planets.' Taking Mädler's start-point for Martial
+longitudes, that is the longitude-line passing near Dawes' forked bay, I
+found that my results agreed pretty fairly with those in Prof. Phillips'
+map, so far as the latter went; but there are many details in my charts
+not found in Prof. Phillips' nor in Mädler's earlier charts.
+
+I have applied to the different features the names of those observers
+who have studied the physical peculiarities presented by Mars. Mr.
+Dawes' name naturally occurs more frequently than others. Indeed, if I
+had followed the rule of giving to each feature the name of its
+discoverer, Mr. Dawes' name would have occurred much more frequently
+than it actually does.
+
+On account of the eccentricity of his orbit, Mars is seen much better in
+some oppositions than in others. When best seen the southern hemisphere
+is brought more into view than the northern because the summer of his
+northern hemisphere occurs when he is nearly in aphelion (as is the case
+with the Earth by the way).
+
+The relative dimensions and presentation of Mars, as seen in opposition
+in perihelion, and in opposition in aphelion, are shown in the two rows
+of figures.
+
+In and near quadrature Mars is perceptibly gibbous. He is seen thus
+about two months before or after opposition. In the former case, he
+rises late and comes to the meridian six hours or so after midnight. In
+the latter case, he is well seen in the evening, coming to the meridian
+at six. His appearance and relative dimensions as he passes from
+opposition to quadrature are shown in the last three figures of the
+upper row.
+
+Mars' polar caps may be seen with very moderate powers.
+
+I add four sets of meridians (Plate 6), by filling in which from the
+charts the observer may obtain any number of views of the planet as it
+appears at different times.
+
+Passing over the asteroids, which are not very interesting objects to
+the amateur telescopist, we come to Jupiter, the giant of the solar
+system, surpassing our Earth more than 1400 times in volume, and
+overweighing all the planets taken together twice over.
+
+Jupiter is one of the easiest of all objects of telescopic observation.
+No one can mistake this orb when it shines on a dark sky, and only Venus
+can be mistaken for it when seen as a morning or evening star. Sometimes
+both are seen together on the twilight sky, and then Venus is generally
+the brighter. Seen, however, at her brightest and at her greatest
+elongation from the sun, her splendour scarcely exceeds that with which
+Jupiter shines when high above the southern horizon at midnight.
+
+Jupiter's satellites may be seen with very low powers; indeed the outer
+ones have been seen with the naked eye, and all are visible in a good
+opera-glass. Their dimensions relatively to the disc are shown in Plate
+7. Their greatest elongations are compared with the disc in the
+low-power view.
+
+Jupiter's belts may also be well seen with moderate telescopic power.
+The outer parts of his disc are perceptibly less bright than the centre.
+
+More difficult of observation are the transits of the satellites and of
+their shadows. Still the attentive observer can see the shadows with an
+aperture of two inches, and the satellites themselves with an aperture
+of three inches.
+
+The minute at which the satellites enter on the disc, or pass off, is
+given in 'Dietrichsen's Almanac.' The 'Nautical Almanac' also gives the
+corresponding data for the shadows.
+
+The eclipses of the satellites in Jupiter's shadow, and their
+occultations by his disc, are also given in 'Dietrichsen's Almanac.'
+
+In the inverting telescope the satellites move from right to left in the
+nearer parts of their orbit, and therefore transit Jupiter's disc in
+that direction, and from left to right in the farther parts. Also note
+that _before_ opposition, (i.) the shadows travel in front of the
+satellites in transiting the disc; (ii.) the satellites are eclipsed in
+Jupiter's _shadow_; (iii.) they reappear from behind his _disc_. On the
+other hand, _after_ opposition, (i.) the shadows travel _behind_ the
+satellites in transiting the disc; (ii.) the satellites are occulted by
+the _disc_; (iii.) they reappear from eclipse in Jupiter's _shadow_.
+
+Conjunctions of the satellites are common phenomena, and may be waited
+for by the observer who sees the chance. An eclipse of one satellite by
+the shadow of another is not a common phenomenon; in fact, I have never
+heard of such an eclipse being seen. That a satellite should be quite
+extinguished by another's shadow is a phenomenon not absolutely
+impossible, but which cannot happen save at long intervals.
+
+The shadows are not _black spots_ as is erroneously stated in nearly all
+popular works on astronomy. The shadow of the fourth, for instance, is
+nearly all penumbra, the really black part being quite minute by
+comparison. The shadow of the third has a considerable penumbra, and
+even that of the first is not wholly black. These penumbras may not be
+perceptible, but they affect the appearance of the shadows. For
+instance, the shadow of the fourth is perceptibly larger but less black
+than that of the third, though the third is the larger satellite.
+
+In transit the first satellite moves fastest, the fourth slowest, the
+others in their order. The shadow moves just as fast (appreciably) as
+the satellite it belongs to. Sometimes the shadow of the satellite may
+be seen to overtake (apparently) the disc of another. In such a case the
+shadow does not pass over the disc, but the disc conceals the shadow.
+This is explained by the fact that the shadow, if visible throughout its
+length, would be a line reaching slantwise from the satellite it belongs
+to, and the end of the shadow (that is, the point where it meets the
+disc) is _not_ the point where the shadow crosses the orbit of any inner
+satellite. Thus the latter may be interposed between the end of the
+shadow--the only part of the shadow really visible--and the eye; but the
+end of the shadow _cannot_ be interposed between the satellite and the
+eye. If a satellite _on the disc_ were eclipsed by another satellite,
+the black spot thus formed would be in another place from the black spot
+on the planet's body. I mention all this because, simple as the question
+may seem, I have known careful observers to make mistakes on this
+subject. A shadow is seen crossing the disc and overtaking, apparently,
+a satellite in transit. It seems therefore, on a first view, that the
+shadow will hide the satellite, and observers have even said that they
+have _seen_ this happen. But they are deceived. It is obvious that _if
+one satellite eclipse another, the shadows of both must occupy the same
+point on Jupiter's body_. Thus it is the overtaking of one _shadow_ by
+another on the disc, and not the overtaking of a _satellite_ by a
+shadow, which determines the occurrence of that as yet unrecorded
+phenomenon, the eclipse of one satellite by another.[13]
+
+The satellites when far from Jupiter seem to lie in a straight line
+through his centre. But as a matter of fact they do not in general lie
+in an exact straight line. If their orbits could be seen as lines of
+light, they would appear, in general, as very long ellipses. The orbit
+of the fourth would frequently be seen to be _quite clear_ of Jupiter's
+disc, and the orbit of the third might in some very exceptional
+instances pass _just_ clear of the disc. The satellites move most nearly
+in a straight line (apparently) when Jupiter comes to opposition in the
+beginning of February or August, and they appear to depart most from
+rectilinear motion when opposition occurs in the beginning of May and
+November. At these epochs the fourth satellite may be seen to pass above
+and below Jupiter's disc at a distance equal to about one-sixth of the
+disc's radius.
+
+The shadows do not travel in the same apparent paths as the satellites
+themselves across the disc, but (in an inverting telescope) _below_ from
+August to January, and _above_ from February to July.
+
+We come now to the most charming telescopic object in the heavens--the
+planet Saturn. Inferior only to Jupiter in mass and volume, this planet
+surpasses him in the magnificence of his system. Seen in a telescope of
+adequate power, Saturn is an object of surpassing loveliness. He must be
+an unimaginative man who can see Saturn for the first time in such a
+telescope, without a feeling of awe and amazement. If there is any
+object in the heavens--I except not even the Sun--calculated to impress
+one with a sense of the wisdom and omnipotence of the Creator it is
+this. "His fashioning hand" is indeed visible throughout space, but in
+Saturn's system it is most impressively manifest.
+
+Saturn, to be satisfactorily seen, requires a much more powerful
+telescope than Jupiter. A good 2-inch telescope will do much, however,
+in exhibiting his rings and belts. I have never seen him satisfactorily
+myself with such an aperture, but Mr. Grover has not only seen the
+above-named features, but even a penumbra to the shadow on the rings
+with a 2-inch telescope.
+
+Saturn revolving round the sun in a long period--nearly thirty
+years--presents slowly varying changes of appearance (see Plate 7). At
+one time the edge of his ring is turned nearly towards the earth; seven
+or eight years later his rings are as much open as they can ever be;
+then they gradually close up during a corresponding interval; open out
+again, exhibiting a different face; and finally close up as first seen.
+The last epoch of greatest opening occurred in 1856, the next occurs in
+1870: the last epoch of disappearance occurred in 1862-63, the next
+occurs in 1879. The successive views obtained are as in Plate 7 in order
+from right to left, then back to the right-hand figure (but sloped the
+other way); inverting the page we have this figure thus sloped, and the
+following changes are now indicated by the other figures in order back
+to the first (but sloped the other way and still inverted), thus
+returning to the right-hand figure as seen without inversion.
+
+The division in the ring can be seen in a good 2-inch aperture in
+favourable weather. The dark ring requires a good 4-inch and good
+weather.
+
+Saturn's satellites do not, like Jupiter's, form a system of nearly
+equal bodies. Titan, the sixth, is probably larger than any of
+Jupiter's satellites. The eighth also (Japetus) is a large body,
+probably at least equal to Jupiter's third satellite. But Rhea, Dione,
+and Tethys are much less conspicuous, and the other three cannot be seen
+without more powerful telescopes than those we are here dealing with.
+
+So far as my own experience goes, I consider that the five larger
+satellites may be seen distinctly in good weather with a good 3-1/2-inch
+aperture. I have never seen them with such an aperture, but I judge from
+the distinctness with which these satellites may be seen with a 4-inch
+aperture. Titan is generally to be looked for at a considerable distance
+from Saturn--_always_ when the ring is widely open. Japetus is to be
+looked for yet farther from the disc. In fact, when Saturn comes to
+opposition in perihelion (in winter only this can happen) Japetus may be
+as far from Saturn as one-third of the apparent diameter of the moon. I
+believe that under these circumstances, or even under less favourable
+circumstances, Japetus could be seen with a good opera-glass. So also
+might Titan.
+
+Transits, eclipses, and occulations of Saturn's satellites can only be
+seen when the ring is turned nearly edgewise towards the earth. For the
+orbits of the seven inner satellites lying nearly in the plane of the
+rings would (if visible throughout their extent) then only appear as
+straight lines, or as long ellipses cutting the planet's disc.
+
+The belts on Saturn are not very conspicuous. A good 3-1/2-inch is
+required (so far as my experience extends) to show them satisfactorily.
+
+The rings when turned edgewise either towards the earth or sun, are not
+visible in ordinary telescopes, neither can they be seen when the earth
+and sun are on opposite sides of the rings. In powerful telescopes the
+rings seem never entirely to disappear.
+
+The shadow of the planet on the rings may be well seen with a good
+2-inch telescope, which will also show Ball's division in the rings. The
+shadow of the rings on the planet is a somewhat more difficult feature.
+The shadow of the planet on the rings is best seen when the rings are
+well open and the planet is in or near quadrature. It is to be looked
+for to the left of the ball (in an inverting telescope) at quadrature
+preceding opposition, and to the right at quadrature following
+opposition. Saturn is more likely to be studied at the latter than at
+the former quadrature, as in quadrature preceding opposition he is a
+morning star. The shadow of the rings on the planet is best seen when
+the rings are but moderately open, and Saturn is in or near quadrature.
+When the shadow lies outside the rings it is best seen, as the dark ring
+takes off from the sharpness of the contrast when the shadow lies within
+the ring. It would take more space than I can spare here to show how it
+is to be determined (independently) whether the shadow lies within or
+without the ring. But the 'Nautical Almanac' gives the means of
+determining this point. When, in the table for assigning the appearance
+of the rings, _l_ is less than _l'_ the shadow lies outside the ring,
+when _l_ is greater than _l'_ the shadow lies within the ring.
+
+Uranus is just visible to the naked eye when he is in opposition, and
+his place accurately known. But he presents no phenomena of interest. I
+have seen him under powers which made his disc nearly equal to that of
+the moon, yet could see nothing but a faint bluish disc.
+
+Neptune also is easily found if his place be accurately noted on a map,
+and a good finder used. We have only to turn the telescope to a few
+stars seen in the finder nearly in the place marked in our map, and
+presently we shall recognise the one we want by the peculiarity of its
+light. What is the lowest power which will exhibit Neptune as a disc I
+do not know, but I am certain no observer can mistake him for a fixed
+star with a 2-inch aperture and a few minutes' patient scrutiny in
+favourable weather.
+
+[Illustration: PLATE VII.]
+
+
+
+
+CHAPTER VII.
+
+HALF-HOURS WITH THE SUN AND MOON.
+
+
+The moon perhaps is the easiest of all objects of telescopic
+observation. A very moderate telescope will show her most striking
+features, while each increase of power is repaid by a view of new
+details. Yet in one sense the moon is a disappointing object even to the
+possessor of a first-class instrument. For the most careful and
+persistent scrutiny, carried on for a long series of years, too often
+fails to reward the observer by any new discoveries of interest. Our
+observer must therefore rather be prepared to enjoy the observation of
+recognised features than expect to add by his labours to our knowledge
+of the earth's nearest neighbour.
+
+Although the moon is a pleasing and surprising telescopic object when
+full, the most interesting views of her features are obtained at other
+seasons. If we follow the moon as she waxes or wanes, we see the true
+nature of that rough and bleak mountain scenery, which when the moon is
+full is partially softened through the want of sharp contrasts of light
+and shadow. If we watch, even for half an hour only, the changing form
+of the ragged line separating light from darkness on the moon's disc, we
+cannot fail to be interested. "The outlying and isolated peak of some
+great mountain-chain becomes gradually larger, and is finally merged in
+the general luminous surface; great circular spaces, enclosed with rough
+and rocky walls many miles in diameter, become apparent; some with flat
+and perfectly smooth floors, variegated with streaks; others in which
+the flat floor is dotted with numerous pits or covered with broken
+fragments of rock. Occasionally a regularly-formed and unusually
+symmetrical circular formation makes its appearance; the exterior
+surface of the wall bristling with terraces rising gradually from the
+plain, the interior one much more steep, and instead of a flat floor,
+the inner space is concave or cup-shaped, with a solitary peak rising in
+the centre. Solitary peaks rise from the level plains and cast their
+long narrow shadows athwart the smooth surface. Vast plains of a dusky
+tint become visible, not perfectly level, but covered with ripples,
+pits, and projections. Circular wells, which have no surrounding wall
+dip below the plain, and are met with even in the interior of the
+circular mountains and on the tops of their walls. From some of the
+mountains great streams of a brilliant white radiate in all directions
+and can be traced for hundreds of miles. We see, again, great fissures,
+almost perfectly straight and of great length, although very narrow,
+which appear like the cracks in moist clayey soil when dried by the
+sun."[14]
+
+But interesting as these views may be, it was not for such discoveries
+as these that astronomers examined the surface of the moon. The
+examination of mere peculiarities of physical condition is, after all,
+but barren labour, if it lead to no discovery of physical variation. The
+principal charm of astronomy, as indeed of all observational science,
+lies in the study of change--of progress, development, and decay, and
+specially of systematic variations taking place in regularly-recurring
+cycles. And it is in this relation that the moon has been so
+disappointing an object of astronomical observation. For two centuries
+and a half her face has been scanned with the closest possible scrutiny;
+her features have been portrayed in elaborate maps; many an astronomer
+has given a large portion of his life to the work of examining craters,
+plains, mountains, and valleys, for the signs of change; but until
+lately no certain evidence--or rather, no evidence save of the most
+doubtful character--has been afforded that the moon is other than "a
+dead and useless waste of extinct volcanoes." Whether the examination of
+the remarkable spot called Linné--where lately signs were supposed to
+have been seen of a process of volcanic eruption--will prove an
+exception to this rule, remains to be seen. The evidence seems to me
+strongly to favour the supposition of a change of some sort having taken
+place in this neighbourhood.
+
+The sort of scrutiny required for the discovery of changes, or for the
+determination of their extent, is far too close and laborious to be
+attractive to the general observer. Yet the kind of observation which
+avails best for the purpose is perhaps also the most interesting which he
+can apply to the lunar details. The peculiarities presented by a spot upon
+the moon are to be observed from hour to hour (or from day to day,
+according to the size of the spot) as the sun's light gradually sweeps
+across it, until the spot is fully lighted; then as the moon wanes and the
+sun's light gradually passes from the spot, the series of observations is
+to be renewed. A comparison of them is likely--especially if the observer
+is a good artist and has executed several faithful delineations of the
+region under observation, to throw much light upon the real contour of the
+moon's surface at this point.
+
+In the two lunar views in Plate 7 some of the peculiarities I have
+described are illustrated. But the patient observer will easily be able
+to construct for himself a set of interesting views of different
+regions.
+
+It may be noticed that for observation of the waning moon there is no
+occasion to wait for those hours in which only the waning moon is
+visible _during the night_. Of course for the observation of a
+particular region under a particular illumination, the observer has no
+choice as to hour. But for generally interesting observations of the
+waning moon he can wait till morning and observe by daylight. The moon
+is, of course, very easily found by the unaided eye (in the day time)
+when not very near to the sun; and the methods described in Chapter V.
+will enable the observer to find the moon when she is so near to the sun
+as to present the narrowest possible sickle of light.
+
+One of the most interesting features of the moon, when she is observed
+with a good telescope, is the variety of colour presented by different
+parts of her surface. We see regions of the purest white--regions which
+one would be apt to speak of as _snow-covered_, if one could conceive
+the possibility that snow should have fallen where (now, at least) there
+is neither air nor water. Then there are the so-called seas, large grey
+or neutral-tinted regions, differing from the former not merely in
+colour and in tone, but in the photographic quality of the light they
+reflect towards the earth. Some of the seas exhibit a greenish tint, as
+the Sea of Serenity and the Sea of Humours. Where there is a central
+mountain within a circular depression, the surrounding plain is
+generally of a bluish steel-grey colour. There is a region called the
+Marsh of Sleep, which exhibits a pale red tint, a colour seen also near
+the Hyrcinian mountains, within a circumvallation called Lichtenburg.
+The brightest portion of the whole lunar disc is Aristarchus, the peaks
+of which shine often like stars, when the mountain is within the
+unillumined portion of the moon. The darkest regions are Grimaldi and
+Endymion and the great plain called Plato by modern astronomers--but, by
+Hevelius, the Greater Black Lake.
+
+The Sun.--Observation of the sun is perhaps on the whole the most
+interesting work to which the possessor of a moderately good telescope
+can apply his instrument. Those wonderful varieties in the appearance of
+the solar surface which have so long perplexed astronomers, not only
+supply in themselves interesting subjects of observation and
+examination, but gain an enhanced meaning from the consideration that
+they speak meaningly to us of the structure of an orb which is the
+source of light and heat enjoyed by a series of dependent worlds whereof
+our earth is--in size at least--a comparatively insignificant member.
+Swayed by the attraction of this giant globe, Jupiter and Saturn, Uranus
+and Neptune, as well as the four minor planets, and the host of
+asteroids, sweep continuously in their appointed orbits, in ever new but
+ever safe and orderly relations amongst each other. If the sun's light
+and heat were lost, all life and work among the denizens of these orbs
+would at once cease; if his attractive energy were destroyed, these orbs
+would cease to form a _system_.
+
+The sun may be observed conveniently in many ways, some more suited to
+the general observer who has not time or opportunity for systematic
+observation; others more instructive, though involving more of
+preparation and arrangement.
+
+The simplest method of observing the sun is to use the telescope in the
+ordinary manner, protecting the eye by means of dark-green or
+neutral-tinted glasses. Some of the most interesting views I have ever
+obtained of the sun, have resulted from the use of the ordinary
+terrestrial or erecting eye-piece, capped with a dark glass. The
+magnifying power of such an eye-piece is, in general, much lower than
+that available with astronomical eye-pieces. But vision is very pleasant
+and distinct when the sun is thus observed, and a patient scrutiny
+reveals almost every feature which the highest astronomical power
+applicable could exhibit. Then, owing to the greater number of
+intervening lenses, there is not the same necessity for great darkness
+or thickness in the coloured glass, so that the colours of the solar
+features are seen much more satisfactorily than when astronomical
+eye-pieces are employed.
+
+In using astronomical eye-pieces it is convenient to have a rotating
+wheel attached, by which darkening glasses of different power may be
+brought into use as the varying illumination may require.
+
+Those who wish to observe carefully and closely a minute portion of the
+solar disc, should employ Dawes' eye-piece: in this a metallic screen
+placed in the focus keeps away all light but such as passes through a
+minute hole in the diaphragm.
+
+Another convenient method of diminishing the light is to use a glass
+prism, light being partially reflected from one of the exterior
+surfaces, while the refracted portion is thrown out at another.
+
+Very beautiful and interesting views may be obtained by using such a
+pyramidal box as is depicted in fig. 11.
+
+[Illustration: _Fig. 11._]
+
+This box should be made of black cloth or calico fastened over a light
+framework of wire or cane. The base of the pyramid should be covered on
+the inside with a sheet of white glazed paper, or with some other
+uniform white surface. Captain Noble, I believe, makes use of a surface
+of plaster of Paris, smoothed while wet with plate-glass. The door _b
+c_ enables the observer to "change power" without removing the box,
+while larger doors, _d e_ and _g f_, enable him to examine the image; a
+dark cloth, such as photographers use, being employed, if necessary, to
+keep out extraneous light. The image may also be examined from without,
+if the bottom of the pyramid be formed of a sheet of cut-glass or oiled
+tissue-paper.
+
+When making use of the method just described, it is very necessary that
+the telescope-tube should be well balanced. A method by which this may
+be conveniently accomplished has been already described in Chapter I.
+
+But, undoubtedly, for the possessor of a moderately good telescope there
+is no way of viewing the sun's features comparable to that now to be
+described, which has been systematically and successfully applied for a
+long series of years by the Rev. F. Howlett. To use his own words: "Any
+one possessing a good achromatic of not more than three inches'
+aperture, who has a little dexterity with his pencil, and a little time
+at his disposal (all the better if it be at a somewhat early hour of the
+morning)" may by this method "deliberately and satisfactorily view,
+measure, and (if skill suffice) delineate most of those interesting and
+grand solar phenomena of which he may have read, or which he may have
+seen depicted, in various works on physical astronomy."[15]
+
+The method in question depends on the same property which is involved in
+the use of the pyramidal box just described, supplemented (where exact
+and systematic observation is required) by the fact that objects lying
+on or between the lenses of the eye-piece are to be seen faithfully
+projected on the white surface on which the sun's image is received. In
+place, however, of a box carried upon the telescope-tube, a darkened
+room (or true _camera obscura_) contains the receiving sheet.
+
+A chamber is to be selected, having a window looking towards the
+south--a little easterly, if possible, so as to admit of morning
+observation. All windows are to be completely darkened save one, through
+which the telescope is directed towards the sun. An arrangement is to be
+adopted for preventing all light from entering by this window except
+such light as passes down the tube of the telescope. This can readily be
+managed with a little ingenuity. Mr. Howlett describes an excellent
+method. The following, perhaps, will sufficiently serve the purposes of
+the general observer:--A plain frame (portable) is to be constructed to
+fit into the window: to the four sides of this frame triangular pieces
+of cloth (impervious to light) are to be attached, their shape being
+such that when their adjacent edges are sewn together and the flaps
+stretched out, they form a rectangular pyramid of which the frame is the
+base. Through the vertex of this pyramid (near which, of course, the
+cloth flaps are not sewn together) the telescope tube is to be passed,
+and an elastic cord so placed round the ends of the flaps as to prevent
+any light from penetrating between them and the telescope. It will now
+be possible, without disturbing the screen (fixed in the window), to
+move the telescope so as to follow the sun during the time of
+observation. And the same arrangement will serve for all seasons, if so
+managed that the elastic cord is not far from the middle of the
+telescope-tube; for in this case the range of motion is small compared
+to the range of the tube's extremity.
+
+A large screen of good drawing-paper should next be prepared. This
+should be stretched on a light frame of wood, and placed on an easel,
+the legs of which should be furnished with holes and pegs that the
+screen may be set at any required height, and be brought square to the
+tube's axis. A large T-square of light wood will be useful to enable the
+observer to judge whether the screen is properly situated in the last
+respect.
+
+We wish now to direct the tube towards the sun, and this "without
+dazzling the eyes as by the ordinary method." This may be done in two
+ways. We may either, before commencing work--that is, before fastening
+our elastic cord so as to exclude all light--direct the tube so that its
+shadow shall be a perfect circle (when of course it is truly directed),
+then fasten the cord and afterwards we can easily keep the sun in the
+field by slightly shifting the tube as occasion requires. Or (if the
+elastic cord has already been fastened) we may remove the eye-tube and
+shift the telescope-tube about--the direction in which the sun lies
+being roughly known--until we see the spot of light received down the
+telescope's axis grow brighter and brighter and finally become a _spot
+of sun-light_. If a card be held near the focus of the telescope there
+will be seen in fact an image of the sun. The telescope being now
+properly directed, the eye-tube may be slipped in again, and the sun may
+be kept in the field as before.
+
+There will now be seen upon the screen a picture of the sun very
+brilliant and pleasing, but perhaps a little out of focus. The focusing
+should therefore next be attended to, the increase of clearness in the
+image being the test of approach to the true focus. And again, it will
+be well to try the effect of slight changes of distance between the
+screen and the telescope's eye-piece. Mr. Howlett considers one yard as
+a convenient distance for producing an excellent effect with almost any
+eye-piece that the state of the atmosphere will admit of. Of course, the
+image becomes more sharply defined if we bring the screen nearer to the
+telescope, while all the details are enlarged when we move the screen
+away. The enlargement has no limits save those depending on the amount
+of light in the image. But, of course, the observer must not expect
+enlargement to bring with it a view of new details, after a certain
+magnitude of image has been attained. Still there is something
+instructive, I think, in occasionally getting a very magnified view of
+some remarkable spot. I have often looked with enhanced feelings of awe
+and wonder on the gigantic image of a solar spot thrown by means of the
+diagonal eye-piece upon the ceiling of the observing-room. Blurred and
+indistinct through over-magnifying, yet with a new meaning to me,
+_there_ the vast abysm lies pictured; vague imaginings of the vast and
+incomprehensible agencies at work in the great centre of our system
+crowd unbidden into my mind; and I seem to _feel_--not merely think
+about--the stupendous grandeur of that life-emitting orb.
+
+To return, however, to observation:--By slightly shifting the tube,
+different parts of the solar disc can be brought successively upon the
+screen and scrutinized as readily as if they were drawn upon a chart.
+"With a power of--say about 60 or 80 linear--the most minute solar spot,
+properly so called, that is capable of formation" (Mr. Howlett believes
+"they are never less than three seconds in length or breadth) will be
+more readily detected than by any other method," see Plate 7; "as also
+will any faculæ, mottling, or in short, any other phenomena that may
+then be existing on the disc." "Drifting clouds frequently sweep by, to
+vary the scene, and occasionally an aërial hail- or snow-storm." Mr.
+Howlett has more than once seen a distant flight of rooks pass slowly
+across the disc with wonderful distinctness, when the sun has been at a
+low altitude, and likewise, much more frequently, the rapid dash of
+starlings, which, very much closer at hand, frequent his church-tower."
+
+An eclipse of the sun, or a transit of an inferior planet, is also much
+better seen in this way than by any other method of observing the solar
+disc. In Plate 7 are presented several solar spots as they have appeared
+to Mr. Howlett, with an instrument of moderate power. The grotesque
+forms of some of these are remarkable; and the variations the spots
+undergo from day to day are particularly interesting to the thoughtful
+observer.
+
+A method of measuring the spots may now be described. It is not likely
+indeed that the ordinary observer will care to enter upon any systematic
+series of measurements. But even in his case, the means of forming a
+general comparison between the spots he sees at different times cannot
+fail to be valuable. Also the knowledge--which a simple method of
+measurement supplies--of the actual dimensions of a spot in miles
+(roughly) is calculated to enhance our estimate of the importance of
+these features of the solar disc. I give Mr. Howlett's method in his own
+words:--
+
+"Cause your optician to rule for you on a circular piece of glass a
+number of fine graduations, the 200th part of an inch apart, each fifth
+and tenth line being of a different length in order to assist the eye in
+their enumeration. Insert this between the anterior and posterior lenses
+of a Huygenian eye-piece of moderate power, say 80 linear. Direct your
+telescope upon the sun, and having so arranged it that the whole disc of
+the sun may be projected on the screen, count carefully the number of
+graduations that are seen to exactly occupy the solar diameter.... It
+matters not in which direction you measure your diameter, provided only
+the sun has risen some 18° or 20° above the horizon, and so escaped the
+distortion occasioned by refraction.[16]
+
+"Next let us suppose that our observer has been observing the sun on any
+day of the year, say, if you choose, at the time of its mean apparent
+diameter, namely about the first of April or first of October, and has
+ascertained that" (as is the case with Mr. Howlett's instrument)
+"sixty-four graduations occupy the diameter of the projected image. Now
+the semi-diameter of the sun, at the epochs above mentioned, according
+to the tables given for every day of the year in the 'Nautical Almanac'
+(the same as in Dietrichsen and Hannay's very useful compilation) is
+16' 2", and consequently his mean total diameter is 32' 4" or 1924". If
+now we divide 1924" by 64" this will, of course, award as nearly as
+possible 30" as the value in celestial arc of each graduation, either as
+seen on the screen, or as applied directly to the sun or any heavenly
+body large enough to be measured by it."
+
+Since the sun's diameter is about 850,000 miles, each graduation (in the
+case above specified) corresponds to one-64th part of 850,000
+miles--that is, to a length of 13,256 miles on the sun's surface. Any
+other case can be treated in precisely the same manner.
+
+It will be found easy so to place the screen that the distance between
+successive graduations (as seen projected upon the screen) may
+correspond to any desired unit of linear measurement--say an inch. Then
+if the observer use transparent tracing-paper ruled with faint lines
+forming squares half-an-inch in size, he can comfortably copy directly
+from the screen any solar phenomena he may be struck with. A variety of
+methods of drawing will suggest themselves. Mr. Howlett, in the paper I
+have quoted from above, describes a very satisfactory method, which
+those who are anxious to devote themselves seriously to solar
+observation will do well to study.
+
+It is necessary that the observer should be able to determine
+approximately where the sun's equator is situated at the time of any
+observation, in order that he may assign to any spot or set of spots its
+true position in relation to solar longitude and latitude. Mr. Howlett
+shows how this may be done by three observations of the sun made at any
+fixed hour on successive days. Perhaps the following method will serve
+the purpose of the general observer sufficiently well:--
+
+The hour at which the sun crosses the meridian must be taken for the
+special observation now to be described. This hour can always be learnt
+from 'Dietrichsen's Almanac'; but noon, civil time, is near enough for
+practical purposes. Now it is necessary first to know the position of
+the ecliptic with reference to the celestial equator. Of course, at noon
+a horizontal line across the sun's disc is parallel to the equator, but
+the position of that diameter of the sun which coincides with the
+ecliptic is not constant: at the summer and winter solstices this
+diameter coincides with the other, or is horizontal at noon; at the
+spring equinox the sun (which travels on the ecliptic) is passing
+towards the north of the equator, crossing that curve at an angle of
+23-1/2°, so that the ecliptic coincides with that diameter of the sun
+which cuts the horizontal one at an angle of 23-1/2° and has its _left_
+end above the horizontal diameter; and at the autumn equinox the sun is
+descending and the same description applies, only that the diameter
+(inclined 23-1/2° to the horizon) which has its _right_ end uppermost,
+now represents the ecliptic. For intermediate dates, use the following
+little table:--
+
+--------------------------------------------------------------------------
+Date.              |Dec. 22|Jan. 5|Jan. 20|Feb. 4|Feb. 19|Mar. 5 |Mar. 21
+(Circiter.)        |       |June 6|May 21 |May 5 |Apr. 20|Apr. 5 |
+-------------------+-------+------+-------+------+-------+-------+--------
+Inclination of     |Left   |Left  |Left   |Left  |Left   |Left   |Left
+Ecliptical Diameter|       |      |       |      |       |       |
+of Sun to the      |0° 0'  |6°24' |12°14' |17°3' |20°36' |22°44' |23°27'
+Horizon.[17]       |Right  |Right |Right  |Right |Right  |Right  |Right
+-------------------+-------+------+-------+------+-------+-------+--------
+Date.              |       |Dec. 7|Nov. 22|Nov. 7|Oct. 23|Oct. 8 |
+(Circiter.)        |Jan. 21|July 7|July 23|Aug. 6|Aug. 23|Sept. 7|Sept. 23
+--------------------------------------------------------------------------
+
+Now if our observer describe a circle, and draw a diameter inclined
+according to above table, this diameter would represent the sun's
+equator if the axis of the sun were square to the ecliptic-plane. But
+this axis is slightly inclined, the effect of which is, that on or about
+June 10 the sun is situated as shown in fig. 14 with respect to the
+ecliptic _ab_; on or about September 11 he is situated as shown in fig.
+13; on or about December 11 as shown in fig. 12; and on or about March
+10 as shown in fig. 15. The inclination of his equator to the ecliptic
+being so small, the student can find little difficulty in determining
+with sufficient approximation the relation of the sun's polar axis to
+the ecliptic on intermediate days, since the equator is never more
+_inclined_ than in figs. 12 and 14, never more _opened out_ than in
+figs. 13 and 15. Having then drawn a line to represent the sun's
+ecliptical diameter inclined to the horizontal diameter as above
+described, and having (with this line to correspond to _ab_ in figs.
+12-15) drawn in the sun's equator suitably inclined and opened out, he
+has the sun's actual presentation (at noon) as seen with an erecting
+eye-piece. Holding his picture upside down, he has the sun's
+presentation as seen with an astronomical eye-piece--and, finally,
+looking at his picture from behind (without inverting it), he has the
+presentation seen when the sun is projected on the screen. Hence, if he
+make a copy of this last view of his diagram upon the centre of his
+screen, and using a low power, bring the whole of the sun's image to
+coincide with the circle thus drawn (to a suitable scale) on the screen,
+he will at once see what is the true position of the different
+sun-spots. After a little practice the construction of a suitably sized
+and marked circle on the screen will not occupy more than a minute or
+two.
+
+[Illustration: _Fig. 12._]
+
+[Illustration: _Fig. 13._]
+
+[Illustration: _Fig. 14._]
+
+[Illustration: _Fig. 15._]
+
+It must be noticed that the sun's apparent diameter is not always the
+same. He is nearer to us in winter than in summer, and, of course, his
+apparent diameter is greater at the former than at the latter season.
+The variation of the apparent diameter corresponds (inversely) to the
+variation of distance. As the sun's greatest distance from the earth is
+93,000,000 miles (pretty nearly) and his least 90,000,000, his greatest,
+mean, and least apparent diameters are as 93, 91-1/2, and 90
+respectively; that is, as 62, 61, and 60 respectively.
+
+Mr. Howlett considers that with a good 3-inch telescope, applied in the
+manner we have described, all the solar features may be seen, except the
+separate granules disclosed by first-class instruments in the hands of
+such observers as Dawes, Huggins, or Secchi. Faculæ may, of course, be
+well seen. They are to be looked for near spots which lie close to the
+sun's limb.
+
+When the sun's general surface is carefully scrutinised, it is found to
+present a mottled appearance. This is a somewhat delicate feature. It
+results, undoubtedly, from the combined effect of the granules
+separately seen in powerful instruments. Sir John Herschel has stated
+that he cannot recognise the marbled appearance of the sun with an
+achromatic. Mr. Webb, however, has seen this appearance with such a
+telescope, of moderate power, used with direct vision; and certainly I
+can corroborate Mr. Howlett in the statement that this appearance may be
+most distinctly seen when the image of the sun is received within a
+well-darkened room.
+
+My space will not permit me to enter here upon the discussion of any of
+those interesting speculations which have been broached concerning solar
+phenomena. We may hope that the great eclipse of August, 1868, which
+promises to be the most favourable (for effective observation) that has
+ever taken place, will afford astronomers the opportunity of resolving
+some important questions. It seems as if we were on the verge of great
+discoveries,--and certainly, if persevering and well-directed labour
+would seem in any case to render such discoveries due as man's just
+reward, we may well say that he deserves shortly to reap a harvest of
+exact knowledge respecting solar phenomena.
+
+
+
+
+THE END.
+
+
+
+FOOTNOTES:
+
+[Footnote 1: Such a telescope is most powerful with the shortest sight.
+It may be remarked that the use of a telescope often reveals a
+difference in the sight of the two eyes. In my own case, for instance, I
+have found that the left eye is very short-sighted, the sight of the
+right eye being of about the average range. Accordingly with my left eye
+a 5-1/2-foot object-glass, alone, forms an effective telescope, with
+which I can see Jupiter's moons quite distinctly, and under favourable
+circumstances even Saturn's rings. I find that the full moon is too
+bright to be observed in this way without pain, except at low
+altitudes.]
+
+[Footnote 2: Betelgeuse--commonly interpreted the Giant's
+Shoulder--_ibt-al-jauza_. The words, however, really signify, "the
+armpit of the central one," Orion being so named because he is divided
+centrally by the equator.]
+
+[Footnote 3: I have never been able to see more than four with a
+3-3/4-inch aperture. I give a view of the trapezium as seen with an
+8-inch equatorial.]
+
+[Footnote 4: Sir W. Herschel several times saw [epsilon] Lyræ as a
+double. Bessel also relates that when he was a lad of thirteen he could
+see this star double. I think persons having average eye-sight could see
+it double if they selected a suitable hour for observation. My own
+eye-sight is not good enough for this, but I can distinctly see this
+star wedged whenever the line joining the components is inclined about
+45° to the horizon, and also when Lyra is near the zenith.]
+
+[Footnote 5: They were so described by Admiral Smyth in 1839. Mr. Main,
+in 1862, describes them as straw-coloured and reddish, while Mr. Webb,
+in 1865, saw them pale-yellow and _lilac_!]
+
+[Footnote 6: Or the observer may sweep from [omicron] towards [nu],
+looking for R about two-fifths of the way from [omicron] to [nu].]
+
+[Footnote 7: Here a single period only is taken, to get back to a
+convenient hour of the evening.]
+
+[Footnote 8: Here a single period only is taken, to get back to a
+convenient hour of the evening.]
+
+[Footnote 9: I have constructed a zodiac-chart, which will enable the
+student to mark in the path of a planet, at any season of the year, from
+the recorded places in the almanacs.]
+
+[Footnote 10: It is convenient to remember that through precession a
+star near the ecliptic shifts as respects the R.A. and Dec. lines,
+through an arc of one degree--or nearly twice the moon's diameter--in
+about 72 years, all other stars through a less arc.]
+
+[Footnote 11: Mercury is best seen when in quadrature to the sun, but
+_not_ (as I have seen stated) at those quadratures in which he attains
+his maximum elongation from the sun. This will appear singular, because
+the maximum elongation is about 27°, the minimum only about 18°. But it
+happens that in our northern latitudes Mercury is always _south_ of the
+sun when he attains his maximum elongation, and this fact exercises a
+more important effect than the mere amount of elongation.]
+
+[Footnote 12: It does not seem to me that the difficulty of detecting
+Mercury is due to the difficulty "of identifying it amongst the
+surrounding stars, during the short time that it can be seen" (Hind's
+'Introduction to Astronomy'). There are few stars which are comparable
+with Mercury in brilliancy, when seen under the same light.]
+
+[Footnote 13: I may notice another error sometimes made. It is said that
+the shadow of a satellite _appears_ elliptical when near the edge of the
+disc. The shadow is _in reality_ elliptical when thus situated, but
+_appears_ circular. A moment's consideration will show that this should
+be so. The part of the disc concealed by a _satellite_ near the limb is
+also elliptical, but of course appears round.]
+
+[Footnote 14: From a paper by Mr. Breen, in the 'Popular Science
+Review,' October, 1864.]
+
+[Footnote 15: 'Intellectual Observer' for July, 1867, to which magazine
+the reader is referred for full details of Mr. Howlett's method of
+observation, and for illustrations of the appliances he made use of, and
+of some of his results.]
+
+[Footnote 16: As the sun does not attain such an altitude as 18° during
+two months in the year, it is well to notice that the true length of the
+sun's apparent solar diameter is determinable even immediately after
+sun-rise, if the line of graduation is made to coincide with the
+_horizontal_ diameter of the picture on the screen--for refraction does
+not affect the length of this diameter.]
+
+[Footnote 17: The words "Left" and "Right" indicate which end of the
+sun's ecliptical diameter is uppermost at the dates in upper or lower
+row respectively.]
+
+
+
+
+LONDON:
+
+PRINTED BY W. CLOWES AND SONS, DUKE STREET, STAMFORD STREET, AND CHARING
+CROSS.
+
+
+
+
+
+End of the Project Gutenberg EBook of Half-hours with the Telescope
+by Richard A. Proctor
+
+*** END OF THIS PROJECT GUTENBERG EBOOK HALF-HOURS WITH THE TELESCOPE ***
+
+***** This file should be named 16767-8.txt or 16767-8.zip *****
+This and all associated files of various formats will be found in:
+        https://www.gutenberg.org/1/6/7/6/16767/
+
+Produced by Jason Isbell and the Online Distributed
+Proofreading Team at https://www.pgdp.net
+
+
+Updated editions will replace the previous one--the old editions
+will be renamed.
+
+Creating the works from public domain print editions 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.  Special rules,
+set forth in the General Terms of Use part of this license, apply to
+copying and distributing Project Gutenberg-tm electronic works to
+protect the PROJECT GUTENBERG-tm concept and trademark.  Project
+Gutenberg is a registered trademark, and may not be used if you
+charge for the eBooks, unless you receive specific permission.  If you
+do not charge anything for copies of this eBook, complying with the
+rules is very easy.  You may use this eBook for nearly any purpose
+such as creation of derivative works, reports, performances and
+research.  They may be modified and printed and given away--you may do
+practically ANYTHING with public domain eBooks.  Redistribution is
+subject to the trademark license, especially commercial
+redistribution.
+
+
+
+*** START: FULL LICENSE ***
+
+THE FULL PROJECT GUTENBERG LICENSE
+PLEASE READ THIS BEFORE YOU DISTRIBUTE OR USE THIS WORK
+
+To protect the Project Gutenberg-tm mission of promoting the free
+distribution of electronic works, by using or distributing this work
+(or any other work associated in any way with the phrase "Project
+Gutenberg"), you agree to comply with all the terms of the Full Project
+Gutenberg-tm License (available with this file or online at
+https://gutenberg.org/license).
+
+
+Section 1.  General Terms of Use and Redistributing Project Gutenberg-tm
+electronic works
+
+1.A.  By reading or using any part of this Project Gutenberg-tm
+electronic work, you indicate that you have read, understand, agree to
+and accept all the terms of this license and intellectual property
+(trademark/copyright) agreement.  If you do not agree to abide by all
+the terms of this agreement, you must cease using and return or destroy
+all copies of Project Gutenberg-tm electronic works in your possession.
+If you paid a fee for obtaining a copy of or access to a Project
+Gutenberg-tm electronic work and you do not agree to be bound by the
+terms of this agreement, you may obtain a refund from the person or
+entity to whom you paid the fee as set forth in paragraph 1.E.8.
+
+1.B.  "Project Gutenberg" is a registered trademark.  It may only be
+used on or associated in any way with an electronic work by people who
+agree to be bound by the terms of this agreement.  There are a few
+things that you can do with most Project Gutenberg-tm electronic works
+even without complying with the full terms of this agreement.  See
+paragraph 1.C below.  There are a lot of things you can do with Project
+Gutenberg-tm electronic works if you follow the terms of this agreement
+and help preserve free future access to Project Gutenberg-tm electronic
+works.  See paragraph 1.E below.
+
+1.C.  The Project Gutenberg Literary Archive Foundation ("the Foundation"
+or PGLAF), owns a compilation copyright in the collection of Project
+Gutenberg-tm electronic works.  Nearly all the individual works in the
+collection are in the public domain in the United States.  If an
+individual work is in the public domain in the United States and you are
+located in the United States, we do not claim a right to prevent you from
+copying, distributing, performing, displaying or creating derivative
+works based on the work as long as all references to Project Gutenberg
+are removed.  Of course, we hope that you will support the Project
+Gutenberg-tm mission of promoting free access to electronic works by
+freely sharing Project Gutenberg-tm works in compliance with the terms of
+this agreement for keeping the Project Gutenberg-tm name associated with
+the work.  You can easily comply with the terms of this agreement by
+keeping this work in the same format with its attached full Project
+Gutenberg-tm License when you share it without charge with others.
+
+1.D.  The copyright laws of the place where you are located also govern
+what you can do with this work.  Copyright laws in most countries are in
+a constant state of change.  If you are outside the United States, check
+the laws of your country in addition to the terms of this agreement
+before downloading, copying, displaying, performing, distributing or
+creating derivative works based on this work or any other Project
+Gutenberg-tm work.  The Foundation makes no representations concerning
+the copyright status of any work in any country outside the United
+States.
+
+1.E.  Unless you have removed all references to Project Gutenberg:
+
+1.E.1.  The following sentence, with active links to, or other immediate
+access to, the full Project Gutenberg-tm License must appear prominently
+whenever any copy of a Project Gutenberg-tm work (any work on which the
+phrase "Project Gutenberg" appears, or with which the phrase "Project
+Gutenberg" is associated) is accessed, displayed, performed, viewed,
+copied or distributed:
+
+This eBook is for the use of anyone anywhere 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
+
+1.E.2.  If an individual Project Gutenberg-tm electronic work is derived
+from the public domain (does not contain a notice indicating that it is
+posted with permission of the copyright holder), the work can be copied
+and distributed to anyone in the United States without paying any fees
+or charges.  If you are redistributing or providing access to a work
+with the phrase "Project Gutenberg" associated with or appearing on the
+work, you must comply either with the requirements of paragraphs 1.E.1
+through 1.E.7 or obtain permission for the use of the work and the
+Project Gutenberg-tm trademark as set forth in paragraphs 1.E.8 or
+1.E.9.
+
+1.E.3.  If an individual Project Gutenberg-tm electronic work is posted
+with the permission of the copyright holder, your use and distribution
+must comply with both paragraphs 1.E.1 through 1.E.7 and any additional
+terms imposed by the copyright holder.  Additional terms will be linked
+to the Project Gutenberg-tm License for all works posted with the
+permission of the copyright holder found at the beginning of this work.
+
+1.E.4.  Do not unlink or detach or remove the full Project Gutenberg-tm
+License terms from this work, or any files containing a part of this
+work or any other work associated with Project Gutenberg-tm.
+
+1.E.5.  Do not copy, display, perform, distribute or redistribute this
+electronic work, or any part of this electronic work, without
+prominently displaying the sentence set forth in paragraph 1.E.1 with
+active links or immediate access to the full terms of the Project
+Gutenberg-tm License.
+
+1.E.6.  You may convert to and distribute this work in any binary,
+compressed, marked up, nonproprietary or proprietary form, including any
+word processing or hypertext form.  However, if you provide access to or
+distribute copies of a Project Gutenberg-tm work in a format other than
+"Plain Vanilla ASCII" or other format used in the official version
+posted on the official Project Gutenberg-tm web site (www.gutenberg.org),
+you must, at no additional cost, fee or expense to the user, provide a
+copy, a means of exporting a copy, or a means of obtaining a copy upon
+request, of the work in its original "Plain Vanilla ASCII" or other
+form.  Any alternate format must include the full Project Gutenberg-tm
+License as specified in paragraph 1.E.1.
+
+1.E.7.  Do not charge a fee for access to, viewing, displaying,
+performing, copying or distributing any Project Gutenberg-tm works
+unless you comply with paragraph 1.E.8 or 1.E.9.
+
+1.E.8.  You may charge a reasonable fee for copies of or providing
+access to or distributing Project Gutenberg-tm electronic works provided
+that
+
+- You pay a royalty fee of 20% of the gross profits you derive from
+     the use of Project Gutenberg-tm works calculated using the method
+     you already use to calculate your applicable taxes.  The fee is
+     owed to the owner of the Project Gutenberg-tm trademark, but he
+     has agreed to donate royalties under this paragraph to the
+     Project Gutenberg Literary Archive Foundation.  Royalty payments
+     must be paid within 60 days following each date on which you
+     prepare (or are legally required to prepare) your periodic tax
+     returns.  Royalty payments should be clearly marked as such and
+     sent to the Project Gutenberg Literary Archive Foundation at the
+     address specified in Section 4, "Information about donations to
+     the Project Gutenberg Literary Archive Foundation."
+
+- You provide a full refund of any money paid by a user who notifies
+     you in writing (or by e-mail) within 30 days of receipt that s/he
+     does not agree to the terms of the full Project Gutenberg-tm
+     License.  You must require such a user to return or
+     destroy all copies of the works possessed in a physical medium
+     and discontinue all use of and all access to other copies of
+     Project Gutenberg-tm works.
+
+- You provide, in accordance with paragraph 1.F.3, a full refund of any
+     money paid for a work or a replacement copy, if a defect in the
+     electronic work is discovered and reported to you within 90 days
+     of receipt of the work.
+
+- You comply with all other terms of this agreement for free
+     distribution of Project Gutenberg-tm works.
+
+1.E.9.  If you wish to charge a fee or distribute a Project Gutenberg-tm
+electronic work or group of works on different terms than are set
+forth in this agreement, you must obtain permission in writing from
+both the Project Gutenberg Literary Archive Foundation and Michael
+Hart, the owner of the Project Gutenberg-tm trademark.  Contact the
+Foundation as set forth in Section 3 below.
+
+1.F.
+
+1.F.1.  Project Gutenberg volunteers and employees expend considerable
+effort to identify, do copyright research on, transcribe and proofread
+public domain works in creating the Project Gutenberg-tm
+collection.  Despite these efforts, Project Gutenberg-tm electronic
+works, and the medium on which they may be stored, may contain
+"Defects," such as, but not limited to, incomplete, inaccurate or
+corrupt data, transcription errors, a copyright or other intellectual
+property infringement, a defective or damaged disk or other medium, a
+computer virus, or computer codes that damage or cannot be read by
+your equipment.
+
+1.F.2.  LIMITED WARRANTY, DISCLAIMER OF DAMAGES - Except for the "Right
+of Replacement or Refund" described in paragraph 1.F.3, the Project
+Gutenberg Literary Archive Foundation, the owner of the Project
+Gutenberg-tm trademark, and any other party distributing a Project
+Gutenberg-tm electronic work under this agreement, disclaim all
+liability to you for damages, costs and expenses, including legal
+fees.  YOU AGREE THAT YOU HAVE NO REMEDIES FOR NEGLIGENCE, STRICT
+LIABILITY, BREACH OF WARRANTY OR BREACH OF CONTRACT EXCEPT THOSE
+PROVIDED IN PARAGRAPH F3.  YOU AGREE THAT THE FOUNDATION, THE
+TRADEMARK OWNER, AND ANY DISTRIBUTOR UNDER THIS AGREEMENT WILL NOT BE
+LIABLE TO YOU FOR ACTUAL, DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE OR
+INCIDENTAL DAMAGES EVEN IF YOU GIVE NOTICE OF THE POSSIBILITY OF SUCH
+DAMAGE.
+
+1.F.3.  LIMITED RIGHT OF REPLACEMENT OR REFUND - If you discover a
+defect in this electronic work within 90 days of receiving it, you can
+receive a refund of the money (if any) you paid for it by sending a
+written explanation to the person you received the work from.  If you
+received the work on a physical medium, you must return the medium with
+your written explanation.  The person or entity that provided you with
+the defective work may elect to provide a replacement copy in lieu of a
+refund.  If you received the work electronically, the person or entity
+providing it to you may choose to give you a second opportunity to
+receive the work electronically in lieu of a refund.  If the second copy
+is also defective, you may demand a refund in writing without further
+opportunities to fix the problem.
+
+1.F.4.  Except for the limited right of replacement or refund set forth
+in paragraph 1.F.3, this work is provided to you 'AS-IS' WITH NO OTHER
+WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO
+WARRANTIES OF MERCHANTIBILITY OR FITNESS FOR ANY PURPOSE.
+
+1.F.5.  Some states do not allow disclaimers of certain implied
+warranties or the exclusion or limitation of certain types of damages.
+If any disclaimer or limitation set forth in this agreement violates the
+law of the state applicable to this agreement, the agreement shall be
+interpreted to make the maximum disclaimer or limitation permitted by
+the applicable state law.  The invalidity or unenforceability of any
+provision of this agreement shall not void the remaining provisions.
+
+1.F.6.  INDEMNITY - You agree to indemnify and hold the Foundation, the
+trademark owner, any agent or employee of the Foundation, anyone
+providing copies of Project Gutenberg-tm electronic works in accordance
+with this agreement, and any volunteers associated with the production,
+promotion and distribution of Project Gutenberg-tm electronic works,
+harmless from all liability, costs and expenses, including legal fees,
+that arise directly or indirectly from any of the following which you do
+or cause to occur: (a) distribution of this or any Project Gutenberg-tm
+work, (b) alteration, modification, or additions or deletions to any
+Project Gutenberg-tm work, and (c) any Defect you cause.
+
+
+Section  2.  Information about the Mission of Project Gutenberg-tm
+
+Project Gutenberg-tm is synonymous with the free distribution of
+electronic works in formats readable by the widest variety of computers
+including obsolete, old, middle-aged and new computers.  It exists
+because of the efforts of hundreds of volunteers and donations from
+people in all walks of life.
+
+Volunteers and financial support to provide volunteers with the
+assistance they need, is critical to reaching Project Gutenberg-tm's
+goals and ensuring that the Project Gutenberg-tm collection will
+remain freely available for generations to come.  In 2001, the Project
+Gutenberg Literary Archive Foundation was created to provide a secure
+and permanent future for Project Gutenberg-tm and future generations.
+To learn more about the Project Gutenberg Literary Archive Foundation
+and how your efforts and donations can help, see Sections 3 and 4
+and the Foundation web page at https://www.pglaf.org.
+
+
+Section 3.  Information about the Project Gutenberg Literary Archive
+Foundation
+
+The Project Gutenberg Literary Archive Foundation is a non profit
+501(c)(3) educational corporation organized under the laws of the
+state of Mississippi and granted tax exempt status by the Internal
+Revenue Service.  The Foundation's EIN or federal tax identification
+number is 64-6221541.  Its 501(c)(3) letter is posted at
+https://pglaf.org/fundraising.  Contributions to the Project Gutenberg
+Literary Archive Foundation are tax deductible to the full extent
+permitted by U.S. federal laws and your state's laws.
+
+The Foundation's principal office is located at 4557 Melan Dr. S.
+Fairbanks, AK, 99712., but its volunteers and employees are scattered
+throughout numerous locations.  Its business office is located at
+809 North 1500 West, Salt Lake City, UT 84116, (801) 596-1887, email
+business@pglaf.org.  Email contact links and up to date contact
+information can be found at the Foundation's web site and official
+page at https://pglaf.org
+
+For additional contact information:
+     Dr. Gregory B. Newby
+     Chief Executive and Director
+     gbnewby@pglaf.org
+
+
+Section 4.  Information about Donations to the Project Gutenberg
+Literary Archive Foundation
+
+Project Gutenberg-tm depends upon and cannot survive without wide
+spread public support and donations to carry out its mission of
+increasing the number of public domain and licensed works that can be
+freely distributed in machine readable form accessible by the widest
+array of equipment including outdated equipment.  Many small donations
+($1 to $5,000) are particularly important to maintaining tax exempt
+status with the IRS.
+
+The Foundation is committed to complying with the laws regulating
+charities and charitable donations in all 50 states of the United
+States.  Compliance requirements are not uniform and it takes a
+considerable effort, much paperwork and many fees to meet and keep up
+with these requirements.  We do not solicit donations in locations
+where we have not received written confirmation of compliance.  To
+SEND DONATIONS or determine the status of compliance for any
+particular state visit https://pglaf.org
+
+While we cannot and do not solicit contributions from states where we
+have not met the solicitation requirements, we know of no prohibition
+against accepting unsolicited donations from donors in such states who
+approach us with offers to donate.
+
+International donations are gratefully accepted, but we cannot make
+any statements concerning tax treatment of donations received from
+outside the United States.  U.S. laws alone swamp our small staff.
+
+Please check the Project Gutenberg Web pages for current donation
+methods and addresses.  Donations are accepted in a number of other
+ways including including checks, online payments and credit card
+donations.  To donate, please visit: https://pglaf.org/donate
+
+
+Section 5.  General Information About Project Gutenberg-tm electronic
+works.
+
+Professor Michael S. Hart was the originator of the Project Gutenberg-tm
+concept of a library of electronic works that could be freely shared
+with anyone.  For thirty years, he produced and distributed Project
+Gutenberg-tm eBooks with only a loose network of volunteer support.
+
+
+Project Gutenberg-tm eBooks are often created from several printed
+editions, all of which are confirmed as Public Domain in the U.S.
+unless a copyright notice is included.  Thus, we do not necessarily
+keep eBooks in compliance with any particular paper edition.
+
+
+Most people start at our Web site which has the main PG search facility:
+
+     https://www.gutenberg.org
+
+This Web site includes information about Project Gutenberg-tm,
+including how to make donations to the Project Gutenberg Literary
+Archive Foundation, how to help produce our new eBooks, and how to
+subscribe to our email newsletter to hear about new eBooks.
diff --git a/16767-8.zip b/16767-8.zip
new file mode 100644
index 0000000..046e2b7
--- /dev/null
+++ b/16767-8.zip
diff --git a/16767-h.zip b/16767-h.zip
new file mode 100644
index 0000000..5c3c11d
--- /dev/null
+++ b/16767-h.zip
diff --git a/16767-h/16767-h.htm b/16767-h/16767-h.htm
new file mode 100644
index 0000000..3627c36
--- /dev/null
+++ b/16767-h/16767-h.htm
@@ -0,0 +1,4214 @@
+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
+    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
+
+<html xmlns="http://www.w3.org/1999/xhtml">
+  <head>
+    <meta http-equiv="Content-Type" content="text/html;charset=iso-8859-1" />
+    <title>
+      The Project Gutenberg eBook of Half-hours With the Telescope, by Richard A. Proctor, B.A., F.R.A.S..
+    </title>
+    <style type="text/css">
+/*<![CDATA[  XML blockout */
+<!--
+    p {  margin-top: .75em;
+         text-align: justify;
+         margin-bottom: .75em;
+	 text-indent: 1em;
+         }
+    h1,h2,h3,h4,h5,h6 {
+         text-align: center; /* all headings centered */
+         clear: both;
+         }
+    hr { width: 33%;
+	 margin-top: 2em;
+	 margin-bottom: 2em;
+         margin-left: auto;
+         margin-right: auto;
+         clear: both;
+       }
+    img {border:none;}
+
+    table {margin-left: auto; margin-right: auto; text-align: left}
+
+    body{margin-left: 10%; margin-right: 10%;   }
+
+    .blockquot{margin-left: 5%; margin-right: 10%;}
+    .pagenum  {position: absolute; left: 92%; font-size: smaller; text-align: right; color: #C0C0C0} /* page numbers */
+    a.pagenum:link, a.pagenum:visited 	      {text-decoration: none; color: #C0C0C0}
+    .bl       {border-left: solid 1px;}
+
+    .center   {text-align: center;}
+    .right   {text-align: right;}
+    .smcap    {font-variant: small-caps;}
+
+    .caption  {font-weight: bold;text-align: center;}
+    .plate    {border: dashed 1px; margin: auto; text-align: center;}
+
+    .figcenter   {margin: auto; text-align: center;}
+
+    .figleft     {float: left; clear: left;margin-left: 0; margin-bottom: 1em;
+		 margin-top: 1em; margin-right: 1em; padding: 0; text-align: center;}
+
+    .figright    {float: right; margin-left: 1em; margin-bottom: 1em;
+		 margin-top: 1em; margin-right: 0; padding: 0; text-align: center;}
+
+    .footnotes        {border: dashed 1px;}
+    .footnote         { margin-left: 5%; margin-right: 5%; font-size: 0.9em; margin-top: .75em;
+                        text-align: justify; margin-bottom: .75em; text-indent: -2em;}
+    .footnote .label  {text-decoration:  none; text-align: right;}
+    a.fnlink:link, a.footnote:visited {text-decoration: none;}
+    .fnanchor         {vertical-align: super; font-size: .8em; text-decoration: none;}
+
+    // -->
+    /* XML end  ]]>*/
+    </style>
+  </head>
+<body>
+
+
+<pre>
+
+Project Gutenberg's Half-hours with the Telescope, by Richard A. Proctor
+
+This eBook is for the use of anyone anywhere 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
+
+
+Title: Half-hours with the Telescope
+       Being a Popular Guide to the Use of the Telescope as a
+              Means of Amusement and Instruction.
+
+Author: Richard A. Proctor
+
+Release Date: September 28, 2005 [EBook #16767]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK HALF-HOURS WITH THE TELESCOPE ***
+
+
+
+
+Produced by Jason Isbell and the Online Distributed
+Proofreading Team at https://www.pgdp.net
+
+
+
+
+
+
+</pre>
+
+<div class="figcenter">
+<img src="images/front.jpg" alt="Front Cover" title="Front Cover" />
+</div>
+
+<p><span class="pagenum"><a href="#Page_i" class="pagenum">i</a></span><a name="Page_i" id="Page_i"></a></p>
+<h1>HALF-HOURS WITH THE TELESCOPE</h1>
+
+<h3>Being a popular guide to the use of the telescope
+as a means of amusement and instruction.</h3>
+
+<h4>BY</h4>
+
+<h2>Richard A. Proctor, B.A., F.R.A.S.,</h2>
+<h4>Author of "Saturn and its System," Etc.</h4>
+
+<h4>With Illustrations on Stone and Wood.</h4>
+
+
+<hr style='width: 45%;' />
+
+<p class="figcenter">An undevout astronomer is mad:<br />
+True, all things speak a God; but, in the small<br />
+Men trace out Him: in great He seizes man.<br />
+<span style="margin-left: 17em">YOUNG.</span></p>
+
+<hr style='width: 45%;' />
+
+<h5>New York:<br/>
+G.P. Putnam's Sons.<br/>
+1873.</h5>
+<p><span class="pagenum"><a href="#Page_ii" class="pagenum">ii</a></span><a name="Page_ii" id="Page_ii"></a></p>
+<h5>London:<br/>
+Printed by William Clowes and Sons, Stamford Street
+and Charing Cross.</h5>
+
+<div class="plate">
+<a name="plate_I" id="plate_I"></a>
+<span class="caption">Plate I.<br />
+Fronticepeice</span>
+
+<table summary="display of plates">
+ <tr>
+  <td>
+   <span class="caption">Map I.<br/><br/>
+   The Sky<br/>
+   Jan. 20, 10 P.M.<br/>
+   Feb. 19, 8 P.M.<br />
+   Mar. 21, 6 P.M.</span>
+  </td>
+  <td align="center"><a href="images/platei1_lg.jpg"><img src="images/platei1.jpg" alt="Map I" width="65%" /></a></td>
+ </tr>
+ <tr>
+  <td>
+   <span class="caption">Map II.<br/><br/>
+   The Sky<br/>
+   Apr. 20, 10 P.M.<br/>
+   May 21, 8 P.M.<br />
+   Jun. 21, 6 P.M.</span>
+  </td>
+  <td align="center"><a href="images/platei2_lg.jpg"><img src="images/platei2.jpg" alt="Map II" width="65%" /></a></td>
+ </tr>
+ <tr>
+  <td>
+   <span class="caption">Map III.<br/><br/>
+   The Sky<br/>
+   Jul. 22, 10 P.M.<br/>
+   Aug. 23, 8 P.M.<br />
+   Sep. 23, 6 P.M.</span>
+  </td>
+  <td align="center"><a href="images/platei3_lg.jpg"><img src="images/platei3.jpg" alt="Map III" width="65%" /></a></td>
+ </tr>
+ <tr>
+  <td>
+   <span class="caption">Map IV.<br/><br/>
+   The Sky<br/>
+   Oct. 23, 10 P.M.<br/>
+   Nov. 22, 8 P.M.<br />
+   Dec. 21, 6 P.M.</span>
+  </td>
+  <td align="center"><a href="images/platei4_lg.jpg"><img src="images/platei4.jpg" alt="Map IV" width="65%" /></a></td>
+ </tr>
+</table>
+
+</div>
+
+<hr style="width: 65%;" /><p><span class="pagenum"><a href="#Page_iii" class="pagenum">iii</a></span><a name="Page_iii" id="Page_iii"></a></p>
+<h2><a name="PREFACE" id="PREFACE"></a>PREFACE.</h2>
+<div class="figcenter">
+<img src="images/hr.jpg" alt="Horizontal Rule" title="Horizontal Rule" />
+</div>
+
+<p>The object which the Author and Publisher of this little work have
+proposed to themselves, has been the production, at a moderate price, of
+a useful and reliable guide to the amateur telescopist.</p>
+
+<p>Among the celestial phenomena described or figured in this treatise, by
+far the larger number may be profitably examined with small telescopes,
+and there are none which are beyond the range of a good 3-inch
+achromatic.</p>
+
+<p>The work also treats of the construction of telescopes, the nature and
+use of star-maps, and other subjects connected with the requirements of
+amateur observers.</p>
+
+<p class="right">R.A.P.</p>
+
+<p><i>January</i>, 1868.</p>
+
+
+
+<hr style="width: 65%;" /><p><span class="pagenum"><a href="#Page_iv" class="pagenum">iv</a></span><a name="Page_iv" id="Page_iv"></a></p>
+<h2><a name="CONTENTS" id="CONTENTS"></a>CONTENTS.</h2>
+<div class="figcenter">
+<img src="images/hr.jpg" alt="Horizontal Rule" title="Horizontal Rule" />
+</div>
+<table border="0" cellpadding="4" cellspacing="0" summary="Table of Contents">
+<tr>
+  <td></td><td class="right">PAGE</td>
+</tr>
+<tr>
+  <td><a href="#CHAPTER_I"><b>CHAPTER I.</b></a></td>
+</tr>
+<tr>
+  <td>A HALF-HOUR ON THE STRUCTURE OF THE TELESCOPE</td><td class="right">1</td>
+</tr>
+<tr>
+  <td><a href="#CHAPTER_II"><b>CHAPTER II.</b></a></td>
+</tr>
+<tr>
+  <td>A HALF-HOUR WITH ORION, LEPUS, TAURUS, ETC.</td><td class="right">33</td>
+</tr>
+<tr>
+  <td><a href="#CHAPTER_III"><b>CHAPTER III.</b></a></td>
+</tr>
+<tr>
+  <td>A HALF-HOUR WITH LYRA, HERCULES, CORVUS, CRATER, ETC.</td><td class="right">47</td>
+</tr>
+<tr>
+  <td><a href="#CHAPTER_IV"><b>CHAPTER IV.</b></a></td>
+</tr>
+<tr>
+  <td>A HALF-HOUR WITH BOOTES, SCORPIO, OPHIUCHUS, ETC.</td><td class="right">56</td>
+</tr>
+<tr>
+  <td><a href="#CHAPTER_V"><b>CHAPTER V.</b></a></td>
+</tr>
+<tr>
+  <td>A HALF-HOUR WITH ANDROMEDA, CYGNUS, ETC.</td><td class="right">66</td>
+</tr>
+<tr>
+  <td><a href="#CHAPTER_VI"><b>CHAPTER VI.</b></a></td>
+</tr>
+<tr>
+  <td>HALF-HOURS WITH THE PLANETS</td><td class="right">74</td>
+</tr>
+<tr>
+  <td><a href="#CHAPTER_VII"><b>CHAPTER VII.</b></a></td>
+</tr>
+<tr>
+  <td>HALF-HOURS WITH THE SUN AND MOON</td><td class="right">93</td>
+</tr>
+</table>
+
+
+<hr style="width: 65%;" /><p><span class="pagenum"><a href="#Page_v" class="pagenum">v</a></span><a name="Page_v" id="Page_v"></a></p>
+<h2><a name="DESCRIPTION_OF_PLATES" id="DESCRIPTION_OF_PLATES"></a>DESCRIPTION OF PLATES.</h2>
+
+<div class="figcenter">
+<img src="images/hr.jpg" alt="Horizontal Rule" title="Horizontal Rule" />
+</div>
+<h3>PLATE <a href="#plate_I">I.</a>&mdash;<i>Frontispiece.</i></h3>
+
+<p>This plate presents the aspect of the heavens at the four seasons, dealt
+with in Chapters <a href="#CHAPTER_II">II.</a>, <a href="#CHAPTER_III">III.</a>,
+<a href="#CHAPTER_IV">IV.</a>, and <a href="#CHAPTER_V">V.</a> In each map of this plate the
+central point represents the point vertically over the observer's head,
+and the circumference represents his horizon. The plan of each map is
+such that the direction of a star or constellation, as respects the
+compass-points, and its elevation, also, above the horizon, at the given
+season, can be at once determined. Two illustrations of the use of the
+maps will serve to explain their nature better than any detailed
+description. Suppose first, that&mdash;at one of the hours named under Map
+I.&mdash;the observer wishes to find Castor and Pollux:&mdash;Turning to Map I. he
+sees that these stars lie in the lower left-hand quadrant, and very
+nearly towards the point marked S.E.; that is, they are to be looked for
+on the sky towards the south-east. Also, it is seen that the two stars
+lie about one-fourth of the way from the centre towards the
+circumference. Hence, on the sky, the stars will be found about
+one-fourth of the way from the zenith towards the horizon: Castor will
+be seen immediately above Pollux. Next, suppose that at one of the hours
+named the observer wishes to learn what stars are visible towards the
+west and north-west:&mdash;Turning the map until the portion of the
+circumference marked W ... N.W. is lowermost, he sees that in the
+direction named the square of Pegasus lies not very high above the
+horizon, one diagonal of the square being vertical, the other nearly
+horizontal. Above the square is <span class="pagenum"><a href="#Page_vi" class="pagenum">vi</a></span><a name="Page_vi" id="Page_vi"></a>Andromeda, to the right of which lies
+Cassiopeia, the stars &beta; and &epsilon; of this constellation lying
+directly towards the north-west, while the star &alpha; lies almost
+exactly midway between the zenith and the horizon. Above Andromeda, a
+little towards the left, lies Perseus, Algol being almost exactly
+towards the west and one-third of the way from the zenith towards the
+horizon (because one-third of the way from the centre towards the
+circumference of the map). Almost exactly in the zenith is the star
+&delta; Aurig&aelig;.</p>
+
+<p>The four maps are miniatures of Maps I., IV., VII., and X. of my
+'Constellation Seasons,' fourth-magnitude stars, however, being omitted.</p>
+
+
+<h3>PLATES <a href="#plate_II">II.</a>, <a href="#plate_III">III.</a>, <a href="#plate_IV">IV.</a>, and <a href="#plate_V">V.</a>, illustrating Chapters <a href="#CHAPTER_II">II.</a>, <a href="#CHAPTER_III">III.</a>,
+<a href="#CHAPTER_IV">IV.</a>, and <a href="#CHAPTER_V">V.</a></h3>
+
+<p>Plates <a href="#plate_II">II.</a> and <a href="#plate_IV">IV.</a> contain four star-maps. They not only serve to
+indicate the configuration of certain important star-groups, but they
+illustrate the construction of maps, such as the observer should make
+for himself when he wishes to obtain an accurate knowledge of particular
+regions of the sky. They are all made to one scale, and on the conical
+projection&mdash;the simplest and best of all projections for maps of this
+sort. The way in which the meridians and parallels for this projection
+are laid down is described in my 'Handbook of the Stars.' With a little
+practice a few minutes will suffice for sweeping out the equidistant
+circular arcs which mark the parallels and ruling in the straight
+meridians.</p>
+
+<p>The dotted line across three of the maps represents a portion of the
+horizontal circle midway between the zenith and the horizon at the hour
+at which the map is supposed to be used. At other hours, of course, this
+line would be differently situated.</p>
+
+<p>Plates <a href="#plate_III">III.</a> and <a href="#plate_V">V.</a> represent fifty-two of the objects mentioned in the
+above-named chapters. As reference is made to these figures in the text,
+little comment is here required. It is to be remarked, however, that the
+circles, and especially <span class="pagenum"><a href="#Page_vii" class="pagenum">vii</a></span><a name="Page_vii" id="Page_vii"></a>the small circles, do not represent the whole
+of the telescope's field of view, only a small portion of it. The object
+of these figures is to enable the observer to know what to expect when
+he turns his telescope towards a difficult double star. Many of the
+objects depicted are very easy doubles: these are given as objects of
+reference. The observer having seen the correspondence between an easy
+double and its picture, as respects the relation between the line
+joining the components and the apparent path of the double across the
+telescope's field of view, will know how to interpret the picture of a
+difficult double in this respect. And as all the small figures are drawn
+to one scale, he will also know how far apart he may expect to find the
+components of a difficult double. Thus he will have an exact conception
+of the sort of duplicity he is to look for, and this is&mdash;<i>crede
+experto</i>&mdash;a great step towards the detection of the star's duplicity.</p>
+
+
+<h3>PLATES <a href="#plate_VI">VI.</a> and <a href="#plate_VII">VII.</a>, illustrating Chapters <a href="#CHAPTER_VI">VI.</a> and <a href="#CHAPTER_VII">VII.</a></h3>
+
+<p>The views of Mercury, Venus, and Mars in these plates (except the
+smaller view of Jupiter in Plate <a href="#plate_VII">VII.</a>) are supposed to be seen with the
+same "power."</p>
+
+<p>The observer must not expect to see the details presented in the views
+of Mars with anything like the distinctness I have here given to them.
+If he place the plate at a distance of six or seven yards he will see
+the views more nearly as Mars is likely to appear in a good three-inch
+aperture.</p>
+
+<p>The chart of Mars is a reduction of one I have constructed from views by
+Mr. Dawes. I believe that nearly all the features included in the chart
+are permanent, though not always visible. I take this opportunity of
+noting that the eighteen orthographic pictures of Mars presented with my
+shilling chart are to be looked on rather as maps than as representing
+telescopic views. They illustrate usefully the varying presentation of
+Mars towards the earth. The observer can obtain other such illustrations
+for himself by filling in outlines, traced from those given at the foot
+of Plate <a href="#plate_VI">VI.</a>, <span class="pagenum"><a href="#Page_viii" class="pagenum">viii</a></span><a name="Page_viii" id="Page_viii"></a>with details from the chart. It is to be noted that Mars
+varies in presentation, not only as respects the greater or less opening
+out of his equator towards the north or south, but as respects the
+apparent slope of his polar axis to the right or left. The four
+projections as shown, or inverted, or seen from the back of the plate
+(held up to the light) give presentations of Mars towards the sun at
+twelve periods of the Martial year,&mdash;viz., at the autumnal and vernal
+equinoxes, at the two solstices, and at intermediate periods
+corresponding to our terrestrial months.</p>
+
+<p>In fact, by means of these projections one might readily form a series
+of sun-views of Mars resembling my 'Sun-views of the Earth.'</p>
+
+<p>In the first view of Jupiter it is to be remarked that the three
+satellites outside the disc are supposed to be moving in directions
+appreciably parallel to the belts on the disc&mdash;the upper satellites from
+right to left, the lower one from left to right. In general the
+satellites, when so near to the disc, are not seen in a straight line,
+as the three shown in the figure happen to be. Of the three spots on the
+disc, the faintest is a satellite, the neighbouring dark spot its
+shadow, the other dark spot the shadow of the satellite close to the
+planet's disc.</p>
+
+
+
+<hr style="width: 65%;" /><p><span class="pagenum"><a href="#Page_1" class="pagenum">1</a></span><a name="Page_1" id="Page_1"></a></p>
+<h2><a name="HALF-HOURS_WITH_THE_TELESCOPE" id="HALF-HOURS_WITH_THE_TELESCOPE"></a>HALF-HOURS WITH THE TELESCOPE.</h2>
+
+<div class="figcenter">
+<img src="images/hr.jpg" alt="Horizontal Rule" title="Horizontal Rule" />
+</div>
+
+<h2><a name="CHAPTER_I" id="CHAPTER_I"></a>CHAPTER I.</h2>
+
+<h3>A HALF-HOUR ON THE STRUCTURE OF THE TELESCOPE.</h3>
+
+
+<p>There are few instruments which yield more pleasure and instruction than
+the Telescope. Even a small telescope&mdash;only an inch and a half or two
+inches, perhaps, in aperture&mdash;will serve to supply profitable amusement
+to those who know how to apply its powers. I have often seen with
+pleasure the surprise with which the performance even of an opera-glass,
+well steadied, and directed towards certain parts of the heavens, has
+been witnessed by those who have supposed that nothing but an expensive
+and colossal telescope could afford any views of interest. But a
+well-constructed achromatic of two or three inches in aperture will not
+merely supply amusement and instruction,&mdash;it may be made to do useful
+work.</p>
+
+<p>The student of astronomy is often deterred from telescopic observation
+by the thought that in a field wherein so many have laboured, with
+abilities and means perhaps far surpassing those he may possess, he is
+little likely to reap results of any utility. He argues that, since the
+planets, stars, and nebul&aelig; have been scanned by Herschel and Rosse, with
+their gigantic mirrors, and at Pulkova and Greenwich with refractors
+whose construction has taxed <span class="pagenum"><a href="#Page_2" class="pagenum">2</a></span><a name="Page_2" id="Page_2"></a>to the utmost the ingenuity of the
+optician and mechanic, it must be utterly useless for an unpractised
+observer to direct a telescope of moderate power to the examination of
+these objects.</p>
+
+<p>Now, passing over the consideration that a small telescope may afford
+its possessor much pleasure of an intellectual and elevated character,
+even if he is never able by its means to effect original discoveries,
+two arguments may be urged in favour of independent telescopic
+observation. In the first place, the student who wishes to appreciate
+the facts and theories of astronomy should familiarize himself with the
+nature of that instrument to which astronomers have been most largely
+indebted. In the second place, some of the most important discoveries in
+astronomy have been effected by means of telescopes of moderate power
+used skilfully and systematically. One instance may suffice to show what
+can be done in this way. The well-known telescopist Goldschmidt (who
+commenced astronomical observation at the age of forty-eight, in 1850)
+added fourteen asteroids to the solar system, not to speak of important
+discoveries of nebul&aelig; and variable stars, by means of a telescope only
+five feet in focal length, mounted on a movable tripod stand.</p>
+
+<p>The feeling experienced by those who look through a telescope for the
+first time,&mdash;especially if it is directed upon a planet or nebula&mdash;is
+commonly one of disappointment. They have been told that such and such
+powers will exhibit Jupiter's belts, Saturn's rings, and the
+continent-outlines on Mars; yet, though perhaps a higher power is
+applied, they fail to detect these appearances, and can hardly believe
+that they are perfectly distinct to the practised eye.</p>
+
+<p>The expectations of the beginner are especially <span class="pagenum"><a href="#Page_3" class="pagenum">3</a></span><a name="Page_3" id="Page_3"></a>liable to
+disappointment in one particular. He forms an estimate of the view he is
+to obtain of a planet by multiplying the apparent diameter of the planet
+by the magnifying power of his telescope, and comparing the result with
+the apparent diameter of the sun or moon. Let us suppose, for instance,
+that on the day of observation Jupiter's apparent diameter is 45", and
+that the telescopic power applied is 40, then in the telescope Jupiter
+should appear to have a diameter of 1800", or half a degree, which is
+about the same as the moon's apparent diameter. But when the observer
+looks through the telescope he obtains a view&mdash;interesting, indeed, and
+instructive&mdash;but very different from what the above calculation would
+lead him to expect. He sees a disc apparently much smaller than the
+moon's, and not nearly so well-defined in outline; in a line with the
+disc's centre there appear three or four minute dots of light, the
+satellites of the planet; and, perhaps, if the weather is favourable and
+the observer watchful, he will be able to detect faint traces of belts
+across the planet's disc.</p>
+
+<p>Yet in such a case the telescope is not in fault. The planet really
+appears of the estimated size. In fact, it is often possible to prove
+this in a very simple manner. If the observer wait until the planet and
+the moon are pretty near together, he will find that it is possible to
+view the planet with one eye through the telescope and the moon with the
+unaided eye, in such a manner that the two discs may coincide, and thus
+their relative apparent dimensions be at once recognised. Nor should the
+indistinctness and incompleteness of the view be attributed to
+imperfection of the telescope; they are partly due to the nature of the
+observation and the low power employed, and partly to the inexperience
+of the beginner.</p>
+
+<p><span class="pagenum"><a href="#Page_4" class="pagenum">4</a></span><a name="Page_4" id="Page_4"></a>It is to such a beginner that the following pages are specially
+addressed, with the hope of affording him aid and encouragement in the
+use of one of the most enchanting of scientific instruments,&mdash;an
+instrument that has created for astronomers a new sense, so to speak, by
+which, in the words of the ancient poet:</p>
+
+<p><span style="margin-left: 2em;">Subjecere oculis distantia sidera nostris,</span><br />
+<span style="margin-left: 3em;">&AElig;theraque ingenio supposuere suo.</span></p>
+
+<p>In the first place, it is necessary that the beginner should rightly
+know what is the nature of the instrument he is to use. And this is the
+more necessary because, while it is perfectly easy to obtain such
+knowledge without any profound acquaintance with the science of optics,
+yet in many popular works on this subject the really important points
+are omitted, and even in scientific works such points are too often left
+to be gathered from a formula. When the observer has learnt what it is
+that his instrument is actually to do for him, he will know how to
+estimate its performance, and how to vary the application of its
+powers&mdash;whether illuminating or magnifying&mdash;according to the nature of
+the object to be observed.</p>
+
+<p>Let us consider what it is that limits the range of <i>natural</i> vision
+applied to distant objects. What causes an object to become invisible as
+its distance increases? Two things are necessary that an object should
+be visible. It must be <i>large</i> enough to be appreciated by the eye, and
+it must <i>send light</i> enough. Thus increase of distance may render an
+object invisible, either through diminution of its apparent size, or
+through diminution in the quantity of light it sends to the eye, or
+through both these causes combined. A telescope, therefore, or (as its
+name implies) an instrument to render <span class="pagenum"><a href="#Page_5" class="pagenum">5</a></span><a name="Page_5" id="Page_5"></a>distant objects visible, must be
+both a magnifying and an illuminating instrument.</p>
+
+<p>
+<a name="fig_1" id="fig_1"></a>
+<span class="figright">
+<a href="images/fig01.jpg"><img src="images/fig01.jpg" width="85px" alt="Fig. 1." title="Figure 1" /></a>
+<br/><span class="caption">Fig. 1.</span>
+</span></p>
+
+<p>Let EF, <a href="#fig_1">fig. 1</a>, be an object, not near to AB as in the figure, but so
+far off that the bounding lines from A and B would meet at the point
+corresponding to the point P. Then if a large convex glass AB (called an
+<i>object-glass</i>) be interposed between the object and the eye, all those
+rays which, proceeding from P, fall on AB, will be caused to converge
+nearly to a point <i>p</i>. The same is true for every point of the object
+EMF, and thus a small image, <i>emf</i>, will be formed. This image will not
+lie exactly on a flat surface, but will be curved about the point midway
+between A and B as a centre. Now if the lens AB is removed, and an eye
+is placed at <i>m</i> to view the distant object EMF, those rays only from
+each point of the object which fall on the pupil of the eye (whose
+diameter is about equal to <i>mp</i> suppose) will serve to render the object
+visible. On the other hand, every point of the image <i>emf</i> has received
+the whole of the light gathered up by the large glass AB. If then we can
+only make this light <i>available</i>, it is clear that we shall have
+acquired a large increase of <i>light</i> from the distant object. Now it
+will be noticed that the light which has converged to <i>p</i>, diverges from
+<i>p</i> so that an eye, placed that this diverging pencil of rays may fall
+upon it, <span class="pagenum"><a href="#Page_6" class="pagenum">6</a></span><a name="Page_6" id="Page_6"></a>would be too small to receive the whole of the pencil. Or, if
+it did receive the whole of this pencil, it clearly could not receive
+the whole of the pencils proceeding from other parts of the image <i>emf</i>.
+<i>Something</i> would be gained, though, even in this case, since it is
+clear that an eye thus placed at a distance of ten inches from <i>emf</i>
+(which is about the average distance of distinct vision) would not only
+receive much more light from the image <i>emf</i>, than it would from the
+object EMF, but see the image much larger than the object. It is in this
+way that a simple object-glass forms a telescope, a circumstance we
+shall presently have to notice more at length. But we want to gain the
+full benefit of the light which has been gathered up for us by our
+object-glass. We therefore interpose a small convex glass <i>ab</i> (called
+an eye-glass) between the image and the eye, at such a distance from the
+image that the divergent pencil of rays is converted into a pencil of
+parallel or nearly parallel rays. Call this an emergent pencil. Then all
+the emergent pencils now converge to a point on the axial line <i>m</i> M
+(produced beyond <i>m</i>), and an eye suitably placed can take in all of
+them at once. Thus the whole, or a large part, of the image is seen at
+once. But the image is seen inverted as shown. This is the Telescope, as
+it was first discovered, and such an arrangement would now be called a
+<i>simple astronomical Telescope</i>.</p>
+
+<p>Let us clearly understand what each part of the astronomical telescope
+does for us:&mdash;</p>
+
+<p>The object-glass AB gives us an illuminated image, the amount of
+illumination depending on the size of the object-glass. The eye-glass
+enables us to examine the image microscopically.</p>
+
+<p>We may apply eye-glasses of different focal length. It is clear that the
+shorter the focal length <span class="pagenum"><a href="#Page_7" class="pagenum">7</a></span><a name="Page_7" id="Page_7"></a>of <i>ab</i>, the nearer must <i>ab</i> be placed to the
+image, and the smaller will the emergent pencils be, but the greater the
+magnifying power of the eye-glass. If the emergent pencils are severally
+larger than the pupil of the eye, light is wasted at the expense of
+magnifying power. Therefore the eye-glass should never be of greater
+focal length than that which makes the emergent pencils about equal in
+diameter to the pupil of the eye. On the other hand, the eye-glass must
+not be of such small focal length that the image appears indistinct and
+contorted, or dull for want of light.</p>
+
+<p><a name="fig_2" id="fig_2"></a><span class="figleft">
+<a href="images/fig02.jpg"><img src="images/fig02.jpg" title="Fig. 2." alt="Figure 2" width="90px" /></a>
+<br/><span class="caption">Fig. 2.</span>
+</span></p>
+
+
+<p>Let us compare with the arrangement exhibited in <a href="#fig_1">fig. 1</a> that adopted by
+Galileo. Surprise is sometimes expressed that this instrument, which in
+the hands of the great Florentine astronomer effected so much, should
+now be known as the <i>non-astronomical Telescope</i>. I think this will be
+readily understood when we compare the two arrangements.</p>
+
+<p>In the Galilean Telescope a small concave eye-glass, <i>ab</i> (<a href="#fig_2">fig. 2</a>), is
+placed between the object-glass and the image. In fact, no image is
+allowed to be formed in this arrangement, but the convergent pencils are
+intercepted by the concave eye-glass, and converted into parallel
+emergent pencils. Now in <a href="#fig_2">fig. 2</a> the concave eye-glass is so placed as to
+receive <span class="pagenum"><a href="#Page_8" class="pagenum">8</a></span><a name="Page_8" id="Page_8"></a>only a part of the convergent pencil A <i>p</i> B, and this is the
+arrangement usually adopted. By using a concave glass of shorter focus,
+which would therefore be placed nearer to <i>m p</i>, the whole of the
+convergent pencil might be received in this as in the former case. But
+then the axis of the emergent pencil, instead of returning (as we see it
+in <a href="#fig_1">fig. 1</a>) <i>towards</i> the axis of the telescope, would depart as much
+<i>from</i> that axis. Thus there would be no point on the axis at which the
+eye could be so placed as to receive emergent pencils showing any
+considerable part of the object. The difference may be compared to that
+between looking through the small end of a cone-shaped roll of paper and
+looking through the large end; in the former case the eye sees at once
+all that is to be seen through the roll (supposed fixed in position), in
+the latter the eye may be moved about so as to command the same range of
+view, but <i>at any instant</i> sees over a much smaller range.</p>
+
+<p>To return to the arrangement actually employed, which is illustrated by
+the common opera-glass. We see that the full illuminating power of the
+telescope is not brought into play. But this is not the only objection
+to the Galilean Telescope. It is obvious that if the part C D of the
+object-glass were covered, the point P would not be visible, whereas, in
+the astronomical arrangement no other effect is produced on the
+visibility of an object, by covering part of the object-glass, than a
+small loss of illumination. In other words, the dimensions of the field
+of view of a Galilean Telescope depend on the size of the object-glass,
+whereas in the astronomical Telescope the field of view is independent
+of the size of the object-glass. The difference may be readily tested.
+If we direct an opera-glass upon any object, we shall find that any
+covering placed over a part of the object-glass <i>becomes visible</i> when
+we look through <span class="pagenum"><a href="#Page_9" class="pagenum">9</a></span><a name="Page_9" id="Page_9"></a>the instrument, interfering therefore <i>pro tanto</i> with
+the range of view. A covering similarly placed on any part of the
+object-glass of an astronomical telescope does not become visible when
+we look through the instrument. The distinction has a very important
+bearing on the theory of telescopic vision.</p>
+
+<p>In considering the application of the telescope to practical
+observation, the circumstance that in the Galilean Telescope no real
+image is formed, is yet more important. A real image admits of
+measurement, linear or angular, while to a <i>virtual</i> image (such an
+image, for instance, as is formed by a common looking-glass) no such
+process can be applied. In simple observation the only noticeable effect
+of this difference is that, whereas in the astronomical Telescope a
+<i>stop</i> or diaphragm can be inserted in the tube so as to cut off what is
+called the <i>ragged edge</i> of the field of view (which includes all the
+part not reached by <i>full pencils of light</i> from the object-glass),
+there is no means of remedying the corresponding defect in the Galilean
+Telescope. It would be a very annoying defect in a telescope intended
+for astronomical observation, since in general the edge of the field of
+view is not perceptible at night. The unpleasant nature of the defect
+may be seen by looking through an opera-glass, and noticing the gradual
+fading away of light round the circumference of the field of view.</p>
+
+<p>The properties of reflection as well as of refraction have been enlisted
+into the service of the astronomical observer. The formation of an image
+by means of a concave mirror is exhibited in <a href="#fig_3">fig. 3</a>. As the observer's
+head would be placed between the object and the mirror, if the image,
+formed as in <a href="#fig_3">fig. 3</a>, were to be microscopically examined, various
+devices are employed in the construction of reflecting telescopes to
+avoid the loss of light which would <span class="pagenum"><a href="#Page_10" class="pagenum">10</a></span><a name="Page_10" id="Page_10"></a>result&mdash;a loss which would be
+important even with the largest mirrors yet constructed. Thus, in
+Gregory's Telescope, a small mirror, having its concavity towards the
+great one, is placed in the axis of the tube and forms an image which is
+viewed through an aperture in the middle of the great mirror. A similar
+plan is adopted in Cassegrain's Telescope, a small convex mirror
+replacing the concave one. In Newton's Telescope a small inclined-plane
+reflector is used, which sends the pencil of light off at right-angles
+to the axis of the tube. In Herschel's Telescope the great mirror is
+inclined so that the image is formed at a slight distance from the axis
+of the telescope. In the two first cases the object is viewed in the
+usual or direct way, the image being erect in Gregory's and inverted in
+Cassegrain's. In the third the observer looks through the side of the
+telescope, seeing an inverted image of the object. In the last the
+observer sees the object inverted, but not altered as respects right and
+left. The last-mentioned method of viewing objects is the only one in
+which the observer's back is turned towards the object, yet this method
+is called the <i>front view</i>&mdash;apparently <i>quasi lucus a non lucendo</i>.</p>
+
+<p><span class="figright">
+<a name="fig_3" id="fig_3"></a>
+<a href="images/fig03.jpg"><img src="images/fig03.jpg" title="Fig. 3." alt="Figure 3" width="85px" /></a>
+<br/><span class="caption">Fig. 3.</span>
+</span>
+</p>
+
+<p>It appears, then, that in all astronomical Telescopes, reflecting or
+refracting, a <i>real image</i> of an object is submitted to microscopical
+examination.</p>
+
+<p><span class="pagenum"><a href="#Page_11" class="pagenum">11</a></span><a name="Page_11" id="Page_11"></a>Of this fact the possessor of a telescope may easily assure himself;
+for if the eye-glass be removed, and a small screen be placed at the
+focus of the object-glass, there will appear upon the screen a small
+picture of any object towards which the tube is turned. But the image
+may be viewed in another way which requires to be noticed. If the eye,
+placed at a distance of five or six inches from the image, be directed
+down the tube, the image will be seen as before; in fact, just as a
+single convex lens of short focus is the simplest microscope, so a
+simple convex lens of long focus is the simplest telescope.<a name="FNanchor_1_1" id="FNanchor_1_1"></a><a href="#Footnote_1_1" class="fnanchor">[1]</a> But a
+singular circumstance will immediately attract the observer's notice. A
+real picture, or the image formed on the screen as in the former case,
+can be viewed at varying distances; but when we view the image directly,
+it will be found that for distinct vision the eye must be placed almost
+exactly at a fixed distance from the image. This peculiarity is more
+important than it might be thought at first sight. In fact, it is
+essential that the observer who would rightly apply the powers of his
+telescope, or fairly test its performance, should understand in what
+respect an image formed by an object-glass or object-mirror differs from
+a real object.</p>
+
+<p>The peculiarities to be noted are the <i>curvature</i>, <i>indistinctness</i>, and
+<i>false colouring</i> of the image.</p>
+
+<p>The curvature of the image is the least important of the three defects
+named&mdash;a fortunate circum<span class="pagenum"><a href="#Page_12" class="pagenum">12</a></span><a name="Page_12" id="Page_12"></a>stance, since this defect admits neither of
+remedy nor modification. The image of a distant object, instead of lying
+in a plane, that is, forming what is technically called a <i>flat field</i>,
+forms part of a spherical surface whose centre is at the centre of the
+object-glass. Hence the centre of the field of view is somewhat nearer
+to the eye than are the outer parts of the field. The amount of
+curvature clearly depends on the extent of the field of view, and
+therefore is not great in powerful telescopes. Thus, if we suppose that
+the angular extent of the field is about half a degree (a large or
+low-power field), the centre is nearer than the boundary of the field by
+about 1-320th part only of the field's diameter.</p>
+
+<p>The indistinctness of the image is partly due to the obliquity of the
+pencils which form parts of the image, and partly to what is termed
+<i>spherical aberration</i>. The first cause cannot be modified by the
+optician's skill, and is not important when the field of view is small.
+Spherical aberration causes those parts of a pencil which fall near the
+boundary of a convex lens to converge to a nearer (<i>i.e.</i> shorter) focus
+than those which fall near the centre. This may be corrected by a proper
+selection of the forms of the two lenses which replace, in all modern
+telescopes, the single lens hitherto considered.</p>
+
+<p>The false colouring of the image is due to <i>chromatic aberration</i>. The
+pencil of light proceeding from a point, converges, not to one point,
+but to a short line of varying colour. Thus a series of coloured images
+is formed, at different distances from the object-glass. So that, if a
+screen were placed to receive the mean image <i>in focus</i>, a coloured
+fringe due to the other images (<i>out of focus, and therefore too large</i>)
+would surround the mean image.</p>
+
+<p>Newton supposed that it was impossible to get rid of this defect, and
+therefore turned his attention to the construction of reflectors. But
+the discovery <span class="pagenum"><a href="#Page_13" class="pagenum">13</a></span><a name="Page_13" id="Page_13"></a>that the <i>dispersive</i> powers of different glasses are not
+proportional to their reflective powers, supplied opticians with the
+means of remedying the defect. Let us clearly understand what is the
+discovery referred to. If with a glass prism of a certain form we
+produce a spectrum of the sun, this spectrum will be thrown a certain
+distance away from the point on which the sun's rays would fall if not
+interfered with. This distance depends on the <i>refractive</i> power of the
+glass. The spectrum will have a certain length, depending on the
+<i>dispersive</i> power of the glass. Now, if we change our prism for another
+of exactly the same shape, but made of a different kind of glass, we
+shall find the spectrum thrown to a different spot. If it appeared that
+the length of the new spectrum was increased or diminished in exactly
+the same proportion as its distance from the line of the sun's direct
+light, it would have been hopeless to attempt to remedy chromatic
+aberration. Newton took it for granted that this was so. But the
+experiments of Hall and the Dollonds showed that there is no such strict
+proportionality between the dispersive and refractive powers of
+different kinds of glass. It accordingly becomes possible to correct the
+chromatic aberration of one glass by superadding that of another.</p>
+
+<p><span class="figleft">
+<a name="fig_4" id="fig_4"></a>
+<a href="images/fig04.jpg"><img src="images/fig04.jpg" title="Fig. 4." alt="Figure 4" width="52px" /></a>
+<br/><span class="caption">Fig. 4.</span>
+</span>
+</p>
+
+<p>This is effected by combining, as shown in <a href="#fig_4">fig. 4</a>, a convex lens of
+<i>crown</i> glass with a concave lens of <i>flint</i> glass, the convex lens
+being placed nearest to the object. A little colour still remains, but
+not enough to interfere seriously with the distinctness of the image.</p>
+
+<p>But even if the image formed by the object-glass were perfect, yet this
+image, viewed <span class="pagenum"><a href="#Page_14" class="pagenum">14</a></span><a name="Page_14" id="Page_14"></a>through a single convex lens of short focus placed as in
+<a href="#fig_1">fig. 1</a>, would appear curved, indistinct, coloured, and also <i>distorted</i>,
+because viewed by pencils of light which do not pass through the centre
+of the eye-glass. These effects can be diminished (but not entirely
+removed <i>together</i>) by using an <i>eye-piece</i> consisting of two lenses
+instead of a single eye-glass. The two forms of eye-piece most commonly
+employed are exhibited in <a href="#fig_5">figs. 5</a> and <a href="#fig_6">6</a>. <a href="#fig_5">Fig. 5</a> is Huyghens' eye-piece,
+called also the <i>negative</i> eye-piece, because a real image is formed
+<i>behind</i> the <i>field-glass</i> (the lens which lies nearest to the
+object-glass). <a href="#fig_6">Fig. 6</a> represents Ramsden's eye-piece, called also the
+<i>positive</i> eye-piece, because the real image formed by the object-glass
+lies <i>in front of</i> the field-glass.</p>
+
+<p><span class="figright">
+<a name="fig_5" id="fig_5"></a>
+<a href="images/fig05.jpg"><img src="images/fig05.jpg" title="Fig. 5." alt="Figure 5" width="102px" /></a>
+<br/><span class="caption">Fig. 5.</span><br/>
+<a name="fig_6" id="fig_6"></a>
+<a href="images/fig06.jpg"><img src="images/fig06.jpg" title="Fig. 6." alt="Figure 6" width="105px" /></a>
+<br/><span class="caption">Fig. 6.</span>
+</span>
+</p>
+
+<p>The course of a slightly oblique pencil through either eye-piece is
+exhibited in the figures. The lenses are usually plano-convex, the
+convexities being turned towards the object-glass in the negative
+eye-piece, and towards each other in the positive eye-piece. Coddington
+has shown, however, that the best forms for the lenses of the negative
+eye-piece are those shown in <a href="#fig_5">fig. 5</a>.</p>
+
+<p>The negative eye-piece, being achromatic, is commonly employed in all
+observations requiring distinct vision only. But as it is clearly unfit
+for observations requiring micrometrical measurement, or reference to
+fixed lines at the focus of the object-glass, the positive eye-piece is
+used for these purposes.</p>
+
+<p><span class="pagenum"><a href="#Page_15" class="pagenum">15</a></span><a name="Page_15" id="Page_15"></a>For observing objects at great elevations the diagonal eye-tube is
+often convenient. Its construction is shown in <a href="#fig_7">fig. 7</a>. ABC is a totally
+reflecting prism of glass. The rays from the object-glass fall on the
+face AB, are totally reflected on the face BC, and emerge through the
+face AC. In using this eye-piece, it must be remembered that it
+lengthens the sliding eye-tube, which must therefore be thrust further
+in, or the object will not be seen in focus. There is an arrangement by
+which the change of direction is made to take place between the two
+glasses of the eye-piece. With this arrangement (known as the <i>diagonal
+eye-piece</i>) no adjustment of the eye-tube is required. However, for
+amateurs' telescopes the more convenient arrangement is the diagonal
+eye-tube, since it enables the observer to apply any eye-piece he
+chooses, just as with the simple sliding eye-tube.</p>
+
+<p><span class="figleft">
+<a name="fig_7" id="fig_7"></a>
+<a href="images/fig07.jpg"><img src="images/fig07.jpg" title="Fig. 7." alt="Figure 7" width="125px" /></a>
+<br/><span class="caption">Fig. 7.</span>
+</span>
+</p>
+
+<p>We come next to the important question of the <i>mounting</i> of our
+telescope.</p>
+
+<p>The best known, and, in some respects, the simplest method of
+mounting a telescope for general observation is that known as the
+<i>altitude-and-azimuth</i> mounting. In this method the telescope is
+pointed towards an object by two motions,&mdash;one giving the tube the
+required <i>altitude</i> (or elevation), the other giving it the required
+<i>azimuth</i> (or direction as respects the compass points).</p>
+
+<p>For small alt-azimuths the ordinary pillar-and-claw stand is
+sufficiently steady. For larger instru<span class="pagenum"><a href="#Page_16" class="pagenum">16</a></span><a name="Page_16" id="Page_16"></a>ments other arrangements are
+needed, both to give the telescope steadiness, and to supply slow
+movements in altitude and azimuth. The student will find no difficulty
+in understanding the arrangement of sliding-tubes and rack-work commonly
+adopted. This arrangement seems to me to be in many respects defective,
+however. The slow movement in altitude is not uniform, but varies in
+effect according to the elevation of the object observed. It is also
+limited in range; and quite a little series of operations has to be gone
+through when it is required to direct the telescope towards a new
+quarter of the heavens. However expert the observer may become by
+practice in effecting these operations, they necessarily take up some
+time (performed as they must be in the dark, or by the light of a small
+lantern), and during this time it often happens that a favourable
+opportunity for observation is lost.</p>
+
+<p>These disadvantages are obviated when the telescope is mounted in such a
+manner as is exhibited in <a href="#fig_8">fig. 8</a>, which represents a telescope of my own
+construction. The slow movement in altitude is given by rotating the rod
+<i>he</i>, the endless screw in which turns the small wheel at <i>b</i>, whose
+axle in turn bears a pinion-wheel working in the teeth of the quadrant
+<i>a</i>. The slow movement in azimuth is given in like manner by rotating
+the rod <i>h'e'</i>, the lantern-wheel at the end of which turns a
+crown-wheel on whose axle is a pinion-wheel working in the teeth of the
+circle <i>c</i>. The casings at <i>e</i> and <i>e'</i>, in which the rods <i>he</i> and
+<i>h'e'</i> respectively work, are so fastened by elastic cords that an
+upward pressure on the handle <i>h</i>, or a downward pressure on the handle
+<i>h'</i>, at once releases the endless screw or the crown-wheel
+respectively, so that the telescope can be swept at once through any
+desired <span class="pagenum"><a href="#Page_17" class="pagenum">17</a></span><a name="Page_17" id="Page_17"></a>angle in altitude or azimuth. This method of mounting has other
+advantages; the handles are conveniently situated and constant in
+position; also, as they do not work directly on the telescope, <span class="pagenum"><a href="#Page_18" class="pagenum">18</a></span><a name="Page_18" id="Page_18"></a>they can
+be turned without setting the tube in vibration.</p>
+
+<p><span class="figright">
+<a name="fig_8" id="fig_8"></a>
+<a href="images/fig08.jpg"><img src="images/fig08.jpg" title="Fig. 8." alt="Figure 8" width="200px" /></a>
+<br/><span class="caption">Fig. 8.</span>
+</span>
+</p>
+
+<p>I do not recommend the mounting to be exactly as shown in <a href="#fig_8">fig. 8</a>. That
+method is much too expensive for an alt-azimuth. But a simple
+arrangement of belted wheels in place of the toothed wheels <i>a</i> and <i>c</i>
+might very readily be prepared by the ingenious amateur telescopist; and
+I feel certain that the comfort and convenience of the arrangement would
+amply repay him for the labour it would cost him. My own
+telescope&mdash;though the large toothed-wheel and the quadrant were made
+inconveniently heavy (through a mistake of the workman who constructed
+the instrument)&mdash;worked as easily and almost as conveniently as an
+equatorial.</p>
+
+<p>Still, it is well for the observer who wishes systematically to survey
+the heavens&mdash;and who can afford the expense&mdash;to obtain a well-mounted
+<i>equatorial</i>. In this method of mounting, the main axis is directed to
+the pole of the heavens; the other axis, at right angles to the first,
+carries the telescope-tube. One of the many methods adopted for mounting
+equatorials is that exhibited&mdash;with the omission of some minor
+details&mdash;in <a href="#fig_9">fig. 9</a>. <i>a</i> is the polar axis, <i>b</i> is the axis (called the
+declination axis) which bears the telescope. The circles <i>c</i> and <i>d</i>
+serve to indicate, by means of verniers revolving with the axes, the
+motion of the telescope in right ascension and declination,
+respectively. The weight <i>w</i> serves to counterpoise the telescope, and
+the screws <i>s</i>, <i>s</i>, <i>s</i>, <i>s</i>, serve to adjust the instrument so that
+the polar axis shall be in its proper position. The advantage gained by
+the equatorial method of mounting is that only one motion is required to
+follow a star. Owing to the diurnal rotation of the earth, the stars
+appear to move uniformly in circles parallel to the celestial equator;
+and it is clear that <span class="pagenum"><a href="#Page_19" class="pagenum">19</a></span><a name="Page_19" id="Page_19"></a>a star so moving will be kept in the field of
+view, if the telescope, once directed to the star, be made to revolve
+uniformly and at a proper rate round the polar axis.</p>
+
+<p><span class="figleft">
+<a name="fig_9" id="fig_9"></a>
+<a href="images/fig09.jpg"><img src="images/fig09.jpg" title="Fig. 9." alt="Figure 9" width="177px" /></a>
+<br/><span class="caption">Fig. 9.</span>
+</span>
+</p>
+
+<p>The equatorial can be directed by means of the <span class="pagenum"><a href="#Page_20" class="pagenum">20</a></span><a name="Page_20" id="Page_20"></a>circles <i>c</i> and <i>d</i> to
+any celestial object whose right ascension and declination are known. On
+the other hand, to bring an object into the field of view of an
+alt-azimuth, it is necessary, either that the object itself should be
+visible to the naked eye, or else that the position of the object should
+be pretty accurately learned from star-maps, so that it may be picked up
+by the alt-azimuth after a little searching. A small telescope called a
+<i>finder</i> is usually attached to all powerful telescopes intended for
+general observation. The finder has a large field of view, and is
+adjusted so as to have its axis parallel to that of the large telescope.
+Thus a star brought to the centre of the large field of the finder
+(indicated by the intersection of two lines placed at the focus of the
+eye-glass) is at, or very near, the centre of the small field of the
+large telescope.</p>
+
+<p>If a telescope has no finder, it will be easy for the student to
+construct one for himself, and will be a useful exercise in optics. Two
+convex lenses not very different in size from those shown in <a href="#fig_1">fig. 1</a>, and
+placed as there shown&mdash;the distance between them being the sum of the
+focal lengths of the two glasses&mdash;in a small tube of card, wood, or tin,
+will serve the purpose of a finder for a small telescope. It can be
+attached by wires to the telescope-tube, and adjusted each night before
+commencing observation. The adjustment is thus managed:&mdash;a low power
+being applied to the telescope, the tube is turned towards a bright
+star; this is easily effected with a low power; then the finder is to be
+fixed, by means of its wires, in such a position that the star shall be
+in the centre of the field of the finder when also in the centre of the
+telescope's field. When this has been done, the finder will greatly help
+the observations of the evening; since with high powers much time would
+be wasted in bringing an object <span class="pagenum"><a href="#Page_21" class="pagenum">21</a></span><a name="Page_21" id="Page_21"></a>into the field of view of the telescope
+without the aid of a finder. Yet more time would be wasted in the case
+of an object not visible to the naked eye, but whose position with
+reference to several visible stars is known; since, while it is easy to
+bring the point required to the centre of the <i>finder's</i> field, in which
+the guiding stars are visible, it is very difficult to direct the
+<i>telescope's</i> tube on a point of this sort. A card tube with wire
+fastenings, such as we have described, may appear a very insignificant
+contrivance to the regular observer, with his well-mounted equatorial
+and carefully-adjusted finder. But to the first attempts of the amateur
+observer it affords no insignificant assistance, as I can aver from my
+own experience. Without it&mdash;a superior finder being wanting&mdash;our
+"half-hours" would soon be wasted away in that most wearisome and
+annoying of all employments, trying to "pick up" celestial objects.</p>
+
+<p>It behoves me at this point to speak of star-maps. Such maps are of many
+different kinds. There are the Observatory maps, in which the places of
+thousands of stars are recorded with an amazing accuracy. Our beginner
+is not likely to make use of, or to want, such maps as these. Then there
+are maps merely intended to give a good general idea of the appearance
+of the heavens at different hours and seasons. Plate <a href="#plate_I">I.</a> presents four
+maps of this sort; but a more complete series of eight maps has been
+published by Messrs. Walton and Maberly in an octavo work; and my own
+'Constellation-Seasons' give, at the same price, twelve quarto maps (of
+four of which those in Plate <a href="#plate_I">I.</a> are miniatures), showing the appearance
+of the sky at any hour from month to month, or on any night, at
+successive intervals of two hours. But maps intermediate in character to
+these and to Observatory maps are required by the <span class="pagenum"><a href="#Page_22" class="pagenum">22</a></span><a name="Page_22" id="Page_22"></a>amateur observer.
+Such are the Society's six gnomonic maps, the set of six gnomonic maps
+in Johnstone's 'Atlas of Astronomy,' and my own set of twelve gnomonic
+maps. The Society's maps are a remarkably good set, containing on the
+scale of a ten-inch globe all the stars in the Catalogue of the
+Astronomical Society (down to the fifth magnitude). The distortion,
+however, is necessarily enormous when the celestial sphere is presented
+in only six gnomonic maps. In my maps all the stars of the British
+Association Catalogue down to the fifth magnitude are included on the
+scale of a six-inch globe. The distortion is scarcely a fourth of that
+in the Society's maps. The maps are so arranged that the relative
+positions of all the stars in each hemisphere can be readily gathered
+from a single view; and black duplicate-maps serve to show the
+appearance of the constellations.</p>
+
+<p>It is often convenient to make small maps of a part of the heavens we
+may wish to study closely. My 'Handbook of the Stars' has been prepared
+to aid the student in the construction of such maps.</p>
+
+<p>In selecting maps it is well to be able to recognise the amount of
+distortion and scale-variation. This may be done by examining the spaces
+included between successive parallels and meridians, near the edges and
+angles of the maps, and comparing these either with those in the centre
+of the map, or with the known figures and dimensions of the
+corresponding spaces on a globe.</p>
+
+<p>We may now proceed to discuss the different tests which the intending
+purchaser of a telescope should apply to the instrument.</p>
+
+<p>The excellence of an object-glass can be satisfactorily determined only
+by testing the performance of the telescope in the manner presently to
+be described. But it is well to examine the quality of <span class="pagenum"><a href="#Page_23" class="pagenum">23</a></span><a name="Page_23" id="Page_23"></a>the glass as
+respects transparency and uniformity of texture. Bubbles, scratches, and
+other such defects, are not very important, since they do not affect the
+distinctness of the field as they would in a Galilean Telescope,&mdash;a
+little light is lost, and that is all. The same remark applies to dust
+upon the glass. The glass should be kept as free as possible from dirt,
+damp, or dust, but it is not advisable to remove every speck which,
+despite such precaution, may accidentally fall upon the object-glass.
+When it becomes necessary to clean the glass, it is to be noted that the
+substance used should be soft, perfectly dry, and free from dust. Silk
+is often recommended, but some silk is exceedingly objectionable in
+texture,&mdash;old silk, perfectly soft to the touch, is perhaps as good as
+anything. If the dust which has fallen on the glass is at all gritty,
+the glass will suffer by the method of cleaning commonly adopted, in
+which the dust is <i>gathered up</i> by pressure. The proper method is to
+clean a small space near the edge of the glass, and to <i>sweep</i> from that
+space as centre. In this way the dust is <i>pushed before</i> the silk or
+wash-leather, and does not cut the glass. It is well always to suspect
+the presence of gritty dust, and adopt this cautious method of cleaning.</p>
+
+<p>The two glasses should on no account be separated.</p>
+
+<p>In examining an eye-piece, the quality of the glass should be noted, and
+care taken that both glasses (but especially the field-glass) are free
+from the least speck, scratch, or blemish of any kind, for these defects
+will be exhibited in a magnified state in the field of view. Hence the
+eye-pieces require to be as carefully preserved from damp and dust as
+the object-glass, and to be more frequently cleaned.</p>
+
+<p>The tube of the telescope should be light, but strong, and free from
+vibration. Its quality in the last respect can be tested by lightly
+striking it <span class="pagenum"><a href="#Page_24" class="pagenum">24</a></span><a name="Page_24" id="Page_24"></a>when mounted; the sound given out should be dead or
+non-resonant. The inside of the tube must absorb extraneous light, and
+should therefore be coloured a dull black; and stops of varying radius
+should be placed along its length with the same object. Sliding tubes,
+rack-work, etc., should work closely, yet easily.</p>
+
+<p>The telescope should be well balanced for vision with the small
+astronomical eye-pieces. But as there is often occasion to use
+appliances which disturb the balance, it is well to have the means of at
+once restoring equilibrium. A cord ring running round the tube (pretty
+tightly, so as to rest still when the tube is inclined), and bearing a
+small weight, will be all that is required for this purpose; it must be
+slipped along the tube until the tube is found to be perfectly balanced.
+Nothing is more annoying than, after getting a star well in the field,
+to see the tube shift its position through defective balance, and thus
+to have to search again for the star. Even with such an arrangement as
+is shown in <a href="#fig_8">fig. 8</a>, though the tube cannot readily shift its position,
+it is better to have it well balanced.</p>
+
+<p>The quality of the stand has a very important influence on the
+performance of a telescope. In fact, a moderately good telescope,
+mounted on a steady stand, working easily and conveniently, will not
+only enable the observer to pass his time much more pleasantly, but will
+absolutely exhibit more difficult objects than a finer instrument on a
+rickety, ill-arranged stand. A good observing-chair is also a matter of
+some importance, the least constraint or awkwardness of position
+detracting considerably from the power of distinct vision. Such, at
+least, is my own experience.</p>
+
+<p>But the mere examination of the glasses, tube, mounting, &amp;c., is only
+the first step in the series of <span class="pagenum"><a href="#Page_25" class="pagenum">25</a></span><a name="Page_25" id="Page_25"></a>tests which should be applied to a
+telescope, since the excellence of the instrument depends, not on its
+size, the beauty of its mounting, or any extraneous circumstances, but
+on its performance.</p>
+
+<p>The observer should first determine whether the chromatic aberration is
+corrected. To ascertain this the telescope should be directed to the
+moon, or (better) to Jupiter, and accurately focussed for distinct
+vision. If, then, on moving the eye-piece towards the object-glass, a
+ring of purple appears round the margin of the object, and on moving the
+eye-glass in the contrary direction a ring of green, the chromatic
+aberration is corrected, since these are the colours of the secondary
+spectrum.</p>
+
+<p>To determine whether the spherical aberration is corrected, the
+telescope should be directed towards a star of the third or fourth
+magnitude, and focussed for distinct vision. A cap with an aperture of
+about one-half its diameter should then be placed over the object-glass.
+If no new adjustment is required for distinct vision, the spherical
+aberration is corrected, since the mean focal length and the focal
+length of the central rays are equal. If, when the cap is on, the
+eye-piece has to be pulled out for distinct vision, the spherical
+aberration has not been fully corrected; if the eye-piece has to be
+pushed in, the aberration has been over-corrected. As a further test, we
+may cut off the central rays, by means of a circular card covering the
+middle of the object-glass, and compare the focal length for distinct
+vision with the focal length when the cap is applied. The extent of the
+spherical aberration may be thus determined; but if the first experiment
+gives a satisfactory result, no other is required.</p>
+
+<p>A star of the first magnitude should next be brought into the field of
+view. If an irradiation from one side is perceived, part of the
+object-glass <span class="pagenum"><a href="#Page_26" class="pagenum">26</a></span><a name="Page_26" id="Page_26"></a>has not the same refractive power as the rest; and the
+part which is defective can be determined by applying in different
+positions a cap which hides half the object-glass. If the irradiation is
+double, it will probably be found that the object-glass has been too
+tightly screwed, and the defect will disappear when the glass is freed
+from such undue pressure.</p>
+
+<p>If the object-glass is not quite at right angles to the axis of the
+tube, or if the eye-tube is at all inclined, a like irradiation will
+appear when a bright star is in the field. The former defect is not
+easily detected or remedied; nor is it commonly met with in the work of
+a careful optician. The latter defect may be detected by cutting out
+three circular cards of suitable size with a small aperture at the
+centre of each, and inserting one at each end of the eye-tube, and one
+over the object-glass. If the tube is rightly placed the apertures will
+of course lie in a right line, so that it will be possible to look
+through all three at once. If not, it will be easy to determine towards
+what part of the object-glass the eye-tube is directed, and to correct
+the position of the tube accordingly.</p>
+
+<p>The best tests for determining the defining power of a telescope are
+close double or multiple stars, the components of which are not very
+unequal. The illuminating power should be tested by directing the
+telescope towards double or multiple stars having one or more minute
+components. Many of the nebul&aelig; serve as tests both for illumination and
+defining power. As we proceed we shall meet with proper objects for
+testing different telescopes. For the present, let the following list
+suffice. It is selected from Admiral Smyth's tests, obtained by
+diminishing the aperture of a 6-in. telescope having a focal length of
+8&frac12; feet:</p>
+
+<p><span class="pagenum"><a href="#Page_27" class="pagenum">27</a></span><a name="Page_27" id="Page_27"></a>A two-inch aperture, with powers of from 60 to 100, should exhibit</p>
+
+<table border="0" cellpadding="4" cellspacing="0" summary="2 Inch Aperature">
+ <tr>
+  <td style="width:40%">&alpha; Piscium (3"&middot;5).</td>
+  <td class="bl" style="width:50%">&delta; Cassiopei&aelig; (9"&middot;5), mag. (4 and 7&frac12;)</td>
+ </tr>
+ <tr>
+  <td>&gamma; Leonis  (3"&middot;2).</td>
+  <td class="bl">Polaris (18"&middot;6), mag. (2&frac12; and 9&frac12;)</td>
+ </tr>
+</table>
+
+<p>A four-inch, powers 80 to 120, should exhibit</p>
+
+<table border="0" cellpadding="4" cellspacing="0" summary="2 Inch Aperature">
+ <tr>
+  <td style="width:40%">&xi; Urs&aelig; Majoris (2"&middot;4).</td>
+  <td class="bl" style="width:50%">&sigma; Cassiopei&aelig; (3"&middot;1), mag. (6 and 8).</td>
+ </tr>
+ <tr>
+  <td>&gamma; Ceti (2"&middot;6).</td>
+  <td class="bl">&delta; Geminorum (7"&middot;1), mag. (4 and 9).</td>
+ </tr>
+</table>
+<p>The tests in the first column are for definition, those in the second
+for illumination. It will be noticed that, though in the case of Polaris
+the smaller aperture may be expected to show the small star of less than
+the 9th magnitude, a larger aperture is required to show the 8th
+magnitude component of &sigma; Cassiopei&aelig;, on account of the greater
+closeness of this double.</p>
+
+<p>In favourable weather the following is a good general test of the
+performance of a telescope:&mdash;A star of the 3rd or 4th magnitude at a
+considerable elevation above the horizon should exhibit a small well
+defined disc, surrounded by two or three fine rings of light.</p>
+
+<p>A telescope should not be mounted within doors, if it can be
+conveniently erected on solid ground, as every movement in the house
+will cause the instrument to vibrate unpleasantly. Further, if the
+telescope is placed in a warm room, currents of cold air from without
+will render observed objects hazy and indistinct. In fact, Sir W.
+Herschel considered that a telescope should not even be erected near a
+house or elevation of any kind round which currents of air are likely to
+be produced. If a tele<span class="pagenum"><a href="#Page_28" class="pagenum">28</a></span><a name="Page_28" id="Page_28"></a>scope is used in a room, the temperature of the
+room should be made as nearly equal as possible to that of the outer
+air.</p>
+
+<p>When a telescope is used out of doors a 'dew-cap,' that is, a tube of
+tin or pasteboard, some ten or twelve inches long, should be placed on
+the end of the instrument, so as to project beyond the object-glass. For
+glass is a good radiator of heat, so that dew falls heavily upon it,
+unless the radiation is in some way checked. The dew-cap does this
+effectually. It should be blackened within, especially if made of metal.
+"After use," says old Kitchener, "the telescope should be kept in a warm
+place long enough for any moisture on the object-glass to evaporate." If
+damp gets between the glasses it produces a fog (which opticians call a
+sweat) or even a seaweed-like vegetation, by which a valuable glass may
+be completely ruined.</p>
+
+<p>The observer should not leave to the precious hours of the night the
+study of the bearing and position of the objects he proposes to examine.
+This should be done by day&mdash;an arrangement which has a twofold
+advantage,&mdash;the time available for observation is lengthened, and the
+eyes are spared sudden changes from darkness to light, and <i>vice vers&acirc;</i>.
+Besides, the eye is ill-fitted to examine difficult objects, after
+searching by candle-light amongst the minute details recorded in maps or
+globes. Of the effect of rest to the eye we have an instance in Sir J.
+Herschel's rediscovery of the satellites of Uranus, which he effected
+after keeping his eyes in darkness for a quarter of an hour. Kitchener,
+indeed, goes so far as to recommend (with a <i>crede experto</i>) an
+<i>interval of sleep</i> in the darkness of the observing-room before
+commencing operations. I have never tried the experiment, but I should
+expect it to have a bad rather than a good <span class="pagenum"><a href="#Page_29" class="pagenum">29</a></span><a name="Page_29" id="Page_29"></a>effect on the eyesight, as
+one commonly sees the eyes of a person who has been sleeping in his
+day-clothes look heavy and bloodshot.</p>
+
+<p>The object or the part of an object to be observed should be brought as
+nearly as possible to the centre of the field of view. When there is no
+apparatus for keeping the telescope pointed upon an object, the best
+plan is so to direct the telescope by means of the finder, that the
+object shall be just out of the field of view, and be brought (by the
+earth's motion) across the centre of the field. Thus the vibrations
+which always follow the adjustment of the tube will have subsided before
+the object appears. The object should then be intently watched during
+the whole interval of its passage across the field of view.</p>
+
+<p>It is important that the student should recognise the fact that the
+highest powers do not necessarily give the best views of celestial
+objects. High powers in all cases increase the difficulty of
+observation, since they diminish the field of view and the illumination
+of the object, increase the motion with which (owing to the earth's
+motion) the image moves across the field, and magnify all defects due to
+instability of the stand, imperfection of the object-glass, or
+undulation of the atmosphere. A good object-glass of three inches
+aperture will in very favourable weather bear a power of about 300, when
+applied to the observation of close double or multiple stars, but for
+all other observations much lower powers should be used. Nothing but
+failure and annoyance can follow the attempt to employ the highest
+powers on unsuitable objects or in unfavourable weather.</p>
+
+<p>The greatest care should be taken in focussing the telescope. When high
+powers are used this is a matter of some delicacy. It would be well if
+the <span class="pagenum"><a href="#Page_30" class="pagenum">30</a></span><a name="Page_30" id="Page_30"></a>eye-pieces intended for a telescope were so constructed that when
+the telescope is focussed for one, this might be replaced by any other
+without necessitating any use of the focussing rack-work. This could be
+readily effected by suitably placing the shoulder which limits the
+insertion of the eye-piece.</p>
+
+<p>It will be found that, even in the worst weather for observation, there
+are instants of distinct vision (with moderate powers) during which the
+careful observer may catch sight of important details; and, similarly,
+in the best observing weather, there are moments of unusually distinct
+vision well worth patient waiting for, since in such weather alone the
+full powers of the telescope can be employed.</p>
+
+<p>The telescopist should not be deterred from observation by the presence
+of fog or haze, since with a hazy sky definition is often singularly
+good.</p>
+
+<p>The observer must not expect distinct vision of objects near the
+horizon. Objects near the eastern horizon during the time of morning
+twilight are especially confused by atmospheric undulations; in fact,
+early morning is a very unfavourable time for the observation of all
+objects.</p>
+
+<p>The same rules which we have been applying to refractors, serve for
+reflectors. The performance of a reflector will be found to differ in
+some respects, however, from that of a refractor. Mr. Dawes is, we
+believe, now engaged in testing reflectors, and his unequalled
+experience of refractors will enable him to pronounce decisively on the
+relative merits of the two classes of telescopes.</p>
+
+<p>We have little to say respecting the construction of telescopes. Whether
+it is advisable or not for an amateur observer to attempt the
+construction of his own telescope is a question depending entirely on
+his mechanical ability and ingenuity. My <span class="pagenum"><a href="#Page_31" class="pagenum">31</a></span><a name="Page_31" id="Page_31"></a>own experience of telescope
+construction is confined to the conversion of a 3-feet into a 5&frac12;-feet
+telescope. This operation involved some difficulties, since the aperture
+had to be increased by about an inch. I found a tubing made of alternate
+layers of card and calico well pasted together, to be both light and
+strong. But for the full length of tube I think a core of metal is
+wanted. A learned and ingenious friend, Mr. Sharp, Fellow of St. John's
+College, informs me that a tube of tin, covered with layers of brown
+paper, well pasted and thicker near the middle of the tube, forms a
+light and strong telescope-tube, almost wholly free from vibration.</p>
+
+<p>Suffer no inexperienced person to deal with your object-glass. I knew a
+valuable glass ruined by the proceedings of a workman who had been told
+to attach three pieces of brass round the cell of the double lens. What
+he had done remained unknown, but ever after a wretched glare of light
+surrounded all objects of any brilliancy.</p>
+
+<p>One word about the inversion of objects by the astronomical telescope.
+It is singular that any difficulty should be felt about so simple a
+matter, yet I have seen in the writings of more than one distinguished
+astronomer, wholly incorrect views as to the nature of the inversion.
+One tells us that to obtain the correct presentation from a picture
+taken with a telescope, the view should be inverted, held up to the
+light, and looked at from the back of the paper. Another tells us to
+invert the picture and hold it opposite a looking-glass. Neither method
+is correct. The simple correction wanted is to hold the picture upside
+down&mdash;the same change which brings the top to the bottom brings the
+right to the left, <i>i.e.</i>, fully corrects the inversion.</p>
+
+<p>In the case, however, of a picture taken by an <span class="pagenum"><a href="#Page_32" class="pagenum">32</a></span><a name="Page_32" id="Page_32"></a>Herschelian reflector,
+the inversion not being complete, a different method must be adopted. In
+fact, either of the above-named processes, incorrect for the ordinary
+astronomical, would be correct for the Herschelian Telescope. The latter
+inverts but does not reverse right and left; therefore after inverting
+our picture we must interchange right and left because they have been
+reversed by the inversion. This is effected either by looking at the
+picture from behind, or by holding it up to a mirror.</p>
+
+<div class="plate">
+<a name="plate_II" id="plate_II"></a>
+<span class="caption">Plate II.</span><br/>
+<a href="images/plateii1_lg.jpg"><img src="images/plateii1.jpg" alt="Plate II" width="65%" /></a>
+<br/>
+<a href="images/plateii2_lg.jpg"><img src="images/plateii2.jpg" alt="Plate II" width="65%" /></a>
+</div>
+
+
+<hr style="width: 65%;" /><p><span class="pagenum"><a href="#Page_33" class="pagenum">33</a></span><a name="Page_33" id="Page_33"></a></p>
+<h2><a name="CHAPTER_II" id="CHAPTER_II"></a>CHAPTER II.</h2>
+
+<h3>A HALF-HOUR WITH ORION, LEPUS TAURUS, ETC.</h3>
+
+
+<p>Any of the half-hours here assigned to the constellation-seasons may be
+taken first, and the rest in seasonal or cyclic order. The following
+introductory remarks are applicable to each:&mdash;</p>
+
+<p>If we stand on an open space, on any clear night, we see above us the
+celestial dome spangled with stars, apparently fixed in position. But
+after a little time it becomes clear that these orbs are slowly shifting
+their position. Those near the eastern horizon are rising, those near
+the western setting. Careful and continuous observation would show that
+the stars are all moving in the same way, precisely, as they would if
+they were fixed to the concave surface of a vast hollow sphere, and this
+sphere rotated about an axis. This axis, in our latitude, is inclined
+about 51&frac12;&deg; to the horizon. Of course only one end of this imaginary
+axis can be above our horizon. This end lies very near a star which it
+will be well for us to become acquainted with at the beginning of our
+operations. It lies almost exactly towards the north, and is raised from
+50&deg; to 53&deg; (according to the season and hour) above the horizon. There
+is an easy method of finding it.</p>
+
+<p>We must first find the Greater Bear. It will be seen from Plate <a href="#plate_I">1</a>, that
+on a spring evening the seven conspicuous stars of this constellation
+are to be looked for towards the north-east, about half way between the
+horizon and the point overhead (or <span class="pagenum"><a href="#Page_34" class="pagenum">34</a></span><a name="Page_34" id="Page_34"></a><i>zenith</i>), the length of the set of
+stars being vertical. On a summer's evening the Great Bear is nearly
+overhead. On an autumn evening he is towards the north-west, the length
+of the set of seven being somewhat inclined to the horizon. Finally, on
+a winter's evening, he is low down towards the north, the length of the
+set of seven stars being nearly in a horizontal direction.</p>
+
+<p>Having found the seven stars, we make use of the pointers &alpha; and
+&beta; (shown in Plate <a href="#plate_I">1</a>) to indicate the place of the Pole-star, whose
+distance from the pointer &alpha; is rather more than three times the
+distance of &alpha; from &beta;.</p>
+
+<p>Now stand facing the Pole-star. Then all the stars are travelling round
+that star <i>in a direction contrary to that in which the hands of a watch
+move</i>. Thus the stars below the pole are moving <i>towards the right</i>,
+those above the pole <i>towards the left</i>, those to the right of the pole
+<i>upwards</i>, those to the left of the pole <i>downwards</i>.</p>
+
+<p>Next face the south. Then all the stars on our left, that is, towards
+the east, are rising slantingly towards the south; those due south are
+moving horizontally to the right, that is, towards the west; and those
+on our right are passing slantingly downwards towards the west.</p>
+
+<p>It is important to familiarise ourselves with these motions, because it
+is through them that objects pass out of the field of view of the
+telescope, and by moving the tube in a proper direction we can easily
+pick up an object that has thus passed away, whereas if we are not
+familiar with the varying motions in different parts of the celestial
+sphere, we may fail in the attempt to immediately recover an object, and
+waste time in the search for it.</p>
+
+<p>The consideration of the celestial motions shows how advantageous it is,
+when using an alt-azimuth, <span class="pagenum"><a href="#Page_35" class="pagenum">35</a></span><a name="Page_35" id="Page_35"></a>to observe objects as nearly as possible due
+south. Of course in many cases this is impracticable, because a
+phenomenon we wish to watch may occur when an object is not situated
+near the meridian. But in examining double stars there is in general no
+reason for selecting objects inconveniently situated. We can wait till
+they come round to the meridian, and then observe them more comfortably.
+Besides, most objects are higher, and therefore better seen, when due
+south.</p>
+
+<p>Northern objects, and especially those within the circle of perpetual
+apparition, often culminate (that is, cross the meridian, or north and
+south line) at too great a height for comfortable vision. In this case
+we should observe them towards the east or west, and remember that in
+the first case they are rising, and in the latter they are setting, and
+that in both cases they have also a motion from left to right.</p>
+
+<p>If we allow an object to pass right across the field of view (the
+telescope being fixed), the apparent direction of its motion is the
+exact reverse of the true direction of the star's motion. This will
+serve as a guide in shifting the alt-azimuth after a star has passed out
+of the field of view.</p>
+
+<p>The following technical terms must be explained. That part of the field
+of view towards which the star appears to move is called the <i>preceding</i>
+part of the field, the opposite being termed the <i>following</i> part. The
+motion for all stars, except those lying in an oval space extending from
+the zenith to the pole of the heavens, is more or less from right to
+left (in the inverted field). Now, if we suppose a star to move along a
+diameter of the field so as to divide the field into two semicircles,
+then in all cases in which this motion takes places from right to left,
+that semicircle which contains the lowest point (apparently) <span class="pagenum"><a href="#Page_36" class="pagenum">36</a></span><a name="Page_36" id="Page_36"></a>of the
+field is the <i>northern</i> half, the other is the <i>southern</i> half. Over the
+oval space just mentioned the reverse holds.</p>
+
+<p>Thus the field is divided into four quadrants, and these are termed
+<i>north following</i> (<i>n.f.</i>) and <i>south following</i> (<i>s.f.</i>); <i>north
+preceding</i> (<i>n.p.</i>), and <i>south preceding</i> (<i>s.p.</i>). The student can
+have no difficulty in interpreting these terms, since he knows which is
+the following and which the preceding <i>semicircle</i>, which the northern
+and which the southern. In the figures of plates <a href="#plate_III">3</a> and <a href="#plate_V">5</a>, the letters
+<i>n.f.</i>, <i>n.p.</i>, &amp;c., are affixed to the proper quadrants. It is to be
+remembered that the quadrants thus indicated are measured either way
+from the point and feather of the diametral arrows.</p>
+
+<p>Next, of the apparent annual motion of the stars. This takes place in
+exactly the same manner as the daily motion. If we view the sky at eight
+o'clock on any day, and again at the same hour one month later, we shall
+find that at the latter observation (as compared with the former) the
+heavens appear to have rotated by the <i>twelfth part</i> of a complete
+circumference, and the appearance presented is precisely the same as we
+should have observed had we waited for two hours (the <i>twelfth part</i> of
+a day) on the day of the first observation.</p>
+
+<hr style='width: 45%;' />
+
+<p>Our survey of the heavens is supposed to be commenced during the first
+quarter of the year, at ten o'clock on the 20th of January, or at nine
+on the 5th of February, or at eight on the 19th of February, or at seven
+on the 6th of March, or at hours intermediate to these on intermediate
+days.</p>
+
+<p>We look first for the Great Bear towards the north-east, as already
+described, and thence find the Pole-star; turning towards which we see,
+towards <span class="pagenum"><a href="#Page_37" class="pagenum">37</a></span><a name="Page_37" id="Page_37"></a>the right and downwards, the two guardians of the pole (&beta;
+and &gamma; Urs&aelig; Minoris). Immediately under the Pole-star is the
+Dragon's Head, a conspicuous diamond of stars. Just on the horizon is
+Vega, scintillating brilliantly. Overhead is the brilliant Capella, near
+which the Milky Way is seen passing down to the horizon on either side
+towards the quarters S.S.E. and N.N.W.</p>
+
+<p>For the present our business is with the southern heavens, however.</p>
+
+<p>Facing the south, we see a brilliant array of stars, Sirius
+unmistakeably overshining the rest. Orion is shining in full glory, his
+leading brilliant, Betelgeuse<a name="FNanchor_2_2" id="FNanchor_2_2"></a><a href="#Footnote_2_2" class="fnanchor">[2]</a> being almost exactly on the meridian,
+and also almost exactly half way between the horizon and the zenith. In
+Plate <a href="#plate_II">2</a> is given a map of this constellation and its neighbourhood.</p>
+
+<p>Let us first turn the tube on Sirius. It is easy to get him in the field
+without the aid of a finder. The search will serve to illustrate a
+method often useful when a telescope has no finder. Having taking out
+the eye-piece&mdash;a low-power one, suppose&mdash;direct the tube nearly towards
+Sirius. On looking through it, a glare of light will be seen within the
+tube. Now, if the tube be slightly moved about, the light will be seen
+to wax and wane, according as the tube is more or less accurately
+directed. Following these indications, it will be found easy to direct
+the tube, so that the object-glass shall appear <i>full of light</i>. When
+this is done, insert the eye-piece, and the star will be seen in the
+field.</p>
+
+<p>But the telescope is out of focus, therefore we must turn the small
+focussing screw. Observe the <span class="pagenum"><a href="#Page_38" class="pagenum">38</a></span><a name="Page_38" id="Page_38"></a>charming chromatic changes&mdash;green, and
+red, and blue light, purer than the hues of the rainbow, scintillating
+and coruscating with wonderful brilliancy. As we get the focus, the
+excursions of these light flashes diminish until&mdash;if the weather is
+favourable&mdash;the star is seen, still scintillating, and much brighter
+than to the naked eye, but reduced to a small disc of light, surrounded
+(in the case of so bright a star as Sirius) with a slight glare. If
+after obtaining the focus the focussing rack work be still turned, we
+see a coruscating image as before. In the case of a very brilliant star
+these coruscations are so charming that we may be excused for calling
+the observer's attention to them. The subject is not without interest
+and difficulty as an optical one. But the astronomer's object is to get
+rid of all these flames and sprays of coloured light, so that he has
+very little sympathy with the admiration which Wordsworth is said to
+have expressed for out-of-focus views of the stars.</p>
+
+<p>We pass to more legitimate observations, noticing in passing that Sirius
+is a double star, the companion being of the tenth magnitude, and
+distant about ten seconds from the primary. But our beginner is not
+likely to see the companion, which is a very difficult object, vowing to
+the overpowering brilliancy of the primary.</p>
+
+<p>Orion affords the observer a splendid field of research. It is a
+constellation rich in double and multiple stars, clusters, and nebul&aelig;.
+We will begin with an easy object.</p>
+
+<p>The star &delta; (Plate <a href="#plate_III">3</a>), or <i>Mintaka</i>, the uppermost of the three
+stars forming the belt, is a wide double. The primary is of the second
+magnitude, the secondary of the seventh, both being white.</p>
+
+<p>The star &alpha; (<i>Betelgeuse</i>) is an interesting object, on account of
+its colour and brilliance, and as one of <span class="pagenum"><a href="#Page_39" class="pagenum">39</a></span><a name="Page_39" id="Page_39"></a>the most remarkable variables
+in the heavens. It was first observed to be variable by Sir John
+Herschel in 1836. At this period its variations were "most marked and
+striking." This continued until 1840, when the changes became "much less
+conspicuous. In January, 1849, they had recommenced, and on December
+5th, 1852, Mr. Fletcher observed &alpha; Orionis brighter than Capella,
+and actually the largest star in the northern hemisphere." That a star
+so conspicuous, and presumably so large, should present such remarkable
+variations, is a circumstance which adds an additional interest to the
+results which have rewarded the spectrum-analysis of this star by Mr.
+Huggins and Professor Miller. It appears that there is decisive evidence
+of the presence in this luminary of many elements known to exist in our
+own sun; amongst others are found sodium, magnesium, calcium, iron, and
+bismuth. Hydrogen appears to be absent, or, more correctly, there are no
+lines in the star's spectrum corresponding to those of hydrogen in the
+solar spectrum. Secchi considers that there is evidence of an actual
+change in the spectrum of the star, an opinion in which Mr. Huggins does
+not coincide. In the telescope Betelgeuse appears as "a rich and
+brilliant gem," says Lassell, "a rich topaz, in hue and brilliancy
+differing from any that I have seen."</p>
+
+<p>Turn next to &beta; (Rigel), the brightest star below the belt. This is
+a very noted double, and will severely test our observer's telescope, if
+small. The components are well separated (see Plate <a href="#plate_III">3</a>), compared with
+many easier doubles; the secondary is also of the seventh magnitude, so
+that neither as respects closeness nor smallness of the secondary, is
+Rigel a difficult object. It is the combination of the two features
+which makes it a test-object. Kitchener says a 1&frac34;-inch object-glass
+should show <span class="pagenum"><a href="#Page_40" class="pagenum">40</a></span><a name="Page_40" id="Page_40"></a>Rigel double; in earlier editions of his work he gave
+2&frac34;-inches as the necessary aperture. Smyth mentions Rigel as a test
+for a 4-inch aperture, with powers of from 80 to 120. A 3-inch aperture,
+however, will certainly show the companion. Rigel is an orange star, the
+companion blue.</p>
+
+<p>Turn next to &lambda; the northernmost of the set of three stars in the
+head of Orion. This is a triple star, though an aperture of 3 inches
+will show it only as a double. The components are 5" apart, the colours
+pale white and violet. With the full powers of a 3&frac12;-inch glass a
+faint companion may be seen above &lambda;.</p>
+
+<p>The star &zeta;, the lowest in the belt, may be tried with a 3&frac12;-inch
+glass. It is a close double, the components being nearly equal, and
+about 2&frac12;" apart (see Plate <a href="#plate_III">3</a>).</p>
+
+<p>For a change we will now try our telescope on a nebula, selecting the
+great nebula in the Sword. The place of this object is indicated in
+Plate <a href="#plate_II">2</a>. There can be no difficulty in finding it since it is clearly
+visible to the naked eye on a moonless night&mdash;the only sort of night on
+which an observer would care to look at nebul&aelig;. A low power should be
+employed.</p>
+
+<p>The nebula is shown in Plate <a href="#plate_III">3</a> as I have seen it with a 3-inch aperture.
+We see nothing of those complex streams of light which are portrayed in
+the drawings of Herschel, Bond, and Lassell, but enough to excite our
+interest and wonder. What is this marvellous light-cloud? One could
+almost imagine that there was a strange prophetic meaning in the words
+which have been translated "Canst thou loose the bands of Orion?"
+Telescope after telescope had been turned on this wonderful object with
+the hope of resolving its light into stars. But it proved intractable to
+Herschel's great reflector, to <span class="pagenum"><a href="#Page_41" class="pagenum">41</a></span><a name="Page_41" id="Page_41"></a>Lassell's 2-feet reflector, to Lord
+Rosse's 3-feet reflector, and even partially to the great 6-feet
+reflector. Then we hear of its supposed resolution into stars, Lord
+Rosse himself writing to Professor Nichol, in 1846, "I may safely say
+there can be little, if any, doubt as to the resolvability of the
+nebula;&mdash;all about the trapezium is a mass of stars, the rest of the
+nebula also abounding with stars, and exhibiting the characteristics of
+resolvability strongly marked."</p>
+
+<p>It was decided, therefore, that assuredly the great nebula is a
+congeries of stars, and not a mass of nebulous matter as had been
+surmised by Sir W. Herschel. And therefore astronomers were not a little
+surprised when it was proved by Mr. Huggins' spectrum-analysis that the
+nebula consists of gaseous matter. How widely extended this gaseous
+universe may be we cannot say. The general opinion is that the nebul&aelig;
+are removed far beyond the fixed stars. If this were so, the dimensions
+of the Orion nebula would be indeed enormous, far larger probably than
+those of the whole system whereof our sun is a member. I believe this
+view is founded on insufficient evidence, but this would not be the
+place to discuss the subject. I shall merely point out that the nebula
+occurs in a region rich in stars, and if it is not, like the great
+nebula in Argo, clustered around a remarkable star, it is found
+associated in a manner which I cannot look upon as accidental with a set
+of small-magnitude stars, and notably with the trapezium which surrounds
+that very remarkable black gap within the nebula. The fact that the
+nebula shares the proper motion of the trapezium appears inexplicable if
+the nebula is really far out in space beyond the trapezium. A very small
+proper motion of the trapezium (alone) would long since have destroyed
+the remarkable <span class="pagenum"><a href="#Page_42" class="pagenum">42</a></span><a name="Page_42" id="Page_42"></a>agreement in the position of the dark gap and the
+trapezium which has been noticed for so many years.</p>
+
+<p>But whether belonging to our system or far beyond it, the great nebula
+must have enormous dimensions. A vast gaseous system it is, sustained by
+what arrangements or forces we cannot tell, nor can we know what
+purposes it subserves. Mr. Huggins' discovery that comets have gaseous
+nuclei, (so far as the two he has yet examined show) may suggest the
+speculation that in the Orion nebula we see a vast system of comets
+travelling in extensive orbits around nuclear stars, and so slowly as to
+exhibit for long intervals of time an unchanged figure. "But of such
+speculations" we may say with Sir J. Herschel "there is no end."</p>
+
+<p>To return to our telescopic observations:&mdash;The trapezium affords a
+useful test for the light-gathering power of the telescope. Large
+instruments exhibit nine stars. But our observer may be well satisfied
+with his instrument and his eye-sight if he can see five with a
+3&frac12;-inch aperture.<a name="FNanchor_3_3" id="FNanchor_3_3"></a><a href="#Footnote_3_3" class="fnanchor">[3]</a> A good 3-inch glass shows four distinctly. But
+with smaller apertures only three are visible.</p>
+
+<p>The whole neighbourhood of the great nebula will well repay research.
+The observer may sweep over it carefully on any dark night with profit.
+Above the nebula is the star-cluster 362 H. The star &iota; (double as
+shown in Plate <a href="#plate_III">3</a>) below the nebula is involved in a strong nebulosity.
+And in searching over this region we meet with delicate double, triple,
+and multiple stars, which make the survey interesting with almost any
+power that may be applied.</p>
+
+<p>Above the nebula is the star &sigma;, a multiple star. <span class="pagenum"><a href="#Page_43" class="pagenum">43</a></span><a name="Page_43" id="Page_43"></a>To an observer
+with a good 3&frac12;-inch glass &sigma; appears as an octuple star. It is
+well seen, however, as a fine multiple star with a smaller aperture.
+Some of the stars of this group appear to be variable.</p>
+
+<p>The star &rho; Orionis is an unequal, easy double, the components being
+separated by nearly seven seconds. The primary is orange, the smaller
+star smalt-blue (see Plate <a href="#plate_III">3</a>).</p>
+
+<p>The middle star of the belt (&epsilon;) has a distant blue companion.
+This star, like &iota;, is nebulous. In fact, the whole region within
+the triangle formed by stars &gamma;, &kappa; and &beta; is full of
+nebulous double and multiple stars, whose aggregation in this region I
+do not consider wholly accidental.</p>
+
+<p>We have not explored half the wealth of Orion, but leave much for future
+observation. We must turn, however, to other constellations.</p>
+
+<p>Below Orion is Lepus, the Hare, a small constellation containing some
+remarkable doubles. Among these we may note &xi;, a white star with a
+scarlet companion; &gamma;, a yellow and garnet double; and &iota;, a
+double star, white and pale violet, with a distant red companion. The
+star &kappa; Leporis is a rather close double, white with a small green
+companion. The intensely red star R Leporis (a variable) will be found
+in the position indicated in the map.</p>
+
+<p>Still keeping within the boundary of our map, we may next turn to the
+fine cluster 2 H (vii.) in Monoceros. This cluster is visible to the
+naked eye, and will be easily found. The nebula 2 H (iv.) is a
+remarkable one with a powerful telescope.</p>
+
+<p>The star 11 Monocerotis is a fine triple star described by the elder
+Herschel as one of the finest sights in the heavens. Our observer,
+however, will see it as a double (see Plate <a href="#plate_III">3</a>). &delta; Monocerotis is
+an easy double, yellow and lavender.</p>
+
+<p>We may now leave the region covered by the <span class="pagenum"><a href="#Page_44" class="pagenum">44</a></span><a name="Page_44" id="Page_44"></a>map and take a survey of the
+heavens for some objects well seen at this season.</p>
+
+<p>Towards the south-east, high up above the horizon, we see the twin-stars
+Castor and Pollux. The upper is Castor, the finest double star visible
+in the northern heavens. The components are nearly equal and rather more
+than 5" apart (see Plate <a href="#plate_III">3</a>). Both are white according to the best
+observers, but the smaller is thought by some to be slightly greenish.</p>
+
+<p>Pollux is a coarse but fine triple star (in large instruments multiple).
+The components orange, grey, and lilac.</p>
+
+<p>There are many other fine objects in Gemini, but we pass to Cancer.</p>
+
+<p>The fine cluster Pr&aelig;sepe in Cancer may easily be found as it is
+distinctly visible to the naked eye in the position shown in Plate <a href="#plate_I">1</a>,
+Map I. In the telescope it is seen as shown in Plate <a href="#plate_III">3</a>.</p>
+
+<p>The star &iota; Cancri is a wide double, the colours orange and blue.</p>
+
+<p>Procyon, the first-magnitude star between Pr&aelig;sepe and Sirius, is finely
+coloured&mdash;yellow with a distant orange companion, which appears to be
+variable.</p>
+
+<p>Below the Twins, almost in a line with them, is the star &alpha; Hydr&aelig;,
+called Al Fard, or "the Solitary One." It is a 2nd magnitude variable. I
+mention it, however, not on its own account, but as a guide to the fine
+double &epsilon; Hydr&aelig;. This star is the middle one of a group of three,
+lying between Pollux and Al Fard rather nearer the latter. The
+components of &epsilon; Hydr&aelig; are separated by about 3&frac12;" (see Plate
+<a href="#plate_III">3</a>). The primary is of the fourth, the companion of the eighth magnitude;
+the former is yellow, the latter a ruddy purple. The period of &epsilon;
+Hydr&aelig; is about 450 years.</p>
+
+<p><span class="pagenum"><a href="#Page_45" class="pagenum">45</a></span><a name="Page_45" id="Page_45"></a>The constellation Leo Minor, now due east and about midway between the
+horizon and the zenith, is well worth sweeping over. It contains several
+fine fields.</p>
+
+<p>Let us next turn to the western heavens. Here there are some noteworthy
+objects.</p>
+
+<p>To begin with, there are the Pleiades, showing to the naked eye only six
+or seven stars. In the telescope the Pleiades appear as shown in Plate
+<a href="#plate_III">3</a>.</p>
+
+<p>The Hyades also show some fine fields with low powers.</p>
+
+<p>Aldebaran, the principal star of the Hyades, as also of the
+constellation Taurus, is a noted red star. It is chiefly remarkable for
+the close spectroscopic analysis to which it has been subjected by
+Messrs. Huggins and Miller. Unlike Betelgeuse, the spectrum of Aldebaran
+exhibits the lines corresponding to hydrogen, and no less than eight
+metals&mdash;sodium, magnesium, calcium, iron, bismuth, tellurium, antimony,
+and mercury, are proved to exist in the constitution of this brilliant
+red star.</p>
+
+<p>On the right of Aldebaran, in the position indicated in Plate <a href="#plate_I">1</a>, Map I.,
+are the stars &zeta; and &beta; Tauri. If with a low power the observer
+sweep from &zeta; towards &beta;, he will soon find&mdash;not far from &zeta;
+(at a distance of about one-sixth of the distance separating &beta; from
+&zeta;), the celebrated Crab nebula, known as 1 M. This was the first
+nebula discovered by Messier, and its discovery led to the formation of
+his catalogue of 103 nebul&aelig;. In a small telescope this object appears as
+a nebulous light of oval form, no traces being seen of the wisps and
+sprays of light presented in Lord Rosse's well known picture of the
+nebula.</p>
+
+<p>Here I shall conclude the labours of our first half-hour among the
+stars, noticing that the examination of Plate <a href="#plate_I">1</a> will show what other
+constella<span class="pagenum"><a href="#Page_46" class="pagenum">46</a></span><a name="Page_46" id="Page_46"></a>tions besides those here considered are well situated for
+observation at this season. It will be remarked that many constellations
+well seen in the third half-hour (Chapter <a href="#CHAPTER_IV">IV.</a>) are favourably seen in
+the first also, and <i>vice vers&acirc;</i>. For instance, the constellation Ursa
+Major well-placed towards the north-east in the first quarter of the
+year, is equally well-placed towards the north-west in the third, and
+similarly of the constellation Cassiopeia. The same relation connects
+the second and fourth quarters of the year.</p>
+
+<div class="plate">
+<a name="plate_III" id="plate_III"></a>
+<span class="caption">Plate III</span><br/>
+<a href="images/plateiii1_lg.jpg"><img src="images/plateiii1.jpg" alt="Plate III" width="65%" /></a>
+<br/>
+<a href="images/plateiii2_lg.jpg"><img src="images/plateiii2.jpg" alt="Plate III" width="65%" /></a>
+</div>
+
+<hr style="width: 65%;" /><p><span class="pagenum"><a href="#Page_47" class="pagenum">47</a></span><a name="Page_47" id="Page_47"></a></p>
+<h2><a name="CHAPTER_III" id="CHAPTER_III"></a>CHAPTER III.</h2>
+
+<h3>A HALF-HOUR WITH LYRA, HERCULES, CORVUS, CRATER, ETC.</h3>
+
+
+<p>The observations now to be commenced are supposed to take place during
+the second quarter of the year,&mdash;at ten o'clock on the 20th of April, or
+at nine on the 5th of May, or at eight on the 21st of May, or at seven
+on the 5th of June, or at hours intermediate to these on intermediate
+days.</p>
+
+<p>We again look first for the Great Bear, now near the zenith, and thence
+find the Pole-star. Turning towards the north, we see Cassiopeia between
+the Pole-star and the horizon. Towards the north-west is the brilliant
+Capella, and towards the north-east the equally brilliant Vega, beneath
+which, and somewhat northerly, is the cross in Cygnus. The Milky Way
+passes from the eastern horizon towards the north (low down), and so
+round to the western horizon.</p>
+
+<p>In selecting a region for special observation, we shall adopt a
+different plan from that used in the preceding "half-hour." The region
+on the equator and towards the south is indeed particularly interesting,
+since it includes the nebular region in Virgo. Within this space nebul&aelig;
+are clustered more closely than over any corresponding space in the
+heavens, save only the greater Magellanic cloud. But to the observer
+with telescopes of moderate power these nebul&aelig; present few features of
+special interest; and there are regions of the sky now well situated for
+observation, which, at most other epochs are either <span class="pagenum"><a href="#Page_48" class="pagenum">48</a></span><a name="Page_48" id="Page_48"></a>low down towards
+the horizon or inconveniently near to the zenith. We shall therefore
+select one of these, the region included in the second map of Plate <a href="#plate_II">2</a>,
+and the neighbouring part of the celestial sphere.</p>
+
+<p>At any of the hours above named, the constellation Hercules lies towards
+the east. A quadrant taken from the zenith to the eastern horizon passes
+close to the last star (&eta;) of the Great Bear's tail, through &beta;,
+a star in Bootes' head, near &beta; Herculis, between the two "Alphas"
+which mark the heads of Hercules and Ophiuchus, and so past &beta;
+Ophiuchi, a third-magnitude star near the horizon. And here we may turn
+aside for a moment to notice the remarkable vertical row of six
+conspicuous stars towards the east-south-east; these are, counting them
+in order from the horizon, &zeta;, &epsilon;, and &delta; Ophiuchi,
+&epsilon;, &alpha;, and &delta; Serpentis.</p>
+
+<p>Let the telescope first be directed towards Vega. This orb presents a
+brilliant appearance in the telescope. Its colour is a bluish-white. In
+an ordinary telescope Vega appears as a single star, but with a large
+object-glass two distant small companions are seen. A nine-inch glass
+shows also two small companions within a few seconds of Vega. In the
+great Harvard refractor Vega is seen with no less than thirty-five
+companions. I imagine that all these stars, and others which can be seen
+in neighbouring fields, indicate the association of Vega with the
+neighbouring stream of the Milky Way.</p>
+
+<p>Let our observer now direct his telescope to the star &epsilon; Lyr&aelig;. Or
+rather, let him first closely examine this star with the naked eye. The
+star is easily identified, since it lies to the left of Vega, forming
+with &zeta; a small equilateral triangle. A careful scrutiny suffices to
+indicate a peculiarity in this <span class="pagenum"><a href="#Page_49" class="pagenum">49</a></span><a name="Page_49" id="Page_49"></a>star. If our observer possesses very
+good eye-sight, he will distinctly recognise it as a "naked-eye double";
+but more probably he will only notice that it appears lengthened in a
+north and south direction.<a name="FNanchor_4_4" id="FNanchor_4_4"></a><a href="#Footnote_4_4" class="fnanchor">[4]</a> In the finder the star is easily divided.
+Applying a low power to the telescope itself, we see &epsilon; Lyr&aelig; as a
+wide double, the line joining the components lying nearly north and
+south. The southernmost component (the upper in the figure) is called
+&epsilon;<sup>1</sup>, the other &epsilon;<sup>2</sup>. Seen as a double, both
+components appear white.</p>
+
+<p>Now, if the observer's telescope is sufficiently powerful, each of the
+components may be seen to be itself double. First try &epsilon;<sup>1</sup>, the
+northern pair. The line joining the components is directed as shown in
+Plate <a href="#plate_III">3</a>. The distance between them is 3"&middot;2, their magnitudes 5 and
+6&frac12;, and their colours yellow and ruddy. If the observer succeeds in
+seeing &epsilon;<sup>1</sup> fairly divided, he will probably not fail in
+detecting the duplicity of &epsilon;<sup>2</sup>, though this is a rather closer
+pair, the distance between the components being only 2"&middot;6. The
+magnitudes are 5 and 5&frac12;, both being white. Between &epsilon;<sup>1</sup> and
+&epsilon;<sup>2</sup> are three faint stars, possibly forming with the quadruple
+a single system.</p>
+
+<p>Let us next turn to the third star of the equilateral triangle mentioned
+above&mdash;viz. to the star &zeta; Lyr&aelig;. This is a splendid but easy double.
+It is figured in Plate <a href="#plate_III">3</a>, but it must be noticed that <span class="pagenum"><a href="#Page_50" class="pagenum">50</a></span><a name="Page_50" id="Page_50"></a>the figure of
+&zeta; and the other nine small figures are not drawn on the same scale
+as &epsilon; Lyr&aelig;. The actual distance between the components of &zeta;
+Lyra is 44", or about one-fourth of the distance separating
+&epsilon;<sup>1</sup> from &epsilon;<sup>2</sup>. The components of &zeta; are very
+nearly equal in magnitude, in colour topaz and green, the topaz
+component being estimated as of the fifth magnitude, the green component
+intermediate between the fifth and sixth magnitudes.</p>
+
+<p>We may now turn to a star not figured in the map, but readily found. It
+will be noticed that the stars &epsilon;, &alpha;, &beta;, and &gamma;
+form, with two small stars towards the left, a somewhat regular
+hexagonal figure&mdash;a hexagon, in fact, having three equal long sides and
+three nearly equal short sides alternating with the others. The star
+&eta; Lyr&aelig; forms the angle next to &epsilon;. It is a wide and unequal
+double, as figured in Plate <a href="#plate_III">3</a>. The larger component is azure blue; the
+smaller is violet, and, being only of the ninth magnitude, is somewhat
+difficult to catch with apertures under 3 inches.</p>
+
+<p>The star &delta;<sup>2</sup> Lyr&aelig; is orange, &delta;<sup>1</sup> blue. In the same field
+with these are seen many other stars.</p>
+
+<p>The stars &gamma;<sup>1</sup> and &gamma;<sup>2</sup> may also be seen in a single
+field, the distance between them being about half the moon's mean
+diameter. The former is quadruple, the components being yellow, bluish,
+pale blue, and blue.</p>
+
+<p>Turn next to the stars &beta; and &gamma; Lyr&aelig;, the former a multiple,
+the latter an unequal double star. It is not, however, in these respects
+that these stars are chiefly interesting, but for their variability. The
+variability of &gamma; has not indeed been fully established, though it
+is certain that, having once been less bright, &gamma; is now
+considerably brighter than &beta;. The change, however, may be due to
+the variation of &beta; alone. This star is one of the most remarkable
+<span class="pagenum"><a href="#Page_51" class="pagenum">51</a></span><a name="Page_51" id="Page_51"></a>variables known. Its period is 12d. 21h. 53m. 10s. In this time it
+passes from a maximum brilliancy&mdash;that of a star of the 3&middot;4
+magnitude&mdash;to a minimum lustre equal to that of a star of the 4&middot;3
+magnitude, thence to the same maximum brilliancy as before, thence to
+another minimum of lustre&mdash;that of a star of the 4&middot;5 magnitude&mdash;and so
+to its maximum lustre again, when the cycle of changes recommences.
+These remarkable changes seem to point to the existence of two unequal
+dark satellites, whose dimensions bear a much greater proportion to
+those of the bright components of &beta; Lyr&aelig; than the dimensions of the
+members of the Solar System bear to those of the sun. In this case, at
+any rate, the conjecture hazarded about Algol, that the star revolves
+around a dark central orb, would be insufficient to account for the
+observed variation.</p>
+
+<p>Nearly midway between &beta; and &gamma; lies the wonderful ring-nebula
+57 M, of which an imperfect idea will be conveyed by the last figure of
+Plate <a href="#plate_III">3</a>. This nebula was discovered in 1772, by Darquier, at Toulouse.
+It is seen as a ring of light with very moderate telescopic power. In a
+good 3&frac12;-inch telescope the nebula exhibits a mottled appearance and a
+sparkling light. Larger instruments exhibit a faint light within the
+ring; and in Lord Rosse's great Telescope "wisps of stars" are seen
+within, and faint streaks of light stream from the outer border of the
+ring. This nebula has been subjected to spectrum-analysis by Mr.
+Huggins. It turns out to be a gaseous nebula! In fact, ring-nebul&aelig;&mdash;of
+which only seven have been detected&mdash;seem to belong to the same class as
+the planetary nebul&aelig;, all of which exhibit the line-spectrum indicative
+of gaseity. The brightest of the three lines seen in the spectrum of the
+ring-nebula in Lyra presents a rather peculiar appearance, "since it
+consists," <span class="pagenum"><a href="#Page_52" class="pagenum">52</a></span><a name="Page_52" id="Page_52"></a>says Mr. Huggins, "of two bright dots, corresponding to
+sections of the ring, and between these there is not darkness, but an
+excessively faint line joining them. This observation makes it probable
+that the faint nebulous matter occupying the central portion is similar
+in constitution to that of the ring."</p>
+
+<p>The constellation Hercules also contains many very interesting objects.
+Let us first inspect a nebula presenting a remarkable contrast with that
+just described. I refer to the nebula 13 M, known as Halley's nebula
+(Plate <a href="#plate_III">3</a>). This nebula is visible to the naked eye, and in a good
+telescope it is a most wonderful object: "perhaps no one ever saw it for
+the first time without uttering a shout of wonder." It requires a very
+powerful telescope completely to resolve this fine nebula, but the
+outlying streamers may be resolved with a good 3-inch telescope. Sir W.
+Herschel considered that the number of the stars composing this
+wonderful object was at least 14,000. The accepted views respecting
+nebul&aelig; would place this and other clusters far beyond the limits of our
+sidereal system, and would make the component stars not very unequal (on
+the average) to our own sun. It seems to me far more probable, on the
+contrary, that the cluster belongs to our own system, and that its
+components are very much smaller than the average of separate stars.
+Perhaps the whole mass of the cluster does not exceed that of an average
+first-magnitude star.</p>
+
+<p>The nebul&aelig; 92 M and 50 H may be found, after a little searching, from
+the positions indicated in the map. They are both well worthy of study,
+the former being a very bright globular cluster, the latter a bright and
+large round nebula. The spectra of these, as of the great cluster,
+resemble the solar spectrum, being continuous, though, of course, very
+much fainter.</p>
+
+<p><span class="pagenum"><a href="#Page_53" class="pagenum">53</a></span><a name="Page_53" id="Page_53"></a>The star &delta; Herculis (seen at the bottom of the map) is a wide and
+easy double&mdash;a beautiful object. The components, situated as shown in
+Plate <a href="#plate_III">3</a>, are of the fourth and eighth magnitude, and coloured
+respectively greenish-white and grape-red.</p>
+
+<p>The star &kappa; Herculis is not shown in the map, but may be very
+readily found, lying between the two gammas, &gamma; Herculis and
+&gamma; Serpentis (<i>see</i> Frontispiece, Map 2), rather nearer the latter.
+It is a wide double, the components of fifth and seventh magnitude, the
+larger yellowish-white, the smaller ruddy yellow.<a name="FNanchor_5_5" id="FNanchor_5_5"></a><a href="#Footnote_5_5" class="fnanchor">[5]</a></p>
+
+<p>Ras Algethi, or &alpha; Herculis, is also beyond the limits of the map,
+but may be easily found by means of Map 2, Frontispiece. It is, properly
+speaking, a multiple star. Considered as a double, the arrangement of
+the components is that shown in Plate <a href="#plate_III">3</a>. The larger is of magnitude
+3&frac12;, the smaller of magnitude 5&frac12;; the former orange, the latter
+emerald. The companion stars are small, and require a good telescope to
+be well seen. Ras Algethi is a variable, changing from magnitude 3 to
+magnitude 3&frac12; in a period of 66&#8531; days.</p>
+
+<p>The star &rho; Herculis is a closer double. The components are 3"&middot;7
+apart, and situated as shown in Plate <a href="#plate_III">3</a>. The larger is of magnitude 4,
+the smaller 5&frac12;; the former bluish-white, the latter pale emerald.</p>
+
+<p>There are other objects within the range of our map which are well
+worthy of study. Such are &mu; Draconis, a beautiful miniature of
+Castor; &gamma;<sup>1</sup> and &gamma;<sup>2</sup> Draconis, a wide double, the
+distance between the components being nearly 62" (both grey); and
+&gamma;<sup>1</sup> and &gamma;<sup>2</sup> Coron&aelig;, a naked-eye double, the components<span class="pagenum"><a href="#Page_54" class="pagenum">54</a></span><a name="Page_54" id="Page_54"></a>
+being 6' apart, and each double with a good 3-inch telescope.</p>
+
+<p>We turn, however, to another region of the sky. Low down, towards the
+south is seen the small constellation Corvus, recognised by its
+irregular quadrilateral of stars. Of the two upper stars, the left-hand
+one is Algorab, a wide double, the components placed as in Plate <a href="#plate_III">3</a>,
+23"&middot;5 apart, the larger of magnitude 3, the smaller 8&frac12;, the colours
+pale yellow and purple.</p>
+
+<p>There is a red star in this neighbourhood which is well worth looking
+for. To the right of Corvus is the constellation Crater, easily
+recognised as forming a tolerably well-marked small group. The star
+Alkes, or &alpha; Crateris, must first be found. It is far from being
+the brightest star in the constellation, and may be assumed to have
+diminished considerably in brilliancy since it was entitled &alpha; by
+Bayer. It will easily be found, however, by means of the observer's star
+maps. If now the telescope be directed to Alkes, there will be found,
+following him at a distance of 42&middot;5 s, and about one minute southerly, a
+small red star, R. Crateris. Like most red stars, this one is a
+variable. A somewhat smaller blue star may be seen in the same field.</p>
+
+<p>There is another red star which may be found pretty easily at this
+season. First find the stars &eta; and &omicron; Leonis, the former
+forming with Regulus (now lying towards the south-west, and almost
+exactly midway between the zenith and the horizon) the handle of the
+Sickle in Leo, the other farther off from Regulus towards the right, but
+lower down. Now sweep from &omicron; towards &eta; with a low power.<a name="FNanchor_6_6" id="FNanchor_6_6"></a><a href="#Footnote_6_6" class="fnanchor">[6]</a>
+There will be found a sixth-magnitude star about <span class="pagenum"><a href="#Page_55" class="pagenum">55</a></span><a name="Page_55" id="Page_55"></a>one-fourth of the way
+from &omicron; to &eta;. South, following this, will be found a group of
+four stars, of which one is crimson. This is the star R Leonis. Like R
+Crateris and R Leporis it is variable.</p>
+
+<p>Next, let the observer turn towards the south again. Above Corvus, in
+the position shown in the Frontispiece, there are to be seen five stars,
+forming a sort of wide V with somewhat bowed legs. At the angle is the
+star &gamma; Virginis, a noted double. In 1756 the components were 6&frac12;
+seconds apart. They gradually approached till, in 1836, they could not
+be separated by the largest telescopes. Since then they have been
+separating, and they are now 4&frac12; seconds apart, situated as shown in
+Plate <a href="#plate_III">3</a>. They are nearly equal in magnitude (4), and both pale yellow.</p>
+
+<p>The star &gamma; Leonis is a closer and more beautiful double. It will
+be found above Regulus, and is the brightest star on the blade of the
+Sickle. The components are separated by about 3&#8533; seconds, the larger
+of the second, the smaller of the fourth magnitude; the former
+yellow-orange, the latter greenish-yellow.</p>
+
+<p>Lastly, the star &iota; Leonis may be tried. It will be a pretty severe
+test for our observer's telescope, the components being only 2"&middot;4 apart,
+and the smaller scarcely exceeding the eighth magnitude. The brighter
+(fourth magnitude) is pale yellow, the other light blue.</p>
+
+
+
+<hr style="width: 65%;" /><p><span class="pagenum"><a href="#Page_56" class="pagenum">56</a></span><a name="Page_56" id="Page_56"></a></p>
+<h2><a name="CHAPTER_IV" id="CHAPTER_IV"></a>CHAPTER IV.</h2>
+
+<h3>A HALF-HOUR WITH BOOTES, SCORPIO, OPHIUCHUS, ETC.</h3>
+
+
+<p>We now commence a series of observations suited to the third quarter of
+the year, and to the following hours:&mdash;Ten o'clock on the 22nd of July;
+nine on the 8th of August; eight on the 23rd of August; seven on the 8th
+of October; and intermediate hours on days intermediate to these.</p>
+
+<p>We look first for the Great Bear towards the north-west, and thence find
+the Pole-star. Turning towards the north we see Capella and &beta;
+Aurig&aelig; low down and slightly towards the left of the exact north point.
+The Milky Way crosses the horizon towards the north-north-east and
+passes to the opposite point of the compass, attaining its highest point
+above the horizon towards east-south-east. This part of the Milky Way is
+well worth observing, being marked by singular variations of brilliancy.
+Near Arided (the principal star of Cygnus, and now lying due east&mdash;some
+twenty-five degrees from the zenith) there is seen a straight dark rift,
+and near this space is another larger cavity, which has been termed the
+northern Coal-sack. The space between &gamma;, &delta;, and &beta; Cygni
+is covered by a large oval mass, exceedingly rich and brilliant. The
+neighbouring branch, extending from &epsilon; Cygni, is far less
+conspicuous here, but near Sagitta becomes brighter than the other,
+which in this neighbourhood suddenly loses its brilliancy and fading
+gradually beyond this point becomes invisible near
+<span class="pagenum"><a href="#Page_57" class="pagenum">57</a></span><a name="Page_57" id="Page_57"></a>
+&beta; Ophiuchi.
+The continuous stream becomes patchy&mdash;in parts very brilliant&mdash;where it
+crosses Aquila and Clypeus. In this neighbourhood the other stream
+reappears, passing over a region very rich in stars. We see now the
+greatest extent of the Milky Way, towards this part of its length, ever
+visible in our latitudes&mdash;just as in spring we see its greatest extent
+towards Monoceros and Argo.</p>
+
+<div class="plate">
+<span class="caption">Plate IV</span><br/>
+<a name="plate_IV" id="plate_IV"></a>
+<a href="images/plateiv1_lg.jpg"><img src="images/plateiv1.jpg" alt="Plate IV" width="65%" /></a>
+<br/>
+<a href="images/plateiv2_lg.jpg"><img src="images/plateiv2.jpg" alt="Plate IV" width="65%" /></a>
+<br/>
+</div>
+
+<p>I may note here in passing that Sir John Herschel's delineation of the
+northern portion of the Milky Way, though a great improvement on the
+views given in former works, seems to require revision, and especially
+as respects the very remarkable patches and streaks which characterise
+the portion extending over Cepheus and Cygnus. It seems to me, also,
+that the evidence on which it has been urged that the stars composing
+the Milky Way are (on an average) comparable in magnitude to our own
+sun, or to stars of the leading magnitudes, is imperfect. I believe, for
+instance, that the brilliant oval of milky light in Cygnus comes from
+stars intimately associated with the leading stars in that
+constellation, and not far removed in space (proportionately) beyond
+them. Of course, if this be the case, the stars, whose combined light
+forms the patch of milky light, must be far smaller than the leading
+brilliants of Cygnus. However, this is not the place to enter on
+speculations of this sort; I return therefore to the business we have
+more immediately in hand.</p>
+
+<p>Towards the east is the square of Pegasus low down towards the horizon.
+Towards the south is Scorpio, distinguished by the red and brilliant
+Antares, and by a train of conspicuous stars. Towards the west is
+Bootes, his leading brilliant&mdash;the ruddy Arcturus&mdash;lying somewhat nearer
+the horizon than the zenith, and slightly south of west. Bootes as <span class="pagenum"><a href="#Page_58" class="pagenum">58</a></span><a name="Page_58" id="Page_58"></a>a
+constellation is easily found if we remember that he is delineated as
+chasing away the Greater Bear. Thus at present he is seen in a slightly
+inclined position, his head (marked by the third-magnitude star &beta;)
+lying due west, some thirty degrees from the zenith. It has always
+appeared to me, by the way, that Bootes originally had nobler
+proportions than astronomers now assign to him. It is known that Canes
+Venatici now occupy the place of an upraised arm of Bootes, and I
+imagine that Corona Borealis, though undoubtedly a very ancient
+constellation, occupies the place of his other arm. Giving to the
+constellation the extent thus implied, it exhibits (better than most
+constellations) the character assigned to it. One can readily picture to
+oneself the figure of a Herdsman with upraised arms driving Ursa Major
+before him. This view is confirmed, I think, by the fact that the Arabs
+called this constellation the Vociferator.</p>
+
+<p>Bootes contains many beautiful objects. Partly on this account, and
+partly because this is a constellation with which the observer should be
+specially familiar, a map of it is given in Plate <a href="#plate_IV">4</a>.</p>
+
+<p>Arcturus has a distant pale lilac companion, and is in other respects a
+remarkable and interesting object. It is of a ruddy yellow colour.
+Schmidt, indeed, considers that the star has changed colour of late
+years, and that whereas it was once very red it is now a yellow star.
+This opinion does not seem well grounded, however. The star <i>may</i> have
+been more ruddy once than now, though no other observer has noticed such
+a peculiarity; but it is certainly not a pure yellow star at present (at
+any rate as seen in our latitude). Owing probably to the difference of
+colour between Vega, Capella and Arcturus, photometricians have not been
+perfectly agreed as to the relative brilliancy of these objects. <span class="pagenum"><a href="#Page_59" class="pagenum">59</a></span><a name="Page_59" id="Page_59"></a>Some
+consider Vega the most brilliant star in the northern heavens, while
+others assign the superiority to Capella. The majority, however,
+consider Arcturus the leading northern brilliant, and in the whole
+heavens place three only before him, viz., Sirius, Canopus, and &alpha;
+Centauri. Arcturus is remarkable in other respects. His proper motion is
+very considerable, so great in fact that since the time of Ptolemy the
+southerly motion (alone) of Arcturus has carried him over a space nearly
+half as great again as the moon's apparent diameter. One might expect
+that so brilliant a star, apparently travelling at a rate so great
+compared with the average proper motions of the stars, must be
+comparatively near to us. This, however, has not been found to be the
+case. Arcturus is, indeed, one of the stars whose distance it has been
+found possible to estimate roughly. But he is found to be some three
+times as far from us as the small star 61 Cygni, and more than seven
+times as far from us as &alpha; Centauri.</p>
+
+<p>The star &delta; Bootis is a wide and unequal double, the smaller
+component being only of the ninth magnitude.</p>
+
+<p>Above Alkaid the last star in the tail of the Greater Bear, there will
+be noticed three small stars. These are &theta;, &iota;, and &kappa;
+Bootis, and are usually placed in star-maps near the upraised hand of
+the Herdsman. The two which lie next to Alkaid, &iota; and &kappa;, are
+interesting doubles. The former is a wide double (see Plate <a href="#plate_V">5</a>), the
+magnitudes of components 4 and 8, their colours yellow and white. The
+larger star of this pair is itself double. The star &kappa; Bootis is
+not so wide a double (see Plate <a href="#plate_V">5</a>), the magnitudes of the components 5
+and 8, their colours white and faint blue&mdash;a beautiful object.</p>
+
+<p>The star &xi; Bootis is an exceedingly interesting <span class="pagenum"><a href="#Page_60" class="pagenum">60</a></span><a name="Page_60" id="Page_60"></a>object. It is
+double, the colours of the components being orange-yellow and ruddy
+purple, their magnitudes 3&frac12; and 6&frac12;. When this star was first
+observed by Herschel in 1780 the position of the components was quite
+different from that presented in Plate <a href="#plate_V">5</a>. They were also much closer,
+being separated by a distance of less than 3&frac12; seconds. Since that
+time the smaller component has traversed nearly a full quadrant, its
+distance from its primary first increasing, till in 1831 the stars were
+nearly 7&frac12; seconds apart, and thence slowly diminishing, so that at
+present the stars are less than 5 seconds apart. The period usually
+assigned to the revolution of this binary system is 117 years, and the
+period of peri-astral passage is said to be 1779. It appears to me,
+however, that the period should be about 108 years, the epoch of last
+peri-astral passage 1777 and of next peri-astral passage, therefore,
+1885. The angular motion of the secondary round the primary is now
+rapidly increasing, and the distance between the components is rapidly
+diminishing, so that in a few years a powerful telescope will be
+required to separate the pair.</p>
+
+<p>Not far from &xi; is &pi; Bootis, represented in Plate <a href="#plate_V">5</a> as a somewhat
+closer double, but in reality&mdash;now at any rate&mdash;a slightly wider pair,
+since the distance between the components of &xi; has greatly diminished
+of late. Both the components of &pi; are white, and their magnitudes are
+3&frac12; and 6.</p>
+
+<p>We shall next turn to an exceedingly beautiful and delicate object, the
+double star &epsilon; Bootis, known also as Mirac and, on account of its
+extreme beauty, called Pulcherrima by Admiral Smyth. The components of
+this beautiful double are of the third and seventh magnitude, the
+primary orange, the secondary sea-green. The distance separating the
+components is about 3 seconds, perhaps more; it <span class="pagenum"><a href="#Page_61" class="pagenum">61</a></span><a name="Page_61" id="Page_61"></a>appears to have been
+slowly increasing during the past ten or twelve years. Smyth assigns to
+this system a period of revolution of 980 years, but there can be little
+doubt that the true period is largely in excess of this estimate.
+Observers in southern latitudes consider that the colours of the
+components are yellow and blue, not orange and green as most of our
+northern observers have estimated them.</p>
+
+<p>A little beyond the lower left-hand corner of the map is the star
+&delta; Serpentis, in the position shown in the Frontispiece, Map 3.
+This is the star which at the hour and season depicted in Map 2 formed
+the uppermost of a vertical row of stars, which has now assumed an
+almost horizontal position. The components of &delta; Serpentis are
+about 3&frac12; seconds apart, their magnitudes 3 and 5, both white.</p>
+
+<p>The stars &theta;<sup>1</sup> and &theta;<sup>2</sup> Serpentis form a wide double, the
+distance between the components being 21&frac12; seconds. They are nearly
+equal in magnitude, the primary being 4&frac12;, the secondary 5. Both are
+yellow, the primary being of a paler yellow colour than the smaller
+star. But the observer may not know where to look for &theta; Serpentis,
+since it falls in a part of the constellation quite separated from that
+part in which &delta; Serpentis lies. In fact &theta; lies on the
+extreme easterly verge of the eastern half of the constellation. It is
+to be looked for at about the same elevation as the brilliant Altair,
+and (as to azimuth) about midway between Altair and the south.</p>
+
+<p>The stars &alpha;<sup>1</sup> and &alpha;<sup>2</sup> Libr&aelig; form a wide double, perhaps
+just separable by the naked eye in very favourable weather. The larger
+component is of the third, the smaller of the sixth magnitude, the
+former yellow the latter light grey.</p>
+
+<p>The star &beta; Libr&aelig; is a beautiful light-green star to the naked eye;
+in the telescope a wide double, pale emerald and light blue.</p>
+
+<p><span class="pagenum"><a href="#Page_62" class="pagenum">62</a></span><a name="Page_62" id="Page_62"></a>In Scorpio there are several very beautiful objects:&mdash;</p>
+
+<p>The star Antares or Cor Scorpionis is one of the most beautiful of the
+red stars. It has been termed the Sirius of red stars, a term better
+merited perhaps by Aldebaran, save for this that, in our latitude,
+Antares is, like Sirius, always seen as a brilliant "scintillator"
+(because always low down), whereas Aldebaran rises high above the
+horizon. Antares is a double star, its companion being a minute green
+star. In southern latitudes the companion of Antares may be seen with a
+good 4-inch, but in our latitudes a larger opening is wanted. Mr. Dawes
+once saw the companion of Antares shining alone for seven seconds, the
+primary being hidden by the moon. He found that the colour of the
+secondary is not merely the effect of contrast, but that this small star
+is really a green sun.</p>
+
+<p>The star &beta; Scorpionis is a fine double, the components 13"&middot;1 apart,
+their magnitudes 2 and 5&frac12;, colours white and lilac. It has been
+supposed that this pair is only an optical double, but a long time must
+elapse before a decisive opinion can be pronounced on such a point.</p>
+
+<p>The star &sigma; Scorpionis is a wider but much more difficult double,
+the smaller component being below the 9th magnitude. The colour of the
+primary (4) is white, that of the secondary maroon.</p>
+
+<p>The star &xi; Scorpionis is a neat double, the components 7"&middot;2 apart,
+their magnitudes 4&frac12; and 7&frac12;, their colours white and grey. This
+star is really triple, a fifth-magnitude star lying close to the
+primary.</p>
+
+<p>In Ophiuchus, a constellation covering a wide space immediately above
+Scorpio, there are several fine doubles. Among others&mdash;</p>
+
+<p>39 Ophiuchi, distance between components 12"&middot;1, <span class="pagenum"><a href="#Page_63" class="pagenum">63</a></span><a name="Page_63" id="Page_63"></a>their magnitudes 5&frac12;
+and 7&frac12;, their colours orange and blue.</p>
+
+<p>The star 70 Ophiuchi, a fourth-magnitude star on the right shoulder of
+Ophiuchus, is a noted double. The distance between the components about
+5&frac12;", their magnitudes 4&frac12; and 7, the colours yellow and red. The
+pair form a system whose period of revolution is about 95 years.</p>
+
+<p>36 Ophiuchi (variable), distance 5"&middot;2, magnitudes 4&frac12; and 6&frac12;,
+colours red and yellow.</p>
+
+<p>&rho; Opiuchi, distance 4", colours yellow and blue, magnitudes 5 and 7.</p>
+
+<p>Between &alpha; and &beta; Scorpionis the fine nebula 80 M may be looked
+for. (Or more closely thus:&mdash;below &beta; is the wide Double &omega;<sup>1</sup>
+and &omega;<sup>2</sup> Scorpionis; about as far to the right of Antares is the
+star &sigma; Scorpionis, and immediately above this is the
+fifth-magnitude star 19.) The nebula we seek lies between 19 and
+&omega;, nearer to 19 (about two-fifths of the way towards &omega;).
+This nebula is described by Sir W. Herschel as "the richest and most
+condensed mass of stars which the firmament offers to the contemplation
+of astronomers."</p>
+
+<p>There are two other objects conveniently situated for observation, which
+the observer may now turn to. The first is the great cluster in the
+sword-hand of Perseus (see Plate <a href="#plate_IV">4</a>), now lying about 28&deg; above the
+horizon between N.E. and N.N.E. The stars &gamma; and &delta; Cassiopei&aelig;
+(see Map 3 of Frontispiece) point towards this cluster, which is rather
+farther from &delta; than &delta; from &gamma;, and a little south of
+the produced line from these stars. The cluster is well seen with the
+naked eye, even in nearly full moonlight. In a telescope of moderate
+power this cluster is a magnificent object, and no telescope has yet
+revealed its full glory. The view in Plate <a href="#plate_V">5</a> gives but the faintest
+conception <span class="pagenum"><a href="#Page_64" class="pagenum">64</a></span><a name="Page_64" id="Page_64"></a>of the glories of &chi; Persei. Sir W. Herschel tried in
+vain to gauge the depths of this cluster with his most powerful
+telescope. He spoke of the most distant parts as sending light to us
+which must have started 4000 or 5000 years ago. But it appears
+improbable that the cluster has in reality so enormous a longitudinal
+extension compared with its transverse section as this view would imply.
+On the contrary, I think we may gather from the appearance of this
+cluster, that stars are far less uniform in size than has been commonly
+supposed, and that the mere irresolvability of a cluster is no proof of
+excessive distance. It is unlikely that the faintest component of the
+cluster is farther off than the brightest (a seventh-magnitude star) in
+the proportion of more than about 20 to 19, while the ordinary estimate
+of star magnitudes, applied by Herschel, gave a proportion of 20 or 30
+to 1 at least. I can no more believe that the components of this cluster
+are stars greatly varying in distance, but accidentally seen in nearly
+the same direction, (or that they form an <i>enormously long system</i>
+turned by accident directly towards the earth), than I could look on the
+association of several thousand persons in the form of a procession as a
+fortuitous arrangement.</p>
+
+<p>Next there is the great nebula in Andromeda&mdash;known as "the
+transcendantly beautiful queen of the nebul&aelig;." It will not be difficult
+to find this object. The stars &epsilon; and &delta; Cassiopei&aelig; (Map 3,
+Frontispiece) point to the star &beta; Andromed&aelig;. Almost in a vertical
+line above this star are two fourth-magnitude stars &mu; and &gamma;,
+and close above &nu;, a little to the right, is the object we
+seek&mdash;visible to the naked eye as a faint misty spot. To tell the truth,
+the transcendantly beautiful queen of the nebul&aelig; is rather a
+disappointing object in an ordi<span class="pagenum"><a href="#Page_65" class="pagenum">65</a></span><a name="Page_65" id="Page_65"></a>nary telescope. There is seen a long
+oval or lenticular spot of light, very bright near the centre,
+especially with low powers. But there is a want of the interest
+attaching to the strange figure of the Great Orion nebula. The Andromeda
+nebula has been partially resolved by Lord Rosse's great reflector, and
+(it is said) more satisfactorily by the great refractor of Harvard
+College. In the spectroscope, Mr. Huggins informs us, the spectrum is
+peculiar. Continuous from the blue to the orange, the light there
+"appears to cease very abruptly;" there is no indication of gaseity.</p>
+
+<p>Lastly, the observer may turn to the pair Mizar and Alcor, the former
+the middle star in the Great Bear's tail, the latter 15' off. It seems
+quite clear, by the way, that Alcor has increased in brilliancy of late,
+since among the Arabians it was considered an evidence of very good
+eyesight to detect Alcor, whereas this star may now be easily seen even
+in nearly full moonlight. Mizar is a double star, and a fourth star is
+seen in the same field of view with the others (see Plate <a href="#plate_V">5</a>). The
+distance between Mizar and its companion is 14"&middot;4; the magnitude of
+Mizar 3, of the companion 5; their colours white and pale green,
+respectively.</p>
+
+
+
+<hr style="width: 65%;" /><p><span class="pagenum"><a href="#Page_66" class="pagenum">66</a></span><a name="Page_66" id="Page_66"></a></p>
+<h2><a name="CHAPTER_V" id="CHAPTER_V"></a>CHAPTER V.</h2>
+
+<h3>A HALF-HOUR WITH ANDROMEDA, CYGNUS, ETC.</h3>
+
+
+<p>Our last half-hour with the double stars, &amp;c., must be a short one, as
+we have already nearly filled the space allotted to these objects. The
+observations now to be made are supposed to take place during the fourth
+quarter of the year,&mdash;at ten o'clock on October 23rd; or at nine on
+November 7th; or at eight on November 22nd; or at seven on December 6th;
+or at hours intermediate to these on intermediate days.</p>
+
+<p>We look first, as in former cases, for the Great Bear, now lying low
+down towards the north. Towards the north-east, a few degrees easterly,
+are the twin-stars Castor and Pollux, in a vertical position, Castor
+uppermost. Above these, a little towards the right, we see the brilliant
+Capella; and between Capella and the zenith is seen the festoon of
+Perseus. Cassiopeia lies near the zenith, towards the north, and the
+Milky Way extends from the eastern horizon across the zenith to the
+western horizon. Low down in the east is Orion, half risen above
+horizon. Turning to the south, we see high up above the horizon the
+square of Pegasus. Low down towards the south-south-west is Fomalhaut,
+pointed to by &beta; and &alpha; Pegasi. Towards the west, about
+half-way between the zenith and the horizon, is the noble cross in
+Cygnus; below which, towards the left, we see Altair, and his companions
+&beta; and &gamma; Aquil&aelig;: while towards the right we see the brilliant
+Vega.</p>
+
+<p>During this half-hour we shall not confine ourselves to any particular
+region of the heavens, but sweep the most conveniently situated
+constellations.</p>
+
+<div class="plate">
+<a name="plate_V" id="plate_V"></a>
+<span class="caption">Plate V.</span><br/>
+<a href="images/platev1_lg.jpg"><img src="images/platev1.jpg" alt="Plate V" width="65%" /></a>
+<br/>
+<a href="images/platev2_lg.jpg"><img src="images/platev2.jpg" alt="Plate V" width="65%" /></a>
+</div>
+
+<p><span class="pagenum"><a href="#Page_67" class="pagenum">67</a></span><a name="Page_67" id="Page_67"></a>First, however, we should recommend the observer to try and get a good
+view of the great nebula in Andromeda, which is <i>not</i> conveniently
+situated for observation, but is so high that after a little trouble the
+observer may expect a more distinct view than in the previous quarter.
+He will see &beta; Andromed&aelig; towards the south-east, about 18&deg; from the
+zenith, &mu; and &nu; nearly in a line towards the zenith, and the
+nebula about half-way between &beta; and the zenith.</p>
+
+<p>With a similar object it will be well to take another view of the great
+cluster in Perseus, about 18&deg; from the zenith towards the
+east-north-east (<i>see</i> the pointers &gamma; and &delta; Cassiopei&aelig; in
+Map 4, Frontispiece), the cluster being between &delta; Cassiopei&aelig; and
+&alpha; Persei.</p>
+
+<p>Not very far off is the wonderful variable Algol, now due east, and
+about 58&deg; above the horizon. The variability of this celebrated object
+was doubtless discovered in very ancient times, since the name Al-gol,
+or "the Demon" seems to point to a knowledge of the peculiarity of this
+"slowly winking eye." To Goodricke, however, is due the rediscovery of
+Algol's variability. The period of variation is 2d. 20h. 48m.; during
+2h. 14m. Algol appears of the second magnitude; the remaining 6&frac34;
+hours are occupied by the gradual decline of the star to the fourth
+magnitude, and its equally gradual return to the second. It will be
+found easy to watch the variations of this singular object, though, of
+course, many of the minima are attained in the daytime. The following
+may help the observer:&mdash;</p>
+
+<p>On October 8th, 1867, at about half-past eleven in the evening, I
+noticed that Algol had reached its minimum of brilliancy. Hence the next
+minimum was attained at about a quarter-past eight on the evening of
+October 11th; the next at about five on <span class="pagenum"><a href="#Page_68" class="pagenum">68</a></span><a name="Page_68" id="Page_68"></a>the evening of October 14th,
+and so on. Now, if this process be carried on, it will be found that the
+next evening minimum occurred at about 10h. (<i>circiter</i>) on the evening
+of October 31st, the next at about 11h. 30m. on the evening of November
+20th. Thus at whatever hour any minimum occurs, another occurs <i>six
+weeks and a day later</i>, at about the same hour. This would be exact
+enough if the period of variation were <i>exactly</i> 2d. 20m. 48s., but the
+period is nearly a minute greater, and as there are fifteen periods in
+six weeks and a day, it results that there is a difference of about 13m.
+in the time at which the successive recurrences at nearly the same hour
+take place. Hence we are able to draw up the two following Tables, which
+will suffice to give all the minima conveniently observable during the
+next two years. Starting from a minimum at about 11h. 45m. on November
+20th, 1867, and noticing that the next 43-day period (with the 13m.
+added) gives us an observation at midnight on January 2nd, 1868, and
+that successive periods would make the hour later yet, we take the
+minimum next after that of January 2nd, viz. that of January 5th, 1868,
+8h. 48m., and taking 43-day periods (with 13m. added to each), we get
+the series&mdash;</p>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="Minima Table">
+<tr>
+  <td></td><td>h.</td><td>m.</td>
+  <td class="bl"></td><td>h.</td><td>m.</td>
+</tr>
+<tr>
+  <td>Jan. 5, 1868,</td><td>8</td><td>45 P.M.</td>
+  <td class="bl">Mar. 10, &mdash;&mdash;,</td><td>10</td><td>25 &mdash;&mdash;</td>
+</tr>
+<tr>
+  <td>Feb. 17, &mdash;&mdash;,</td><td>8</td><td>58 &mdash;&mdash;</td>
+  <td class="bl">Mar.&nbsp;13, &mdash;&mdash;,</td><td>7</td><td>43&nbsp;&mdash;&mdash;<a name="FNanchor_7_7" id="FNanchor_7_7"></a><a href="#Footnote_7_7" class="fnanchor">[7]</a></td>
+</tr>
+<tr>
+  <td>Mar. 31, &mdash;&mdash;,</td><td>9</td><td>11 &mdash;&mdash;</td>
+  <td class="bl">Apr. 25, &mdash;&mdash;,</td><td>7</td><td>56 &mdash;&mdash;</td>
+</tr>
+<tr>
+  <td>May 13, &mdash;&mdash;,</td><td>9</td><td>24 &mdash;&mdash;</td>
+  <td class="bl">June 7, &mdash;&mdash;,</td><td>8</td><td>9 &mdash;&mdash;</td>
+</tr>
+<tr>
+  <td>June 25, &mdash;&mdash;,</td><td>9</td><td>37 &mdash;&mdash;</td>
+  <td class="bl">July 20, &mdash;&mdash;,</td><td>8</td><td>22 &mdash;&mdash;</td>
+</tr>
+<tr>
+  <td>Aug. 7, &mdash;&mdash;,</td><td>9</td><td>50 &mdash;&mdash;</td>
+  <td class="bl">Sept. 1, &mdash;&mdash;,</td><td>8</td><td>35 &mdash;&mdash;</td>
+</tr>
+<tr>
+  <td>Sept. 19, &mdash;&mdash;,</td><td>10</td><td>3 &mdash;&mdash;</td>
+  <td class="bl">Oct. 14, &mdash;&mdash;,</td><td>8</td><td>48 &mdash;&mdash;</td>
+</tr>
+<tr>
+  <td>Nov. 1, &mdash;&mdash;,</td><td>10</td><td>16 &mdash;&mdash;</td>
+  <td class="bl">Nov. 26, &mdash;&mdash;,</td><td>9</td><td>1 &mdash;&mdash;</td>
+</tr>
+<tr>
+  <td>Dec. 14, &mdash;&mdash;,</td><td>10</td><td>29 &mdash;&mdash;</td>
+  <td class="bl">Jan. 8, 1870,</td><td>9</td><td>14 &mdash;&mdash;</td>
+</tr>
+<tr>
+  <td>Jan. 26, 1869,</td><td>10</td><td>42 &mdash;&mdash;</td>
+  <td class="bl">Feb. 20, &mdash;&mdash;,</td><td>9</td><td>27 &mdash;&mdash;</td>
+</tr>
+<tr>
+  <td style="width:50%" colspan="3"></td>
+  <td style="width:50%" colspan="3"></td>
+</tr>
+</table></div>
+
+<p><span class="pagenum"><a href="#Page_69" class="pagenum">69</a></span><a name="Page_69" id="Page_69"></a>From the minimum at about 10 P.M. on October 31st, 1867, we get in like
+manner the series&mdash;</p>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="Minima Table">
+<tr>
+  <td></td><td>h.</td><td>m.</td>
+  <td class="bl"></td><td>h.</td><td>m.</td>
+</tr>
+<tr>
+  <td>Dec. 13, 1867,</td><td>10</td><td>13 P.M.</td>
+  <td class="bl">Jan. 6, 1869,</td><td>8</td><td>58 &mdash;&mdash;</td>
+</tr>
+<tr>
+  <td>Jan. 25, 1868,</td><td>10</td><td>26 &mdash;&mdash;</td>
+  <td class="bl">Feb. 18, &mdash;&mdash;,</td><td>9</td><td>11 &mdash;&mdash;</td>
+</tr>
+<tr>
+  <td>Mar. 8, &mdash;&mdash;,</td><td>10</td><td>39 &mdash;&mdash;</td>
+  <td class="bl">Apr. 2, &mdash;&mdash;,</td><td>9</td><td>24 &mdash;&mdash;</td>
+</tr>
+<tr>
+  <td>Apr. 20, &mdash;&mdash;,</td><td>10</td><td>52 &mdash;&mdash;</td>
+  <td class="bl">May 15, &mdash;&mdash;,</td><td>9</td><td>37 &mdash;&mdash;</td>
+</tr>
+<tr>
+  <td>June 2, &mdash;&mdash;,</td><td>11</td><td>5 &mdash;&mdash;</td>
+  <td class="bl">June 27, &mdash;&mdash;,</td><td>9</td><td>50 &mdash;&mdash;</td>
+</tr>
+<tr>
+  <td>June 5,&nbsp;&mdash;&mdash;,</td><td>7</td><td>53&nbsp;&mdash;&mdash;<a name="FNanchor_8_8" id="FNanchor_8_8"></a><a href="#Footnote_8_8" class="fnanchor">[8]</a></td>
+  <td class="bl">Aug. 9, &mdash;&mdash;,</td><td>10</td><td>3 &mdash;&mdash;</td>
+</tr>
+<tr>
+  <td>July 18, &mdash;&mdash;,</td><td>8</td><td>6 &mdash;&mdash;</td>
+  <td class="bl">Sept.&nbsp;21,&nbsp;&mdash;&mdash;,</td><td>10</td><td>16&nbsp;&mdash;&mdash;</td>
+</tr>
+<tr>
+  <td>Aug. 30, &mdash;&mdash;,</td><td>8</td><td>19 &mdash;&mdash;</td>
+  <td class="bl">Nov. 3, &mdash;&mdash;,</td><td>10</td><td>29 &mdash;&mdash;</td>
+</tr>
+<tr>
+  <td>Oct. 12, &mdash;&mdash;,</td><td>8</td><td>32 &mdash;&mdash;</td>
+  <td class="bl">Dec. 16, &mdash;&mdash;,</td><td>10</td><td>42 &mdash;&mdash;</td>
+</tr>
+<tr>
+  <td>Nov. 24, &mdash;&mdash;,</td><td>8</td><td>45 &mdash;&mdash;</td>
+  <td class="bl">Jan. 28, 1870,</td><td>10</td><td>55 &mdash;&mdash;</td>
+</tr>
+<tr>
+  <td style="width:50%" colspan="3"></td>
+  <td style="width:50%" colspan="3"></td>
+</tr>
+</table></div>
+
+<p>From one or other of these tables every observable minimum can be
+obtained. Thus, suppose the observer wants to look for a minimum during
+the last fortnight in August, 1868. The first table gives him no
+information, the latter gives him a minimum at 8h. 19m. P.M. on August
+30; hence of course there is a minimum at 11h. 31m. P.M. on August 27;
+and there are no other conveniently observable minima during the
+fortnight in question.</p>
+
+<p>The cause of the remarkable variation in this star's brilliancy has been
+assigned by some astronomers to the presence of an opaque secondary,
+which transits Algol at regular intervals; others have adopted the view
+that Algol is a luminous secondary, revolving around an opaque primary.
+Of these views the former seems the most natural and satisfactory. It
+points to a secondary whose mass bears a far greater proportion to that
+of the primary, than the mass even of Jupiter bears to the sun; the
+shortness of the period is also remarkable. It may be noticed that
+observation points to a gradual diminution in the period of Algol's
+varia<span class="pagenum"><a href="#Page_70" class="pagenum">70</a></span><a name="Page_70" id="Page_70"></a>tion, and the diminution seems to be proceeding more and more
+rapidly. Hence (assuming the existence of a dark secondary) we must
+suppose that either it travels in a resisting medium which is gradually
+destroying its motion, or that there are other dependent orbs whose
+attractions affect the period of this secondary. In the latter case the
+decrease in the period will attain a limit and be followed by an
+increase.</p>
+
+<p>However, interesting as the subject may be, it is a digression from
+telescopic work, to which we now return.</p>
+
+<p>Within the confines of the second map in Plate <a href="#plate_IV">4</a> is seen the fine star
+&gamma; Andromed&aelig;. At the hour of our observations it lies high up
+towards E.S.E. It is seen as a double star with very moderate telescopic
+power, the distance between the components being upwards of 10"; their
+magnitudes 3 and 5&frac12;, their colours orange and green. Perhaps there is
+no more interesting double visible with low powers. The smaller star is
+again double in first-class telescopes, the components being yellow and
+blue according to some observers, but according to others, both green.</p>
+
+<p>Below &gamma; Andromed&aelig; lie the stars &beta; and &gamma; Triangulorum,
+&gamma; a fine naked-eye triple (the companions being &delta; and &eta;
+Triangulorum), a fine object with a very low power. To the right is
+&alpha; Triangulorum, certainly less brilliant than &beta;. Below
+&alpha; are the three stars &alpha;, &beta;, and &gamma; Arietis, the
+first an unequal and difficult double, the companion being purple, and
+only just visible (under favourable circumstances) with a good 3-inch
+telescope; the last an easy double, interesting as being the first ever
+discovered (by Hook, in 1664), the colours of components white and grey.</p>
+
+<p>Immediately below &alpha; Arietis is the star &alpha; Ceti, <span class="pagenum"><a href="#Page_71" class="pagenum">71</a></span><a name="Page_71" id="Page_71"></a>towards the
+right of which (a little lower) is Mira, a wonderful variable. This star
+has a period of 331&#8531; days; during a fortnight it appears as a star of
+the 2nd magnitude,&mdash;on each side of this fortnight there is a period of
+three months during one of which the star is increasing, while during
+the other it is diminishing in brightness: during the remaining five
+months of the period the star is invisible to the naked eye. There are
+many peculiarities and changes in the variation of this star, into which
+space will not permit me to enter.</p>
+
+<p>Immediately above Mira is the star &alpha; Piscium at the knot of the
+Fishes' connecting band. This is a fine double, the distance between the
+components being about 3&frac12;", their magnitudes 5 and 6, their colours
+pale green and blue (see Plate <a href="#plate_V">5</a>).</p>
+
+<p>Close to &gamma; Aquarii (see Frontispiece, Map 4), above and to the
+left of it, is the interesting double &zeta; Aquarii; the distance
+between the components is about 3&frac12;", their magnitudes 4 and 4&frac12;,
+both whitish yellow. The period of this binary seems to be about 750
+years.</p>
+
+<p>Turning next towards the south-west we see the second-magnitude star
+&epsilon; Pegasi, some 40&deg; above the horizon. This star is a wide but
+not easy double, the secondary being only of the ninth magnitude; its
+colour is lilac, that of the primary being yellow.</p>
+
+<p>Towards the right of &epsilon; Pegasi and lower down are seen the three
+fourth-magnitude stars which mark the constellation Equuleus. Of these
+the lowest is &alpha;, to the right of which lies &epsilon; Equulei, a
+fifth-magnitude star, really triple, but seen as a double star with
+ordinary telescopes (Plate <a href="#plate_V">5</a>). The distance between the components is
+nearly 11", their colours white and blue, their magnitudes 5&frac12; and
+7&frac12;. The primary is a very close double, which appears, however, to be
+opening out rather rapidly.</p>
+
+<p>Immediately below Equuleus are the stars &alpha;<sup>1</sup> and &alpha;^2<span class="pagenum"><a href="#Page_72" class="pagenum">72</a></span><a name="Page_72" id="Page_72"></a>
+Capricorni, seen as a naked-eye double to the right of and above &beta;.
+Both &alpha;^1 and &alpha;^2 are yellow; &alpha;^2 is of the 3rd,
+&alpha;^1 of the 4th magnitude; in a good telescope five stars are seen,
+the other three being blue, ash-coloured, and lilac. The star &beta;
+Capricorni is also a wide double, the components yellow and blue, with
+many telescopic companions.</p>
+
+<p>To the right of Equuleus, towards the west-south-west is the
+constellation Delphinus. The upper left-hand star of the rhombus of
+stars forming the head of the Delphinus is the star &gamma; Delphini, a
+rather easy double (see Plate <a href="#plate_V">5</a>), the components being nearly 12" apart,
+their magnitudes 4 and 7, their colours golden yellow and flushed grey.</p>
+
+<p>Turn we next to the charming double Albireo, on the beak of Cygnus,
+about 36&deg; above the horizon towards the west. The components are 34&frac12;"
+apart, their magnitudes 3 and 6, their colours orange-yellow, and blue.
+It has been supposed (perhaps on insufficient evidence) that this star
+is merely an optical double. It must always be remembered that a certain
+proportion of stars (amongst those separated by so considerable a
+distance) <i>must</i> be optically combined only.</p>
+
+<p>The star &chi; Cygni is a wide double (variable) star. The components
+are separated by nearly 26", their magnitudes 5 and 9, their colours
+yellow and light blue. &chi; may be found by noticing that there is a
+cluster of small stars in the middle of the triangle formed by the stars
+&gamma;, &delta;, and &beta; Cygni (see Map 4, Frontispiece), and that
+&chi; is the nearest star <i>of the cluster</i> to &beta;. The star &phi;
+Cygni, which is just above and very close to &beta; (Albireo), does not
+belong to the cluster. &chi; is about half as far again from &phi; as
+&phi; from Albireo. But as &chi; descends to the 11th magnitude at its
+minimum the observer must not always expect to find it very easily. It
+has been known to be invisible at the epoch when it should <span class="pagenum"><a href="#Page_73" class="pagenum">73</a></span><a name="Page_73" id="Page_73"></a>have been
+most conspicuous. The period of this variable is 406 days.</p>
+
+<p>The star 61 Cygni is an interesting one. So far as observation has yet
+extended, it would seem to be the nearest to us of all stars visible in
+the northern hemisphere. It is a fine double, the components nearly
+equal (5&frac12; and 6), both yellow, and nearly 19" apart. The period of
+this binary appears to be about 540 years. To find 61 Cygni note that
+&epsilon; and &delta; Cygni form the diameter of a semicircle divided
+into two quadrants by &alpha; Cygni (Arided). On this semicircle, on
+either side of &alpha;, lie the stars &nu; and &alpha; Cygni, &nu;
+towards &epsilon;. Now a line from &alpha; to &nu; produced passes very
+near to 61 Cygni at a distance from &nu; somewhat greater than half the
+distance of &nu; from &alpha;.</p>
+
+<p>The star &mu; Cygni lies in a corner of the constellation, rather
+farther from &zeta; than &zeta; from &epsilon; Cygni. A line from
+&epsilon; to &zeta; produced meets &kappa; Pegasi, a fourth-magnitude
+star; and &mu; Cygni, a fifth-magnitude star, lies close above &kappa;
+Pegasi. The distance between the components is about 5&frac12;", their
+magnitudes 5 and 6, their colours white and pale blue.</p>
+
+<p>The star &psi; Cygni may next be looked for, but for this a good map of
+Cygnus will be wanted, as &psi; is not pointed to by any well-marked
+stars. A line from &alpha;, parallel to the line joining &gamma; and
+&delta;, and about one-third longer than that line, would about mark the
+position of &psi; Cygni. The distance between the components of this
+double is about 3&frac12;", their magnitudes 5&frac12; and 8, their colours
+white and lilac.</p>
+
+<p>Lastly, the observer may turn to the stars &gamma;<sub>1</sub> and &gamma;<sub>2</sub>
+Draconis towards the north-west about 40&deg; above the horizon (they are
+included in the second map of Plate <a href="#plate_II">2</a>). They form a wide double, having
+equal (fifth-magnitude) components, both grey. (See Plate <a href="#plate_V">5</a>.)</p>
+
+
+
+<hr style="width: 65%;" /><p><span class="pagenum"><a href="#Page_74" class="pagenum">74</a></span><a name="Page_74" id="Page_74"></a></p>
+<h2><a name="CHAPTER_VI" id="CHAPTER_VI"></a>CHAPTER VI.</h2>
+
+<h3>HALF-HOURS WITH THE PLANETS.</h3>
+
+
+<p>In observing the stars, we can select a part of the heavens which may be
+conveniently observed; and in this way in the course of a year we can
+observe every part of the heavens visible in our northern hemisphere.
+But with the planets the case is not quite so simple. They come into
+view at no fixed season of the year: some of them can never be seen <i>by
+night</i> on the meridian; and they all shift their place among the stars,
+so that we require some method of determining where to look for them on
+any particular night, and of recognising them from neighbouring fixed
+stars.</p>
+
+<p>The regular observer will of course make use of the 'Nautical Almanac';
+but 'Dietrichsen and Hannay's Almanac' will serve every purpose of the
+amateur telescopist. I will briefly describe those parts of the almanac
+which are useful to the observer.</p>
+
+<p>It will be found that three pages are assigned to each month, each page
+giving different information. If we call these pages I. II. III., then
+in order that page I. for each month may fall to the left of the open
+double page, and also that I. and II. may be open together, the pages
+are arranged in the following order: I. II. III.; III. I. II.; I. II.
+III.; and so on.</p>
+
+<p>Now page III. for any month does not concern the amateur observer. It
+gives information concerning the moon's motions, which is valuable to
+the sailor, and interesting to the student of astronomy, but not
+applicable to amateur observation.</p>
+
+<div class="plate">
+<a name="plate_VI" id="plate_VI"><br/></a>
+<span class="caption">Plate VI</span><br/>
+<a href="images/platevi1_lg.jpg"><img src="images/platevi1.jpg" alt="Mercury" width="10%" /></a>
+<a href="images/platevi2_lg.jpg"><img src="images/platevi2.jpg" alt="Venus" width="55%" /></a>
+<br/>
+<span class="caption">Mercury and Venus</span>
+<br/><br/>
+<a href="images/platevi3_lg.jpg"><img src="images/platevi3.jpg" alt="Mars, Summer of the Southern Hemisphere" width="65%" /></a>
+<br/>
+<span class="caption">Mars, Summer of the Southern Hemisphere</span>
+<br/><br/>
+<a href="images/platevi4_lg.jpg"><img src="images/platevi4.jpg" alt="Mars, Summer of the Northern Hemisphere" width="65%" /></a>
+<br/>
+<span class="caption">Mars, Summer of the Northern Hemisphere</span>
+<br/><br/>
+<a href="images/platevi5_lg.jpg"><img src="images/platevi5.jpg" alt="Chart of Mars from Drawings of Mr. Dawes" width="65%" /></a>
+<br/>
+<span class="caption">Chart of Mars from Drawings of Mr. Dawes</span>
+</div>
+
+<p><span class="pagenum"><a href="#Page_75" class="pagenum">75</a></span><a name="Page_75" id="Page_75"></a>
+We have then only pages I. and II. to consider:&mdash;</p>
+
+<p>Across the top of both pages the right ascension and declination of the
+planets Venus, Jupiter, Mars, Saturn, Mercury, and Uranus are given,
+accompanied by those of two conspicuous stars. This information is very
+valuable to the telescopist. In the first place, as we shall presently
+see, it shows him what planets are well situated for observation, and
+secondly it enables him to map down the path of any planet from day to
+day among the fixed stars. This is a very useful exercise, by the way,
+and also a very instructive one. The student may either make use of the
+regular maps and mark down the planet's path in pencil, taking a light
+curve through the points given by the data in his almanac, or he may lay
+down a set of meridians suited to the part of the heavens traversed by
+the planet, and then proceed to mark in the planet's path and the stars,
+taking the latter either from his maps or from a convenient list of
+stars.<a name="FNanchor_9_9" id="FNanchor_9_9"></a><a href="#Footnote_9_9" class="fnanchor">[9]</a> My 'Handbook of the Stars' has been constructed to aid the
+student in these processes. It must be noticed that old maps are not
+suited for the work, because, through precession, the stars are all out
+of place as respects R.A. and Dec. Even the Society's maps, constructed
+so as to be right for 1830, are beginning to be out of date. But a
+matter of 20 or 30 years either way is not important.<a name="FNanchor_10_10" id="FNanchor_10_10"></a><a href="#Footnote_10_10" class="fnanchor">[10]</a> My Maps,
+Handbook and Zodiac-chart have been constructed for the year 1880, so as
+to be serviceable for the next fifty years or so.</p>
+
+<p><span class="pagenum"><a href="#Page_76" class="pagenum">76</a></span><a name="Page_76" id="Page_76"></a>Next, below the table of the planets, we have a set of vertical
+columns. These are, in order, the days of the month, the calendar&mdash;in
+which are included some astronomical notices, amongst others the
+diameter of Saturn on different dates, the hours at which the sun rises
+and sets, the sun's right ascension, declination, diameter, and
+longitude; then eight columns which do not concern the observer; after
+which come the hours at which the moon rises and sets, the moon's age;
+and lastly (so far as the observer is concerned) an important column
+about Jupiter's system of satellites.</p>
+
+<p>Next, we have, at the foot of the first page, the hours at which the
+planets rise, south, and set; and at the foot of the second page we have
+the dates of conjunctions, oppositions, and of other phenomena, the
+diameters of Venus, Jupiter, Mars, and Mercury, and finally a few words
+respecting the visibility of these four planets.</p>
+
+<p>After the thirty-six pages assigned to the months follow four (pp.
+42-46) in which much important astronomical information is contained;
+but the points which most concern our observer are (i.) a small table
+showing the appearance of Saturn's rings, and (ii.) a table giving the
+hours at which Jupiter's satellites are occulted or eclipsed, re-appear,
+&amp;c.</p>
+
+<p>We will now take the planets in the order of their distance from the
+sun: we shall see that the information given by the almanac is very
+important to the observer.</p>
+
+<p>Mercury is so close to the sun as to be rarely seen with the naked eye,
+since he never sets much more than two hours and a few minutes after the
+sun, or rises by more than that interval before the sun. It must not be
+supposed that at each successive epoch of most favourable appearance
+Mercury sets so long after the sun or rises so long before him. It would
+<span class="pagenum"><a href="#Page_77" class="pagenum">77</a></span><a name="Page_77" id="Page_77"></a>occupy too much of our space to enter into the circumstances which
+affect the length of these intervals. The question, in fact, is not a
+very simple one. All the necessary information is given in the almanac.
+We merely notice that the planet is most favourably seen as an evening
+star in spring, and as a morning star in autumn.<a name="FNanchor_11_11" id="FNanchor_11_11"></a><a href="#Footnote_11_11" class="fnanchor">[11]</a></p>
+
+<p>The observer with an equatorial has of course no difficulty in finding
+Mercury, since he can at once direct his telescope to the proper point
+of the heavens. But the observer with an alt-azimuth might fail for
+years together in obtaining a sight of this interesting planet, if he
+trusted to unaided naked-eye observations in looking for him. Copernicus
+never saw Mercury, though he often looked for him; and Mr. Hind tells me
+he has seen the planet but once with the naked eye&mdash;though this perhaps
+is not a very remarkable circumstance, since the systematic worker in an
+observatory seldom has occasion to observe objects with the unaided eye.</p>
+
+<p>By the following method the observer can easily pick up the planet.</p>
+
+<p>Across two uprights (Fig. 10) nail a straight rod, so that when looked
+at from some fixed point of view the rod may correspond to the sun's
+path near the time of observation. The rod should be at right-angles to
+the line of sight to its centre. Fasten another rod at right angles to
+the first. From the point at which the rods cross measure off and mark
+<span class="pagenum"><a href="#Page_78" class="pagenum">78</a></span><a name="Page_78" id="Page_78"></a>on both rods spaces each subtending a degree as seen from the point of
+view. Thus, if the point of view is 9&frac12; feet off, these spaces must
+each be 2 inches long, and they must be proportionately less or greater
+as the eye is nearer or farther.</p>
+
+<p><span class="figleft">
+<a name="fig_10" id="fig_10"></a>
+<a href="images/fig10.jpg"><img src="images/fig10.jpg" title="Fig. 10." alt="Figure 10"  width="195px"/></a>
+<br/><span class="caption">Fig. 10.</span>
+</span>
+</p>
+
+<p>Now suppose the observer wishes to view Mercury on some day, whereon
+Mercury is an evening star. Take, for instance, June 9th, 1868. We find
+from 'Dietrichsen' that on this day (at noon) Mercury's R.A. is 6h. 53m.
+23s.: and the sun's 5h. 11m. 31s. We need not trouble ourselves about
+the odd hours after noon, and thus we have Mercury's <span class="pagenum"><a href="#Page_79" class="pagenum">79</a></span><a name="Page_79" id="Page_79"></a>R.A. greater than
+the sun's by 1h. 41m. 52s. Now we will suppose that the observer has so
+fixed his uprights and the two rods, that the sun, seen from the fixed
+point of view, appears to pass the point of crossing of the rods at
+half-past seven, then Mercury will pass the cross-rod at 11m. 52s. past
+nine. But where? To learn this we must take out Mercury's declination,
+which is 24&deg; 43' 18" N., and the sun's, which is 22&deg; 59' 10" N. The
+difference, 1&deg; 44' 8" N. gives us Mercury's place, which it appears is
+rather less than 1&frac34; degree north of the sun. Thus, about 1h. 42m.
+after the sun has passed the cross-rod, Mercury will pass it between the
+first and second divisions above the point of fastening. The sun will
+have set about an hour, and Mercury will be easily found when the
+telescope is directed towards the place indicated.</p>
+
+<p>It will be noticed that this method does not require the time to be
+exactly known. All we have to do is to note the moment at which the sun
+passes the point of fastening of the two rods, and to take our 1h. 42m.
+from that moment.</p>
+
+<p>This method, it may be noticed in passing, may be applied to give
+naked-eye observations of Mercury at proper seasons (given in the
+almanac). By a little ingenuity it may be applied as well to morning as
+to evening observations, the sun's passage of the cross-rod being taken
+on one morning and Mercury's on the next, so many minutes <i>before</i> the
+hour of the first observation. In this way several views of Mercury may
+be obtained during the year.</p>
+
+<p>Such methods may appear very insignificant to the systematic observer
+with the equatorial, but that they are effective I can assert from my
+own experience. Similar methods may be applied to determine from the
+position of a known object, that of any neighbouring unknown object even
+at night. The cross-<span class="pagenum"><a href="#Page_80" class="pagenum">80</a></span><a name="Page_80" id="Page_80"></a>rod must be shifted (or else two cross-rods used)
+when the unknown <i>precedes</i> the known object. If two cross-rods are
+used, account must be taken of the gradual diminution in the length of a
+degree of right ascension as we leave the equator.</p>
+
+<p>Even simpler methods carefully applied may serve to give a view of
+Mercury. To show this, I may describe how I obtained my first view of
+this planet. On June 1st, 1863, I noticed, that at five minutes past
+seven the sun, as seen from my study window, appeared from behind the
+gable-end of Mr. St. Aubyn's house at Stoke, Devon. I estimated the
+effect of Mercury's northerly declination (different of course for a
+vertical wall, than for the cross-rod in <a href="#fig_8">fig. 8</a>, which, in fact, agrees
+with a declination-circle), and found that he would pass out opposite a
+particular point of the wall a certain time after the sun. I then turned
+the telescope towards that point, and focussed for distinct vision of
+distant objects, so that the outline of the house was seen out of focus.
+As the calculated time of apparition approached, I moved the telescope
+up and down so that the field swept the neighbourhood of the estimated
+point of apparition. I need hardly say that Mercury did not appear
+exactly at the assigned point, nor did I see him make his first
+appearance; but I picked him up so soon after emergence that the outline
+of the house was in the field of view with him. He appeared as a
+half-disc. I followed him with the telescope until the sun had set, and
+soon after I was able to see him very distinctly with the naked eye. He
+shone with a peculiar brilliance on the still bright sky; but although
+perfectly distinct to the view when his place was indicated, he escaped
+detection by the undirected eye.<a name="FNanchor_12_12" id="FNanchor_12_12"></a><a href="#Footnote_12_12" class="fnanchor">[12]</a></p>
+
+<p><span class="pagenum"><a href="#Page_81" class="pagenum">81</a></span><a name="Page_81" id="Page_81"></a>Mercury does not present any features of great interest in ordinary
+telescopes; though he usually appears better defined than Venus, at
+least as the latter is seen on a dark sky. The phases are pleasingly
+seen (as shown in Plate <a href="#plate_VI">6</a>) with a telescope of moderate power. For their
+proper observation, however, the planet must be looked for with the
+telescope in the manner above indicated, as he always shows a nearly
+semi-circular disc when he is visible to the naked eye.</p>
+
+<p>We come next to Venus, the most splendid of all the planets to the eye.
+In the telescope Venus disappoints the observer, however. Her intense
+lustre brings out every defect of the instrument, and especially the
+chromatic aberration. A dark glass often improves the view, but not
+always. Besides, an interposed glass has an unpleasant effect on the
+field of view.</p>
+
+<p>Perhaps the best method of observing Venus is to search for her when she
+is still high above the horizon, and when therefore the background of
+the sky is bright enough to take off the planet's glare. The method I
+have described for the observation of Mercury will prove very useful in
+the search for Venus when the sun is above the horizon or but just set.
+Of course, when an object is to be looked for high above the horizon,
+the two rods which support the cross-rods must not be upright, but
+square to the line of view to that part of the sky.</p>
+
+<p>But the observer must not expect to see much during his observation of
+Venus. In fact, he can scarcely do more than note her varying phases
+(see <span class="pagenum"><a href="#Page_82" class="pagenum">82</a></span><a name="Page_82" id="Page_82"></a>Plate <a href="#plate_VI">6</a>) and the somewhat uneven boundary of the terminator. Our
+leading observers have done so little with this fascinating but
+disappointing planet, that amateurs must not be surprised at their own
+failure.</p>
+
+<p>I suppose the question whether Venus has a satellite, or at any rate
+whether the object supposed to have been seen by Cassini and other old
+observers were a satellite, must be considered as decided in the
+negative. That Cassini should have seen an object which Dawes and Webb
+have failed to see must be considered utterly improbable.</p>
+
+<p>Leaving the inferior planets, we come to a series of important and
+interesting objects.</p>
+
+<p>First we have the planet Mars, nearly the last in the scale of planetary
+magnitude, but far from being the least interesting of the planets. It
+is in fact quite certain that we obtain a better view of Mars than of
+any object in the heavens, save the Moon alone. He may present a less
+distinguished appearance than Jupiter or Saturn, but we see his surface
+on a larger scale than that of either of those giant orbs, even if we
+assume that we ever obtain a fair view of their real surface.</p>
+
+<p>Nor need the moderately armed observer despair of obtaining interesting
+views of Mars. The telescope with which Beer and M&auml;dler made their
+celebrated series of views was only a 4-inch one, so that with a 3-inch
+or even a 2-inch aperture the attentive observer may expect interesting
+views. In fact, more depends on the observer than on the instrument. A
+patient and attentive scrutiny will reveal features which at the first
+view wholly escape notice.</p>
+
+<p>In Plate <a href="#plate_VI">6</a> I have given a series of views of Mars much more distinct
+than an observer may expect to obtain with moderate powers. I add a
+chart of Mars, a <span class="pagenum"><a href="#Page_83" class="pagenum">83</a></span><a name="Page_83" id="Page_83"></a>miniature of one I have prepared from a charming
+series of tracings supplied me by Mr. Dawes. The views taken by this
+celebrated observer in 1852, 1856, 1860, 1862, and 1864, are far better
+than any others I have seen. The views by Beer and M&auml;dler are good, as
+are some of Secchi's (though they appear badly drawn), Nasmyth's and
+Phillips'; Delarue's two views are also admirable; and Lockyer has given
+a better set of views than any of the others. But there is an amount of
+detail in Mr. Dawes' views which renders them superior to any yet taken.
+I must confess I failed at a first view to see the full value of Mr.
+Dawes' tracings. Faint marks appeared, which I supposed to be merely
+intended to represent shadings scarcely seen. A more careful study
+shewed me that every mark is to be taken as the representative of what
+Mr. Dawes actually saw. The consistency of the views is perfectly
+wonderful, when compared with the vagueness and inconsistency observable
+in nearly all other views. And this consistency is not shown by mere
+resemblance, which might have been an effect rather of memory
+(unconsciously exerted) than observation. The same feature changes so
+much in figure, as it appears on different parts of the disc, that it
+was sometimes only on a careful projection of different views that I
+could determine what certain features near the limb represented. But
+when this had been done, and the distortion through the effect of
+foreshortening corrected, the feature was found to be as true in shape
+as if it had been seen in the centre of the planet's disc.</p>
+
+<p>In examining Mr. Dawes' drawings it was necessary that the position of
+Mars' axis should be known. The data for determining this were taken
+from Dr. Oudemann's determinations given in a valuable paper on Mars
+issued from Mr. Bishop's observatory. <span class="pagenum"><a href="#Page_84" class="pagenum">84</a></span><a name="Page_84" id="Page_84"></a>But instead of calculating Mars'
+presentation by the formul&aelig; there given, I found it convenient rather to
+make use of geometrical constructions applied to my 'Charts of the
+Terrestrial Planets.' Taking M&auml;dler's start-point for Martial
+longitudes, that is the longitude-line passing near Dawes' forked bay, I
+found that my results agreed pretty fairly with those in Prof. Phillips'
+map, so far as the latter went; but there are many details in my charts
+not found in Prof. Phillips' nor in M&auml;dler's earlier charts.</p>
+
+<p>I have applied to the different features the names of those observers
+who have studied the physical peculiarities presented by Mars. Mr.
+Dawes' name naturally occurs more frequently than others. Indeed, if I
+had followed the rule of giving to each feature the name of its
+discoverer, Mr. Dawes' name would have occurred much more frequently
+than it actually does.</p>
+
+<p>On account of the eccentricity of his orbit, Mars is seen much better in
+some oppositions than in others. When best seen the southern hemisphere
+is brought more into view than the northern because the summer of his
+northern hemisphere occurs when he is nearly in aphelion (as is the case
+with the Earth by the way).</p>
+
+<p>The relative dimensions and presentation of Mars, as seen in opposition
+in perihelion, and in opposition in aphelion, are shown in the two rows
+of figures.</p>
+
+<p>In and near quadrature Mars is perceptibly gibbous. He is seen thus
+about two months before or after opposition. In the former case, he
+rises late and comes to the meridian six hours or so after midnight. In
+the latter case, he is well seen in the evening, coming to the meridian
+at six. His appearance and relative dimensions as he passes from
+opposition to quadrature are shown in the last three figures of the
+upper row.</p>
+
+<p><span class="pagenum"><a href="#Page_85" class="pagenum">85</a></span><a name="Page_85" id="Page_85"></a>Mars' polar caps may be seen with very moderate powers.</p>
+
+<p>I add four sets of meridians (Plate <a href="#plate_VI">6</a>), by filling in which from the
+charts the observer may obtain any number of views of the planet as it
+appears at different times.</p>
+
+<p>Passing over the asteroids, which are not very interesting objects to
+the amateur telescopist, we come to Jupiter, the giant of the solar
+system, surpassing our Earth more than 1400 times in volume, and
+overweighing all the planets taken together twice over.</p>
+
+<p>Jupiter is one of the easiest of all objects of telescopic observation.
+No one can mistake this orb when it shines on a dark sky, and only Venus
+can be mistaken for it when seen as a morning or evening star. Sometimes
+both are seen together on the twilight sky, and then Venus is generally
+the brighter. Seen, however, at her brightest and at her greatest
+elongation from the sun, her splendour scarcely exceeds that with which
+Jupiter shines when high above the southern horizon at midnight.</p>
+
+<p>Jupiter's satellites may be seen with very low powers; indeed the outer
+ones have been seen with the naked eye, and all are visible in a good
+opera-glass. Their dimensions relatively to the disc are shown in Plate
+7. Their greatest elongations are compared with the disc in the
+low-power view.</p>
+
+<p>Jupiter's belts may also be well seen with moderate telescopic power.
+The outer parts of his disc are perceptibly less bright than the centre.</p>
+
+<p>More difficult of observation are the transits of the satellites and of
+their shadows. Still the attentive observer can see the shadows with an
+aperture of two inches, and the satellites themselves with an aperture
+of three inches.</p>
+
+<p>The minute at which the satellites enter on the disc, or pass off, is
+given in 'Dietrichsen's Al<span class="pagenum"><a href="#Page_86" class="pagenum">86</a></span><a name="Page_86" id="Page_86"></a>manac.' The 'Nautical Almanac' also gives the
+corresponding data for the shadows.</p>
+
+<p>The eclipses of the satellites in Jupiter's shadow, and their
+occultations by his disc, are also given in 'Dietrichsen's Almanac.'</p>
+
+<p>In the inverting telescope the satellites move from right to left in the
+nearer parts of their orbit, and therefore transit Jupiter's disc in
+that direction, and from left to right in the farther parts. Also note
+that <i>before</i> opposition, (i.) the shadows travel in front of the
+satellites in transiting the disc; (ii.) the satellites are eclipsed in
+Jupiter's <i>shadow</i>; (iii.) they reappear from behind his <i>disc</i>. On the
+other hand, <i>after</i> opposition, (i.) the shadows travel <i>behind</i> the
+satellites in transiting the disc; (ii.) the satellites are occulted by
+the <i>disc</i>; (iii.) they reappear from eclipse in Jupiter's <i>shadow</i>.</p>
+
+<p>Conjunctions of the satellites are common phenomena, and may be waited
+for by the observer who sees the chance. An eclipse of one satellite by
+the shadow of another is not a common phenomenon; in fact, I have never
+heard of such an eclipse being seen. That a satellite should be quite
+extinguished by another's shadow is a phenomenon not absolutely
+impossible, but which cannot happen save at long intervals.</p>
+
+<p>The shadows are not <i>black spots</i> as is erroneously stated in nearly all
+popular works on astronomy. The shadow of the fourth, for instance, is
+nearly all penumbra, the really black part being quite minute by
+comparison. The shadow of the third has a considerable penumbra, and
+even that of the first is not wholly black. These penumbras may not be
+perceptible, but they affect the appearance of the shadows. For
+instance, the shadow of the fourth is perceptibly larger but less black
+than that of the third, though the third is the larger satellite.</p>
+
+<p><span class="pagenum"><a href="#Page_87" class="pagenum">87</a></span><a name="Page_87" id="Page_87"></a>
+In transit the first satellite moves fastest, the fourth slowest, the
+others in their order. The shadow moves just as fast (appreciably) as
+the satellite it belongs to. Sometimes the shadow of the satellite may
+be seen to overtake (apparently) the disc of another. In such a case the
+shadow does not pass over the disc, but the disc conceals the shadow.
+This is explained by the fact that the shadow, if visible throughout its
+length, would be a line reaching slantwise from the satellite it belongs
+to, and the end of the shadow (that is, the point where it meets the
+disc) is <i>not</i> the point where the shadow crosses the orbit of any inner
+satellite. Thus the latter may be interposed between the end of the
+shadow&mdash;the only part of the shadow really visible&mdash;and the eye; but the
+end of the shadow <i>cannot</i> be interposed between the satellite and the
+eye. If a satellite <i>on the disc</i> were eclipsed by another satellite,
+the black spot thus formed would be in another place from the black spot
+on the planet's body. I mention all this because, simple as the question
+may seem, I have known careful observers to make mistakes on this
+subject. A shadow is seen crossing the disc and overtaking, apparently,
+a satellite in transit. It seems therefore, on a first view, that the
+shadow will hide the satellite, and observers have even said that they
+have <i>seen</i> this happen. But they are deceived. It is obvious that <i>if
+one satellite eclipse another, the shadows of both must occupy the same
+point on Jupiter's body</i>. Thus it is the overtaking of one <i>shadow</i> by
+another on the disc, and not the overtaking of a <i>satellite</i> by a
+shadow, which determines the occurrence of that as yet unrecorded
+phenomenon, the eclipse of one satellite by another.<a name="FNanchor_13_13" id="FNanchor_13_13"></a><a href="#Footnote_13_13" class="fnanchor">[13]</a></p>
+
+<p><span class="pagenum"><a href="#Page_88" class="pagenum">88</a></span><a name="Page_88" id="Page_88"></a>The satellites when far from Jupiter seem to lie in a straight line
+through his centre. But as a matter of fact they do not in general lie
+in an exact straight line. If their orbits could be seen as lines of
+light, they would appear, in general, as very long ellipses. The orbit
+of the fourth would frequently be seen to be <i>quite clear</i> of Jupiter's
+disc, and the orbit of the third might in some very exceptional
+instances pass <i>just</i> clear of the disc. The satellites move most nearly
+in a straight line (apparently) when Jupiter comes to opposition in the
+beginning of February or August, and they appear to depart most from
+rectilinear motion when opposition occurs in the beginning of May and
+November. At these epochs the fourth satellite may be seen to pass above
+and below Jupiter's disc at a distance equal to about one-sixth of the
+disc's radius.</p>
+
+<p>The shadows do not travel in the same apparent paths as the satellites
+themselves across the disc, but (in an inverting telescope) <i>below</i> from
+August to January, and <i>above</i> from February to July.</p>
+
+<p>We come now to the most charming telescopic object in the heavens&mdash;the
+planet Saturn. Inferior only to Jupiter in mass and volume, this planet
+surpasses him in the magnificence of his system. Seen in a telescope of
+adequate power, Saturn is an object of surpassing loveliness. He must be
+an unimaginative man who can see Saturn for the first time in such a
+telescope, without a feeling of awe and amazement. If there is any
+object in the heavens&mdash;I except not even the Sun&mdash;calculated to impress
+one with a sense of the wisdom <span class="pagenum"><a href="#Page_89" class="pagenum">89</a></span><a name="Page_89" id="Page_89"></a>and omnipotence of the Creator it is
+this. "His fashioning hand" is indeed visible throughout space, but in
+Saturn's system it is most impressively manifest.</p>
+
+<p>Saturn, to be satisfactorily seen, requires a much more powerful
+telescope than Jupiter. A good 2-inch telescope will do much, however,
+in exhibiting his rings and belts. I have never seen him satisfactorily
+myself with such an aperture, but Mr. Grover has not only seen the
+above-named features, but even a penumbra to the shadow on the rings
+with a 2-inch telescope.</p>
+
+<p>Saturn revolving round the sun in a long period&mdash;nearly thirty
+years&mdash;presents slowly varying changes of appearance (see Plate <a href="#plate_VII">7</a>). At
+one time the edge of his ring is turned nearly towards the earth; seven
+or eight years later his rings are as much open as they can ever be;
+then they gradually close up during a corresponding interval; open out
+again, exhibiting a different face; and finally close up as first seen.
+The last epoch of greatest opening occurred in 1856, the next occurs in
+1870: the last epoch of disappearance occurred in 1862-63, the next
+occurs in 1879. The successive views obtained are as in Plate <a href="#plate_VII">7</a> in order
+from right to left, then back to the right-hand figure (but sloped the
+other way); inverting the page we have this figure thus sloped, and the
+following changes are now indicated by the other figures in order back
+to the first (but sloped the other way and still inverted), thus
+returning to the right-hand figure as seen without inversion.</p>
+
+<p>The division in the ring can be seen in a good 2-inch aperture in
+favourable weather. The dark ring requires a good 4-inch and good
+weather.</p>
+
+<p>Saturn's satellites do not, like Jupiter's, form a system of nearly
+equal bodies. Titan, the sixth, <span class="pagenum"><a href="#Page_90" class="pagenum">90</a></span><a name="Page_90" id="Page_90"></a>is probably larger than any of
+Jupiter's satellites. The eighth also (Japetus) is a large body,
+probably at least equal to Jupiter's third satellite. But Rhea, Dione,
+and Tethys are much less conspicuous, and the other three cannot be seen
+without more powerful telescopes than those we are here dealing with.</p>
+
+<p>So far as my own experience goes, I consider that the five larger
+satellites may be seen distinctly in good weather with a good 3&frac12;-inch
+aperture. I have never seen them with such an aperture, but I judge from
+the distinctness with which these satellites may be seen with a 4-inch
+aperture. Titan is generally to be looked for at a considerable distance
+from Saturn&mdash;<i>always</i> when the ring is widely open. Japetus is to be
+looked for yet farther from the disc. In fact, when Saturn comes to
+opposition in perihelion (in winter only this can happen) Japetus may be
+as far from Saturn as one-third of the apparent diameter of the moon. I
+believe that under these circumstances, or even under less favourable
+circumstances, Japetus could be seen with a good opera-glass. So also
+might Titan.</p>
+
+<p>Transits, eclipses, and occulations of Saturn's satellites can only be
+seen when the ring is turned nearly edgewise towards the earth. For the
+orbits of the seven inner satellites lying nearly in the plane of the
+rings would (if visible throughout their extent) then only appear as
+straight lines, or as long ellipses cutting the planet's disc.</p>
+
+<p>The belts on Saturn are not very conspicuous. A good 3&frac12;-inch is
+required (so far as my experience extends) to show them satisfactorily.</p>
+
+<p>The rings when turned edgewise either towards the earth or sun, are not
+visible in ordinary telescopes, neither can they be seen when the earth
+and sun are on opposite sides of the rings. In powerful <span class="pagenum"><a href="#Page_91" class="pagenum">91</a></span><a name="Page_91" id="Page_91"></a>telescopes the
+rings seem never entirely to disappear.</p>
+
+<p>The shadow of the planet on the rings may be well seen with a good
+2-inch telescope, which will also show Ball's division in the rings. The
+shadow of the rings on the planet is a somewhat more difficult feature.
+The shadow of the planet on the rings is best seen when the rings are
+well open and the planet is in or near quadrature. It is to be looked
+for to the left of the ball (in an inverting telescope) at quadrature
+preceding opposition, and to the right at quadrature following
+opposition. Saturn is more likely to be studied at the latter than at
+the former quadrature, as in quadrature preceding opposition he is a
+morning star. The shadow of the rings on the planet is best seen when
+the rings are but moderately open, and Saturn is in or near quadrature.
+When the shadow lies outside the rings it is best seen, as the dark ring
+takes off from the sharpness of the contrast when the shadow lies within
+the ring. It would take more space than I can spare here to show how it
+is to be determined (independently) whether the shadow lies within or
+without the ring. But the 'Nautical Almanac' gives the means of
+determining this point. When, in the table for assigning the appearance
+of the rings, <i>l</i> is less than <i>l'</i> the shadow lies outside the ring,
+when <i>l</i> is greater than <i>l'</i> the shadow lies within the ring.</p>
+
+<p>Uranus is just visible to the naked eye when he is in opposition, and
+his place accurately known. But he presents no phenomena of interest. I
+have seen him under powers which made his disc nearly equal to that of
+the moon, yet could see nothing but a faint bluish disc.</p>
+
+<p>Neptune also is easily found if his place be accu<span class="pagenum"><a href="#Page_92" class="pagenum">92</a></span><a name="Page_92" id="Page_92"></a>rately noted on a map,
+and a good finder used. We have only to turn the telescope to a few
+stars seen in the finder nearly in the place marked in our map, and
+presently we shall recognise the one we want by the peculiarity of its
+light. What is the lowest power which will exhibit Neptune as a disc I
+do not know, but I am certain no observer can mistake him for a fixed
+star with a 2-inch aperture and a few minutes' patient scrutiny in
+favourable weather.</p>
+
+<div class="plate">
+<span class="caption">Plate VII</span><br/>
+<a name="plate_VII" id="plate_VII"></a>
+<a href="images/platevii1_lg.jpg"><img src="images/platevii1.jpg" alt="Jupiter" width="65%" /></a>
+<br/>
+<span class="caption">Jupiter</span>
+<br/><br/>
+<a href="images/platevii2_lg.jpg"><img src="images/platevii2.jpg" alt="Low Power View" width="65%" /></a>
+<br/>
+<span class="caption">Low Power View</span>
+<br/><br/>
+<a href="images/platevii3_lg.jpg"><img src="images/platevii3.jpg" alt="Saturn" width="65%" /></a>
+<br/>
+<span class="caption">Saturn</span>
+<br/><br/>
+<a href="images/platevii4_lg.jpg"><img src="images/platevii4.jpg" alt="The Lunar Crater Plate - Sunrise" width="32.5%" /></a>
+<a href="images/platevii5_lg.jpg"><img src="images/platevii5.jpg" alt="Plato Sunset" width="32.5%" /></a>
+<br/>
+<span class="caption">The Lunar Crater Plato at Sunrise and Sunset</span>
+<br/><br/>
+<a href="images/platevii6_lg.jpg"><img src="images/platevii6.jpg" alt="Solar Spots" width="65%" /></a>
+<br/>
+<span class="caption">Solar Spots</span>
+</div>
+
+
+<hr style="width: 65%;" /><p><span class="pagenum"><a href="#Page_93" class="pagenum">93</a></span><a name="Page_93" id="Page_93"></a></p>
+<h2><a name="CHAPTER_VII" id="CHAPTER_VII"></a>CHAPTER VII.</h2>
+
+<h3>HALF-HOURS WITH THE SUN AND MOON.</h3>
+
+
+<p>The moon perhaps is the easiest of all objects of telescopic
+observation. A very moderate telescope will show her most striking
+features, while each increase of power is repaid by a view of new
+details. Yet in one sense the moon is a disappointing object even to the
+possessor of a first-class instrument. For the most careful and
+persistent scrutiny, carried on for a long series of years, too often
+fails to reward the observer by any new discoveries of interest. Our
+observer must therefore rather be prepared to enjoy the observation of
+recognised features than expect to add by his labours to our knowledge
+of the earth's nearest neighbour.</p>
+
+<p>Although the moon is a pleasing and surprising telescopic object when
+full, the most interesting views of her features are obtained at other
+seasons. If we follow the moon as she waxes or wanes, we see the true
+nature of that rough and bleak mountain scenery, which when the moon is
+full is partially softened through the want of sharp contrasts of light
+and shadow. If we watch, even for half an hour only, the changing form
+of the ragged line separating light from darkness on the moon's disc, we
+cannot fail to be interested. "The outlying and isolated peak of some
+great mountain-chain becomes gradually larger, and is finally merged in
+the general luminous surface; great circular spaces, enclosed with rough
+and rocky walls many miles in diameter, become apparent; some with flat
+and perfectly <span class="pagenum"><a href="#Page_94" class="pagenum">94</a></span><a name="Page_94" id="Page_94"></a>smooth floors, variegated with streaks; others in which
+the flat floor is dotted with numerous pits or covered with broken
+fragments of rock. Occasionally a regularly-formed and unusually
+symmetrical circular formation makes its appearance; the exterior
+surface of the wall bristling with terraces rising gradually from the
+plain, the interior one much more steep, and instead of a flat floor,
+the inner space is concave or cup-shaped, with a solitary peak rising in
+the centre. Solitary peaks rise from the level plains and cast their
+long narrow shadows athwart the smooth surface. Vast plains of a dusky
+tint become visible, not perfectly level, but covered with ripples,
+pits, and projections. Circular wells, which have no surrounding wall
+dip below the plain, and are met with even in the interior of the
+circular mountains and on the tops of their walls. From some of the
+mountains great streams of a brilliant white radiate in all directions
+and can be traced for hundreds of miles. We see, again, great fissures,
+almost perfectly straight and of great length, although very narrow,
+which appear like the cracks in moist clayey soil when dried by the
+sun."<a name="FNanchor_14_14" id="FNanchor_14_14"></a><a href="#Footnote_14_14" class="fnanchor">[14]</a></p>
+
+<p>But interesting as these views may be, it was not for such discoveries
+as these that astronomers examined the surface of the moon. The
+examination of mere peculiarities of physical condition is, after all,
+but barren labour, if it lead to no discovery of physical variation. The
+principal charm of astronomy, as indeed of all observational science,
+lies in the study of change&mdash;of progress, development, and decay, and
+specially of systematic variations taking place in regularly-recurring
+cycles. And it is in this relation that the moon has been so
+disappointing an object of astronomical observation. For two <span class="pagenum"><a href="#Page_95" class="pagenum">95</a></span><a name="Page_95" id="Page_95"></a>centuries
+and a half her face has been scanned with the closest possible scrutiny;
+her features have been portrayed in elaborate maps; many an astronomer
+has given a large portion of his life to the work of examining craters,
+plains, mountains, and valleys, for the signs of change; but until
+lately no certain evidence&mdash;or rather, no evidence save of the most
+doubtful character&mdash;has been afforded that the moon is other than "a
+dead and useless waste of extinct volcanoes." Whether the examination of
+the remarkable spot called Linn&eacute;&mdash;where lately signs were supposed to
+have been seen of a process of volcanic eruption&mdash;will prove an
+exception to this rule, remains to be seen. The evidence seems to me
+strongly to favour the supposition of a change of some sort having taken
+place in this neighbourhood.</p>
+
+<p>The sort of scrutiny required for the discovery of changes, or for the
+determination of their extent, is far too close and laborious to be
+attractive to the general observer. Yet the kind of observation which
+avails best for the purpose is perhaps also the most interesting which he
+can apply to the lunar details. The peculiarities presented by a spot upon
+the moon are to be observed from hour to hour (or from day to day,
+according to the size of the spot) as the sun's light gradually sweeps
+across it, until the spot is fully lighted; then as the moon wanes and the
+sun's light gradually passes from the spot, the series of observations is
+to be renewed. A comparison of them is likely&mdash;especially if the observer
+is a good artist and has executed several faithful delineations of the
+region under observation, to throw much light upon the real contour of the
+moon's surface at this point.</p>
+
+<p>In the two lunar views in Plate <a href="#plate_VII">7</a> some of the peculiarities I have
+described are illustrated. But <span class="pagenum"><a href="#Page_96" class="pagenum">96</a></span><a name="Page_96" id="Page_96"></a>the patient observer will easily be able
+to construct for himself a set of interesting views of different
+regions.</p>
+
+<p>It may be noticed that for observation of the waning moon there is no
+occasion to wait for those hours in which only the waning moon is
+visible <i>during the night</i>. Of course for the observation of a
+particular region under a particular illumination, the observer has no
+choice as to hour. But for generally interesting observations of the
+waning moon he can wait till morning and observe by daylight. The moon
+is, of course, very easily found by the unaided eye (in the day time)
+when not very near to the sun; and the methods described in Chapter <a href="#CHAPTER_V">V.</a>
+will enable the observer to find the moon when she is so near to the sun
+as to present the narrowest possible sickle of light.</p>
+
+<p>One of the most interesting features of the moon, when she is observed
+with a good telescope, is the variety of colour presented by different
+parts of her surface. We see regions of the purest white&mdash;regions which
+one would be apt to speak of as <i>snow-covered</i>, if one could conceive
+the possibility that snow should have fallen where (now, at least) there
+is neither air nor water. Then there are the so-called seas, large grey
+or neutral-tinted regions, differing from the former not merely in
+colour and in tone, but in the photographic quality of the light they
+reflect towards the earth. Some of the seas exhibit a greenish tint, as
+the Sea of Serenity and the Sea of Humours. Where there is a central
+mountain within a circular depression, the surrounding plain is
+generally of a bluish steel-grey colour. There is a region called the
+Marsh of Sleep, which exhibits a pale red tint, a colour seen also near
+the Hyrcinian mountains, within a circumvallation called Lichtenburg.
+The brightest portion of the whole <span class="pagenum"><a href="#Page_97" class="pagenum">97</a></span><a name="Page_97" id="Page_97"></a>lunar disc is Aristarchus, the peaks
+of which shine often like stars, when the mountain is within the
+unillumined portion of the moon. The darkest regions are Grimaldi and
+Endymion and the great plain called Plato by modern astronomers&mdash;but, by
+Hevelius, the Greater Black Lake.</p>
+
+<p>The Sun.&mdash;Observation of the sun is perhaps on the whole the most
+interesting work to which the possessor of a moderately good telescope
+can apply his instrument. Those wonderful varieties in the appearance of
+the solar surface which have so long perplexed astronomers, not only
+supply in themselves interesting subjects of observation and
+examination, but gain an enhanced meaning from the consideration that
+they speak meaningly to us of the structure of an orb which is the
+source of light and heat enjoyed by a series of dependent worlds whereof
+our earth is&mdash;in size at least&mdash;a comparatively insignificant member.
+Swayed by the attraction of this giant globe, Jupiter and Saturn, Uranus
+and Neptune, as well as the four minor planets, and the host of
+asteroids, sweep continuously in their appointed orbits, in ever new but
+ever safe and orderly relations amongst each other. If the sun's light
+and heat were lost, all life and work among the denizens of these orbs
+would at once cease; if his attractive energy were destroyed, these orbs
+would cease to form a <i>system</i>.</p>
+
+<p>The sun may be observed conveniently in many ways, some more suited to
+the general observer who has not time or opportunity for systematic
+observation; others more instructive, though involving more of
+preparation and arrangement.</p>
+
+<p>The simplest method of observing the sun is to use the telescope in the
+ordinary manner, protecting the eye by means of dark-green or
+neutral-tinted glasses. Some of the most interesting views <span class="pagenum"><a href="#Page_98" class="pagenum">98</a></span><a name="Page_98" id="Page_98"></a>I have ever
+obtained of the sun, have resulted from the use of the ordinary
+terrestrial or erecting eye-piece, capped with a dark glass. The
+magnifying power of such an eye-piece is, in general, much lower than
+that available with astronomical eye-pieces. But vision is very pleasant
+and distinct when the sun is thus observed, and a patient scrutiny
+reveals almost every feature which the highest astronomical power
+applicable could exhibit. Then, owing to the greater number of
+intervening lenses, there is not the same necessity for great darkness
+or thickness in the coloured glass, so that the colours of the solar
+features are seen much more satisfactorily than when astronomical
+eye-pieces are employed.</p>
+
+<p>In using astronomical eye-pieces it is convenient to have a rotating
+wheel attached, by which darkening glasses of different power may be
+brought into use as the varying illumination may require.</p>
+
+<p>Those who wish to observe carefully and closely a minute portion of the
+solar disc, should employ Dawes' eye-piece: in this a metallic screen
+placed in the focus keeps away all light but such as passes through a
+minute hole in the diaphragm.</p>
+
+<p>Another convenient method of diminishing the light is to use a glass
+prism, light being partially reflected from one of the exterior
+surfaces, while the refracted portion is thrown out at another.</p>
+
+<p>Very beautiful and interesting views may be obtained by using such a
+pyramidal box as is depicted in <a href="#fig_11">fig. 11</a>.</p>
+
+<p><span class="figright">
+<a name="fig_11" id="fig_11"></a>
+<a href="images/fig11.jpg"><img src="images/fig11.jpg" title="Fig. 11." alt="Figure 11" width="245px" /></a>
+<br/><span class="caption">Fig. 11.</span>
+</span>
+</p>
+
+<p>This box should be made of black cloth or calico fastened over a light
+framework of wire or cane. The base of the pyramid should be covered on
+the inside with a sheet of white glazed paper, or with some other
+uniform white surface. Captain Noble, I believe, makes use of a surface
+of plaster of Paris, <span class="pagenum"><a href="#Page_99" class="pagenum">99</a></span><a name="Page_99" id="Page_99"></a>smoothed while wet with plate-glass. The door <i>b
+c</i> enables the observer to "change power" without removing the box,
+while larger doors, <i>d e</i> and <i>g f</i>, enable him to examine the image; a
+dark cloth, such as photographers use, being employed, if necessary, to
+keep out extraneous light. The image may also be examined from without,
+if the bottom of the pyramid be formed of a sheet of cut-glass or oiled
+tissue-paper.</p>
+
+<p>When making use of the method just described, it is very necessary that
+the telescope-tube should be well balanced. A method by which this may
+be conveniently accomplished has been already described in Chapter <a href="#CHAPTER_I">I.</a></p>
+
+<p>But, undoubtedly, for the possessor of a moderately good telescope there
+is no way of viewing the sun's features comparable to that now to be
+described, which has been systematically and successfully applied for a
+long series of years by the Rev. F. Howlett. To use his own words: "Any
+one possessing a good achromatic of not more than three inches'
+aperture, who has a little dexterity with his pencil, and a little time
+at his disposal (all the better if it be at a somewhat early hour of the
+morning)" may by this method "deliberately and satisfactorily view,
+measure, and (if skill suffice) <span class="pagenum"><a href="#Page_100" class="pagenum">100</a></span><a name="Page_100" id="Page_100"></a>delineate most of those interesting and
+grand solar phenomena of which he may have read, or which he may have
+seen depicted, in various works on physical astronomy."<a name="FNanchor_15_15" id="FNanchor_15_15"></a><a href="#Footnote_15_15" class="fnanchor">[15]</a></p>
+
+<p>The method in question depends on the same property which is involved in
+the use of the pyramidal box just described, supplemented (where exact
+and systematic observation is required) by the fact that objects lying
+on or between the lenses of the eye-piece are to be seen faithfully
+projected on the white surface on which the sun's image is received. In
+place, however, of a box carried upon the telescope-tube, a darkened
+room (or true <i>camera obscura</i>) contains the receiving sheet.</p>
+
+<p>A chamber is to be selected, having a window looking towards the
+south&mdash;a little easterly, if possible, so as to admit of morning
+observation. All windows are to be completely darkened save one, through
+which the telescope is directed towards the sun. An arrangement is to be
+adopted for preventing all light from entering by this window except
+such light as passes down the tube of the telescope. This can readily be
+managed with a little ingenuity. Mr. Howlett describes an excellent
+method. The following, perhaps, will sufficiently serve the purposes of
+the general observer:&mdash;A plain frame (portable) is to be constructed to
+fit into the window: to the four sides of this frame triangular pieces
+of cloth (impervious to light) are to be attached, their shape being
+such that when their adjacent edges are sewn together and the flaps
+stretched out, they form a rectangular pyramid of which the frame is the
+base. Through the vertex of this pyramid (near <span class="pagenum"><a href="#Page_101" class="pagenum">101</a></span><a name="Page_101" id="Page_101"></a>which, of course, the
+cloth flaps are not sewn together) the telescope tube is to be passed,
+and an elastic cord so placed round the ends of the flaps as to prevent
+any light from penetrating between them and the telescope. It will now
+be possible, without disturbing the screen (fixed in the window), to
+move the telescope so as to follow the sun during the time of
+observation. And the same arrangement will serve for all seasons, if so
+managed that the elastic cord is not far from the middle of the
+telescope-tube; for in this case the range of motion is small compared
+to the range of the tube's extremity.</p>
+
+<p>A large screen of good drawing-paper should next be prepared. This
+should be stretched on a light frame of wood, and placed on an easel,
+the legs of which should be furnished with holes and pegs that the
+screen may be set at any required height, and be brought square to the
+tube's axis. A large T-square of light wood will be useful to enable the
+observer to judge whether the screen is properly situated in the last
+respect.</p>
+
+<p>We wish now to direct the tube towards the sun, and this "without
+dazzling the eyes as by the ordinary method." This may be done in two
+ways. We may either, before commencing work&mdash;that is, before fastening
+our elastic cord so as to exclude all light&mdash;direct the tube so that its
+shadow shall be a perfect circle (when of course it is truly directed),
+then fasten the cord and afterwards we can easily keep the sun in the
+field by slightly shifting the tube as occasion requires. Or (if the
+elastic cord has already been fastened) we may remove the eye-tube and
+shift the telescope-tube about&mdash;the direction in which the sun lies
+being roughly known&mdash;until we see the spot of light received down the
+telescope's axis grow brighter and brighter and finally become <span class="pagenum"><a href="#Page_102" class="pagenum">102</a></span><a name="Page_102" id="Page_102"></a>a <i>spot
+of sun-light</i>. If a card be held near the focus of the telescope there
+will be seen in fact an image of the sun. The telescope being now
+properly directed, the eye-tube may be slipped in again, and the sun may
+be kept in the field as before.</p>
+
+<p>There will now be seen upon the screen a picture of the sun very
+brilliant and pleasing, but perhaps a little out of focus. The focusing
+should therefore next be attended to, the increase of clearness in the
+image being the test of approach to the true focus. And again, it will
+be well to try the effect of slight changes of distance between the
+screen and the telescope's eye-piece. Mr. Howlett considers one yard as
+a convenient distance for producing an excellent effect with almost any
+eye-piece that the state of the atmosphere will admit of. Of course, the
+image becomes more sharply defined if we bring the screen nearer to the
+telescope, while all the details are enlarged when we move the screen
+away. The enlargement has no limits save those depending on the amount
+of light in the image. But, of course, the observer must not expect
+enlargement to bring with it a view of new details, after a certain
+magnitude of image has been attained. Still there is something
+instructive, I think, in occasionally getting a very magnified view of
+some remarkable spot. I have often looked with enhanced feelings of awe
+and wonder on the gigantic image of a solar spot thrown by means of the
+diagonal eye-piece upon the ceiling of the observing-room. Blurred and
+indistinct through over-magnifying, yet with a new meaning to me,
+<i>there</i> the vast abysm lies pictured; vague imaginings of the vast and
+incomprehensible agencies at work in the great centre of our system
+crowd unbidden into my mind; and I seem to <i>feel</i>&mdash;not merely think
+about&mdash;the stupendous grandeur of that life-emitting orb.</p>
+
+<p><span class="pagenum"><a href="#Page_103" class="pagenum">103</a></span><a name="Page_103" id="Page_103"></a>To return, however, to observation:&mdash;By slightly shifting the tube,
+different parts of the solar disc can be brought successively upon the
+screen and scrutinized as readily as if they were drawn upon a chart.
+"With a power of&mdash;say about 60 or 80 linear&mdash;the most minute solar spot,
+properly so called, that is capable of formation" (Mr. Howlett believes
+"they are never less than three seconds in length or breadth) will be
+more readily detected than by any other method," see Plate <a href="#plate_VII">7</a>; "as also
+will any facul&aelig;, mottling, or in short, any other phenomena that may
+then be existing on the disc." "Drifting clouds frequently sweep by, to
+vary the scene, and occasionally an a&euml;rial hail- or snow-storm." Mr.
+Howlett has more than once seen a distant flight of rooks pass slowly
+across the disc with wonderful distinctness, when the sun has been at a
+low altitude, and likewise, much more frequently, the rapid dash of
+starlings, which, very much closer at hand, frequent his church-tower."</p>
+
+<p>An eclipse of the sun, or a transit of an inferior planet, is also much
+better seen in this way than by any other method of observing the solar
+disc. In Plate <a href="#plate_VII">7</a> are presented several solar spots as they have appeared
+to Mr. Howlett, with an instrument of moderate power. The grotesque
+forms of some of these are remarkable; and the variations the spots
+undergo from day to day are particularly interesting to the thoughtful
+observer.</p>
+
+<p>A method of measuring the spots may now be described. It is not likely
+indeed that the ordinary observer will care to enter upon any systematic
+series of measurements. But even in his case, the means of forming a
+general comparison between the spots he sees at different times cannot
+fail to be valuable. Also the knowledge&mdash;which a simple method of
+measurement supplies&mdash;of the actual di<span class="pagenum"><a href="#Page_104" class="pagenum">104</a></span><a name="Page_104" id="Page_104"></a>mensions of a spot in miles
+(roughly) is calculated to enhance our estimate of the importance of
+these features of the solar disc. I give Mr. Howlett's method in his own
+words:&mdash;</p>
+
+<p>"Cause your optician to rule for you on a circular piece of glass a
+number of fine graduations, the 200th part of an inch apart, each fifth
+and tenth line being of a different length in order to assist the eye in
+their enumeration. Insert this between the anterior and posterior lenses
+of a Huygenian eye-piece of moderate power, say 80 linear. Direct your
+telescope upon the sun, and having so arranged it that the whole disc of
+the sun may be projected on the screen, count carefully the number of
+graduations that are seen to exactly occupy the solar diameter.... It
+matters not in which direction you measure your diameter, provided only
+the sun has risen some 18&deg; or 20&deg; above the horizon, and so escaped the
+distortion occasioned by refraction.<a name="FNanchor_16_16" id="FNanchor_16_16"></a><a href="#Footnote_16_16" class="fnanchor">[16]</a></p>
+
+<p>"Next let us suppose that our observer has been observing the sun on any
+day of the year, say, if you choose, at the time of its mean apparent
+diameter, namely about the first of April or first of October, and has
+ascertained that" (as is the case with Mr. Howlett's instrument)
+"sixty-four graduations occupy the diameter of the projected image. Now
+the semi-diameter of the sun, at the epochs above mentioned, according
+to the tables given for every day of the year in the 'Nautical Almanac'
+<span class="pagenum"><a href="#Page_105" class="pagenum">105</a></span><a name="Page_105" id="Page_105"></a>(the same as in Dietrichsen and Hannay's very useful compilation) is
+16' 2", and consequently his mean total diameter is 32' 4" or 1924". If
+now we divide 1924" by 64" this will, of course, award as nearly as
+possible 30" as the value in celestial arc of each graduation, either as
+seen on the screen, or as applied directly to the sun or any heavenly
+body large enough to be measured by it."</p>
+
+<p>Since the sun's diameter is about 850,000 miles, each graduation (in the
+case above specified) corresponds to one-64th part of 850,000
+miles&mdash;that is, to a length of 13,256 miles on the sun's surface. Any
+other case can be treated in precisely the same manner.</p>
+
+<p>It will be found easy so to place the screen that the distance between
+successive graduations (as seen projected upon the screen) may
+correspond to any desired unit of linear measurement&mdash;say an inch. Then
+if the observer use transparent tracing-paper ruled with faint lines
+forming squares half-an-inch in size, he can comfortably copy directly
+from the screen any solar phenomena he may be struck with. A variety of
+methods of drawing will suggest themselves. Mr. Howlett, in the paper I
+have quoted from above, describes a very satisfactory method, which
+those who are anxious to devote themselves seriously to solar
+observation will do well to study.</p>
+
+<p>It is necessary that the observer should be able to determine
+approximately where the sun's equator is situated at the time of any
+observation, in order that he may assign to any spot or set of spots its
+true position in relation to solar longitude and latitude. Mr. Howlett
+shows how this may be done by three observations of the sun made at any
+fixed hour on successive days. Perhaps the following method will serve
+the purpose of the general observer sufficiently well:&mdash;</p>
+
+<p><span class="pagenum"><a href="#Page_106" class="pagenum">106</a></span><a name="Page_106" id="Page_106"></a>The hour at which the sun crosses the meridian must be taken for the
+special observation now to be described. This hour can always be learnt
+from 'Dietrichsen's Almanac'; but noon, civil time, is near enough for
+practical purposes. Now it is necessary first to know the position of
+the ecliptic with reference to the celestial equator. Of course, at noon
+a horizontal line across the sun's disc is parallel to the equator, but
+the position of that diameter of the sun which coincides with the
+ecliptic is not constant: at the summer and winter solstices this
+diameter coincides with the other, or is horizontal at noon; at the
+spring equinox the sun (which travels on the ecliptic) is passing
+towards the north of the equator, crossing that curve at an angle of
+23&frac12;&deg;, so that the ecliptic coincides with that diameter of the sun
+which cuts the horizontal one at an angle of 23&frac12;&deg; and has its <i>left</i>
+end above the horizontal diameter; and at the autumn equinox the sun is
+descending and the same description applies, only that the diameter
+(inclined 23&frac12;&deg; to the horizon) which has its <i>right</i> end uppermost,
+now represents the ecliptic. For intermediate dates, use the following
+little table:&mdash;</p>
+
+
+<div class='center'>
+<table border="1" cellpadding="4" cellspacing="0" summary="Inclination of Ecliptical Diameter of Sun to the Horizon" style="text-align:center">
+<tr><td rowspan="2">Date. (<i>Circiter</i>.)</td><td style="border-bottom: none">Dec. 22</td><td style="border-bottom: none">Jan. 5</td><td style="border-bottom: none">Jan. 20</td><td style="border-bottom: none">Feb. 4</td><td style="border-bottom: none">Feb. 19</td><td style="border-bottom: none">Mar. 5</td><td style="border-bottom: none">&nbsp;</td></tr>
+<tr><td style="border-top: none">&nbsp;</td><td style="border-top: none">June 6</td><td style="border-top: none">May 21</td><td style="border-top: none">May 5</td><td style="border-top: none">Apr. 20</td><td style="border-top: none">Apr. 5</td><td style="border-top: none">Mar. 21</td></tr>
+<tr><td rowspan="3">Inclination of Ecliptical Diameter of Sun to the Horizon.<a name="FNanchor_17_17" id="FNanchor_17_17"></a><a href="#Footnote_17_17" class="fnanchor">[17]</a></td>
+<td style="border-bottom: none">Left</td><td style="border-bottom: none">Left</td><td style="border-bottom: none">Left</td><td style="border-bottom: none">Left</td><td style="border-bottom: none">Left</td><td style="border-bottom: none">Left</td><td style="border-bottom: none">Left</td></tr>
+<tr><td style="border-bottom: none;border-top: none">0&deg; 0'</td><td style="border-bottom: none;border-top: none">6&deg; 24'</td><td style="border-bottom: none;border-top: none">12&deg; 14'</td><td style="border-bottom: none;border-top: none">17&deg; 3'</td><td style="border-bottom: none;border-top: none">20&deg; 36'</td><td style="border-bottom: none;border-top: none">22&deg; 44'</td><td style="border-bottom: none;border-top: none">23&deg; 27'</td></tr>
+<tr><td style="border-top: none">Right</td><td style="border-top: none">Right</td><td style="border-top: none">Right</td><td style="border-top: none">Right</td><td style="border-top: none">Right</td><td style="border-top: none">Right</td><td style="border-top: none">Right</td></tr>
+<tr><td rowspan="2">Date. (<i>Circiter</i>.)</td><td style="border-bottom: none">Jan. 21</td><td style="border-bottom: none">Dec. 7</td><td style="border-bottom: none">Nov. 22</td><td style="border-bottom: none">Nov. 7</td><td style="border-bottom: none">Oct. 23</td><td style="border-bottom: none">Oct. 8</td><td style="border-bottom: none">&nbsp;</td></tr>
+<tr><td style="border-top: none">&nbsp;</td><td style="border-top: none">July 7</td><td style="border-top: none">July 23</td><td style="border-top: none">Aug. 6</td><td style="border-top: none">Aug. 23</td><td style="border-top: none">Sept. 7</td><td style="border-top: none">Sept. 23</td></tr>
+</table></div>
+
+<p><span class="pagenum"><a href="#Page_107" class="pagenum">107</a></span><a name="Page_107" id="Page_107"></a>Now if our observer describe a circle, and draw a diameter inclined
+according to above table, this diameter would represent the sun's
+equator if the axis of the sun were square to the ecliptic-plane. But
+this axis is slightly inclined, the effect of which is, that on or about
+June 10 the sun is situated as shown in <a href="#fig_14">fig. 14</a> with respect to the
+ecliptic <i>ab</i>; on or about September 11 he is situated as shown in <a href="#fig_13">fig.
+13</a>; on or about December 11 as shown in <a href="#fig_12">fig. 12</a>; and on or about March
+10 as shown in <a href="#fig_15">fig. 15</a>. The inclination of his equator to the ecliptic
+being so small, the student can find little difficulty in determining
+with sufficient approximation the relation of the sun's <span class="pagenum"><a href="#Page_108" class="pagenum">108</a></span><a name="Page_108" id="Page_108"></a>polar axis to
+the ecliptic on intermediate days, since the equator is never more
+<i>inclined</i> than in <a href="#fig_12">figs. 12</a> and <a href="#fig_14">14</a>, never more <i>opened out</i> than in
+<a href="#fig_13">figs. 13</a> and <a href="#fig_15">15</a>. Having then drawn a line to represent the sun's
+ecliptical diameter inclined to the horizontal diameter as above
+described, and having (with this line to correspond to <i>ab</i> in <a href="#fig_12">figs.
+12</a>-<a href="#fig_15">15</a>) drawn in the sun's equator suitably inclined and opened out, he
+has the sun's actual presentation (at noon) as seen with an erecting
+eye-piece. Holding his picture upside down, he has the sun's
+presentation as seen with an astronomical eye-piece&mdash;and, finally,
+looking at his picture from behind (without inverting it), he has the
+presentation seen when the sun is projected on the screen. Hence, if he
+make a copy of this last view of his diagram upon the centre of his
+screen, and using a low power, bring the whole of the sun's image to
+coincide with the circle thus drawn (to a suitable scale) on the screen,
+he will at once see what is the true position of the different
+sun-spots. After a little practice the construction of a suitably sized
+and marked circle on the screen will not occupy more than a minute or
+two.</p>
+
+<table style="text-align:center;" border="0" cellspacing="0" cellpadding="0" summary="display of images">
+  <tr>
+    <td><b>Fig. 12.</b></td>
+    <td><b>Fig. 13.</b></td>
+  </tr>
+  <tr>
+    <td>
+      <a name="fig_12" id="fig_12"></a>
+      <a href="images/fig12.jpg"><img src="images/fig12.jpg" title="Fig. 12." alt="Figure 12"  width="114px"/></a>
+    </td>
+    <td>
+      <a name="fig_13" id="fig_13"></a>
+      <a href="images/fig13.jpg"><img src="images/fig13.jpg" title="Fig. 13." alt="Figure 13" width="113px" /></a>
+    </td>
+  </tr>
+  <tr>
+    <td>
+      <a name="fig_14" id="fig_14"></a>
+      <a href="images/fig14.jpg"><img src="images/fig14.jpg" title="Fig. 14." alt="Figure 14" width="114px" /></a>
+    </td>
+    <td>
+      <a name="fig_15" id="fig_15"></a>
+      <a href="images/fig15.jpg"><img src="images/fig15.jpg" title="Fig. 15." alt="Figure 15" width="117px" /></a>
+    </td>
+  </tr>
+  <tr>
+    <td><b>Fig. 14.</b></td>
+    <td><b>Fig. 15.</b></td>
+  </tr>
+</table>
+
+
+<p>It must be noticed that the sun's apparent diameter is not always the
+same. He is nearer to us in winter than in summer, and, of course, his
+apparent diameter is greater at the former than at the latter season.
+The variation of the apparent diameter corresponds (inversely) to the
+variation of distance. As the sun's greatest distance from the earth is
+93,000,000 miles (pretty nearly) and his least 90,000,000, his greatest,
+mean, and least apparent diameters are as 93, 91&frac12;, and 90
+respectively; that is, as 62, 61, and 60 respectively.</p>
+
+<p>Mr. Howlett considers that with a good 3-inch telescope, applied in the
+manner we have described, all the solar features may be seen, except the
+sepa<span class="pagenum"><a href="#Page_109" class="pagenum">109</a></span><a name="Page_109" id="Page_109"></a>rate granules disclosed by first-class instruments in the hands of
+such observers as Dawes, Huggins, or Secchi. Facul&aelig; may, of course, be
+well seen. They are to be looked for near spots which lie close to the
+sun's limb.</p>
+
+<p>When the sun's general surface is carefully scrutinised, it is found to
+present a mottled appearance. This is a somewhat delicate feature. It
+results, undoubtedly, from the combined effect of the granules
+separately seen in powerful instruments. Sir John Herschel has stated
+that he cannot recognise the marbled appearance of the sun with an
+achromatic. Mr. Webb, however, has seen this appearance with such a
+telescope, of moderate power, used with direct vision; and certainly I
+can corroborate Mr. Howlett in the statement that this appearance may be
+most distinctly seen when the image of the sun is received within a
+well-darkened room.</p>
+
+<p>My space will not permit me to enter here upon the discussion of any of
+those interesting speculations which have been broached concerning solar
+phenomena. We may hope that the great eclipse of August, 1868, which
+promises to be the most favourable (for effective observation) that has
+ever taken place, will afford astronomers the opportunity of resolving
+some important questions. It seems as if we were on the verge of great
+discoveries,&mdash;and certainly, if persevering and well-directed labour
+would seem in any case to render such discoveries due as man's just
+reward, we may well say that he deserves shortly to reap a harvest of
+exact knowledge respecting solar phenomena.</p>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="THE_END" id="THE_END"></a>THE END.</h2>
+<p><span class="pagenum"><a href="#Page_110" class="pagenum">110</a></span><a name="Page_110" id="Page_110"></a></p>
+
+
+<div class="footnotes"><h3>FOOTNOTES:</h3>
+
+<div class="footnote"><a name="Footnote_1_1" id="Footnote_1_1"></a><a href="#FNanchor_1_1" class="fnlink"><span class="label">[1]</span></a> Such a telescope is most powerful with the shortest sight.
+It may be remarked that the use of a telescope often reveals a
+difference in the sight of the two eyes. In my own case, for instance, I
+have found that the left eye is very short-sighted, the sight of the
+right eye being of about the average range. Accordingly with my left eye
+a 5&frac12;-foot object-glass, alone, forms an effective telescope, with
+which I can see Jupiter's moons quite distinctly, and under favourable
+circumstances even Saturn's rings. I find that the full moon is too
+bright to be observed in this way without pain, except at low
+altitudes.</div>
+
+<div class="footnote"><a name="Footnote_2_2" id="Footnote_2_2"></a><a href="#FNanchor_2_2" class="fnlink"><span class="label">[2]</span></a> Betelgeuse&mdash;commonly interpreted the Giant's
+Shoulder&mdash;<i>ibt-al-jauza</i>. The words, however, really signify, "the
+armpit of the central one," Orion being so named because he is divided
+centrally by the equator.</div>
+
+<div class="footnote"><a name="Footnote_3_3" id="Footnote_3_3"></a><a href="#FNanchor_3_3" class="fnlink"><span class="label">[3]</span></a> I have never been able to see more than four with a
+3&frac34;-inch aperture. I give a view of the trapezium as seen with an
+8-inch equatorial.</div>
+
+<div class="footnote"><a name="Footnote_4_4" id="Footnote_4_4"></a><a href="#FNanchor_4_4" class="fnlink"><span class="label">[4]</span></a> Sir W. Herschel several times saw &epsilon; Lyr&aelig; as a
+double. Bessel also relates that when he was a lad of thirteen he could
+see this star double. I think persons having average eye-sight could see
+it double if they selected a suitable hour for observation. My own
+eye-sight is not good enough for this, but I can distinctly see this
+star wedged whenever the line joining the components is inclined about
+45&deg; to the horizon, and also when Lyra is near the zenith.</div>
+
+<div class="footnote"><a name="Footnote_5_5" id="Footnote_5_5"></a><a href="#FNanchor_5_5" class="fnlink"><span class="label">[5]</span></a> They were so described by Admiral Smyth in 1839. Mr. Main,
+in 1862, describes them as straw-coloured and reddish, while Mr. Webb,
+in 1865, saw them pale-yellow and <i>lilac</i>!</div>
+
+<div class="footnote"><a name="Footnote_6_6" id="Footnote_6_6"></a><a href="#FNanchor_6_6" class="fnlink"><span class="label">[6]</span></a> Or the observer may sweep from &omicron; towards &nu;,
+looking for R about two-fifths of the way from &omicron; to &nu;.</div>
+
+<div class="footnote"><a name="Footnote_7_7" id="Footnote_7_7"></a><a href="#FNanchor_7_7" class="fnlink"><span class="label">[7]</span></a> Here a single period only is taken, to get back to a
+convenient hour of the evening.</div>
+
+<div class="footnote"><a name="Footnote_8_8" id="Footnote_8_8"></a><a href="#FNanchor_8_8" class="fnlink"><span class="label">[8]</span></a> Here a single period only is taken, to get back to a
+convenient hour of the evening.</div>
+
+<div class="footnote"><a name="Footnote_9_9" id="Footnote_9_9"></a><a href="#FNanchor_9_9" class="fnlink"><span class="label">[9]</span></a> I have constructed a zodiac-chart, which will enable the
+student to mark in the path of a planet, at any season of the year, from
+the recorded places in the almanacs.</div>
+
+<div class="footnote"><a name="Footnote_10_10" id="Footnote_10_10"></a><a href="#FNanchor_10_10" class="fnlink"><span class="label">[10]</span></a> It is convenient to remember that through precession a
+star near the ecliptic shifts as respects the R.A. and Dec. lines,
+through an arc of one degree&mdash;or nearly twice the moon's diameter&mdash;in
+about 72 years, all other stars through a less arc.</div>
+
+<div class="footnote"><a name="Footnote_11_11" id="Footnote_11_11"></a><a href="#FNanchor_11_11" class="fnlink"><span class="label">[11]</span></a> Mercury is best seen when in quadrature to the sun, but
+<i>not</i> (as I have seen stated) at those quadratures in which he attains
+his maximum elongation from the sun. This will appear singular, because
+the maximum elongation is about 27&deg;, the minimum only about 18&deg;. But it
+happens that in our northern latitudes Mercury is always <i>south</i> of the
+sun when he attains his maximum elongation, and this fact exercises a
+more important effect than the mere amount of elongation.</div>
+
+<div class="footnote"><a name="Footnote_12_12" id="Footnote_12_12"></a><a href="#FNanchor_12_12" class="fnlink"><span class="label">[12]</span></a> It does not seem to me that the difficulty of detecting
+Mercury is due to the difficulty "of identifying it amongst the
+surrounding stars, during the short time that it can be seen" (Hind's
+'Introduction to Astronomy'). There are few stars which are comparable
+with Mercury in brilliancy, when seen under the same light.</div>
+
+<div class="footnote"><a name="Footnote_13_13" id="Footnote_13_13"></a><a href="#FNanchor_13_13" class="fnlink"><span class="label">[13]</span></a> I may notice another error sometimes made. It is said that
+the shadow of a satellite <i>appears</i> elliptical when near the edge of the
+disc. The shadow is <i>in reality</i> elliptical when thus situated, but
+<i>appears</i> circular. A moment's consideration will show that this should
+be so. The part of the disc concealed by a <i>satellite</i> near the limb is
+also elliptical, but of course appears round.</div>
+
+<div class="footnote"><a name="Footnote_14_14" id="Footnote_14_14"></a><a href="#FNanchor_14_14" class="fnlink"><span class="label">[14]</span></a> From a paper by Mr. Breen, in the 'Popular Science
+Review,' October, 1864.</div>
+
+<div class="footnote"><a name="Footnote_15_15" id="Footnote_15_15"></a><a href="#FNanchor_15_15" class="fnlink"><span class="label">[15]</span></a> 'Intellectual Observer' for July, 1867, to which magazine
+the reader is referred for full details of Mr. Howlett's method of
+observation, and for illustrations of the appliances he made use of, and
+of some of his results.</div>
+
+<div class="footnote"><a name="Footnote_16_16" id="Footnote_16_16"></a><a href="#FNanchor_16_16" class="fnlink"><span class="label">[16]</span></a> As the sun does not attain such an altitude as 18&deg; during
+two months in the year, it is well to notice that the true length of the
+sun's apparent solar diameter is determinable even immediately after
+sun-rise, if the line of graduation is made to coincide with the
+<i>horizontal</i> diameter of the picture on the screen&mdash;for refraction does
+not affect the length of this diameter.</div>
+
+<div class="footnote"><a name="Footnote_17_17" id="Footnote_17_17"></a><a href="#FNanchor_17_17" class="fnlink"><span class="label">[17]</span></a> The words "Left" and "Right" indicate which end of the
+sun's ecliptical diameter is uppermost at the dates in upper or lower
+row respectively.</div>
+</div>
+
+
+<hr style="width: 65%;" />
+<h2><a name="LONDON" id="LONDON"></a>LONDON:</h2>
+
+<p>PRINTED BY W. CLOWES AND SONS, DUKE STREET, STAMFORD STREET, AND CHARING
+CROSS.</p>
+<div class="figcenter">
+<img src="images/back.jpg" alt="Back Cover" title="Back Cover" />
+</div>
+
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
+End of the Project Gutenberg EBook of Half-hours with the Telescope
+by Richard A. Proctor
+
+*** END OF THIS PROJECT GUTENBERG EBOOK HALF-HOURS WITH THE TELESCOPE ***
+
+***** This file should be named 16767-h.htm or 16767-h.zip *****
+This and all associated files of various formats will be found in:
+        https://www.gutenberg.org/1/6/7/6/16767/
+
+Produced by Jason Isbell and the Online Distributed
+Proofreading Team at https://www.pgdp.net
+
+
+Updated editions will replace the previous one--the old editions
+will be renamed.
+
+Creating the works from public domain print editions 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.  Special rules,
+set forth in the General Terms of Use part of this license, apply to
+copying and distributing Project Gutenberg-tm electronic works to
+protect the PROJECT GUTENBERG-tm concept and trademark.  Project
+Gutenberg is a registered trademark, and may not be used if you
+charge for the eBooks, unless you receive specific permission.  If you
+do not charge anything for copies of this eBook, complying with the
+rules is very easy.  You may use this eBook for nearly any purpose
+such as creation of derivative works, reports, performances and
+research.  They may be modified and printed and given away--you may do
+practically ANYTHING with public domain eBooks.  Redistribution is
+subject to the trademark license, especially commercial
+redistribution.
+
+
+
+*** START: FULL LICENSE ***
+
+THE FULL PROJECT GUTENBERG LICENSE
+PLEASE READ THIS BEFORE YOU DISTRIBUTE OR USE THIS WORK
+
+To protect the Project Gutenberg-tm mission of promoting the free
+distribution of electronic works, by using or distributing this work
+(or any other work associated in any way with the phrase "Project
+Gutenberg"), you agree to comply with all the terms of the Full Project
+Gutenberg-tm License (available with this file or online at
+https://gutenberg.org/license).
+
+
+Section 1.  General Terms of Use and Redistributing Project Gutenberg-tm
+electronic works
+
+1.A.  By reading or using any part of this Project Gutenberg-tm
+electronic work, you indicate that you have read, understand, agree to
+and accept all the terms of this license and intellectual property
+(trademark/copyright) agreement.  If you do not agree to abide by all
+the terms of this agreement, you must cease using and return or destroy
+all copies of Project Gutenberg-tm electronic works in your possession.
+If you paid a fee for obtaining a copy of or access to a Project
+Gutenberg-tm electronic work and you do not agree to be bound by the
+terms of this agreement, you may obtain a refund from the person or
+entity to whom you paid the fee as set forth in paragraph 1.E.8.
+
+1.B.  "Project Gutenberg" is a registered trademark.  It may only be
+used on or associated in any way with an electronic work by people who
+agree to be bound by the terms of this agreement.  There are a few
+things that you can do with most Project Gutenberg-tm electronic works
+even without complying with the full terms of this agreement.  See
+paragraph 1.C below.  There are a lot of things you can do with Project
+Gutenberg-tm electronic works if you follow the terms of this agreement
+and help preserve free future access to Project Gutenberg-tm electronic
+works.  See paragraph 1.E below.
+
+1.C.  The Project Gutenberg Literary Archive Foundation ("the Foundation"
+or PGLAF), owns a compilation copyright in the collection of Project
+Gutenberg-tm electronic works.  Nearly all the individual works in the
+collection are in the public domain in the United States.  If an
+individual work is in the public domain in the United States and you are
+located in the United States, we do not claim a right to prevent you from
+copying, distributing, performing, displaying or creating derivative
+works based on the work as long as all references to Project Gutenberg
+are removed.  Of course, we hope that you will support the Project
+Gutenberg-tm mission of promoting free access to electronic works by
+freely sharing Project Gutenberg-tm works in compliance with the terms of
+this agreement for keeping the Project Gutenberg-tm name associated with
+the work.  You can easily comply with the terms of this agreement by
+keeping this work in the same format with its attached full Project
+Gutenberg-tm License when you share it without charge with others.
+
+1.D.  The copyright laws of the place where you are located also govern
+what you can do with this work.  Copyright laws in most countries are in
+a constant state of change.  If you are outside the United States, check
+the laws of your country in addition to the terms of this agreement
+before downloading, copying, displaying, performing, distributing or
+creating derivative works based on this work or any other Project
+Gutenberg-tm work.  The Foundation makes no representations concerning
+the copyright status of any work in any country outside the United
+States.
+
+1.E.  Unless you have removed all references to Project Gutenberg:
+
+1.E.1.  The following sentence, with active links to, or other immediate
+access to, the full Project Gutenberg-tm License must appear prominently
+whenever any copy of a Project Gutenberg-tm work (any work on which the
+phrase "Project Gutenberg" appears, or with which the phrase "Project
+Gutenberg" is associated) is accessed, displayed, performed, viewed,
+copied or distributed:
+
+This eBook is for the use of anyone anywhere 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
+
+1.E.2.  If an individual Project Gutenberg-tm electronic work is derived
+from the public domain (does not contain a notice indicating that it is
+posted with permission of the copyright holder), the work can be copied
+and distributed to anyone in the United States without paying any fees
+or charges.  If you are redistributing or providing access to a work
+with the phrase "Project Gutenberg" associated with or appearing on the
+work, you must comply either with the requirements of paragraphs 1.E.1
+through 1.E.7 or obtain permission for the use of the work and the
+Project Gutenberg-tm trademark as set forth in paragraphs 1.E.8 or
+1.E.9.
+
+1.E.3.  If an individual Project Gutenberg-tm electronic work is posted
+with the permission of the copyright holder, your use and distribution
+must comply with both paragraphs 1.E.1 through 1.E.7 and any additional
+terms imposed by the copyright holder.  Additional terms will be linked
+to the Project Gutenberg-tm License for all works posted with the
+permission of the copyright holder found at the beginning of this work.
+
+1.E.4.  Do not unlink or detach or remove the full Project Gutenberg-tm
+License terms from this work, or any files containing a part of this
+work or any other work associated with Project Gutenberg-tm.
+
+1.E.5.  Do not copy, display, perform, distribute or redistribute this
+electronic work, or any part of this electronic work, without
+prominently displaying the sentence set forth in paragraph 1.E.1 with
+active links or immediate access to the full terms of the Project
+Gutenberg-tm License.
+
+1.E.6.  You may convert to and distribute this work in any binary,
+compressed, marked up, nonproprietary or proprietary form, including any
+word processing or hypertext form.  However, if you provide access to or
+distribute copies of a Project Gutenberg-tm work in a format other than
+"Plain Vanilla ASCII" or other format used in the official version
+posted on the official Project Gutenberg-tm web site (www.gutenberg.org),
+you must, at no additional cost, fee or expense to the user, provide a
+copy, a means of exporting a copy, or a means of obtaining a copy upon
+request, of the work in its original "Plain Vanilla ASCII" or other
+form.  Any alternate format must include the full Project Gutenberg-tm
+License as specified in paragraph 1.E.1.
+
+1.E.7.  Do not charge a fee for access to, viewing, displaying,
+performing, copying or distributing any Project Gutenberg-tm works
+unless you comply with paragraph 1.E.8 or 1.E.9.
+
+1.E.8.  You may charge a reasonable fee for copies of or providing
+access to or distributing Project Gutenberg-tm electronic works provided
+that
+
+- You pay a royalty fee of 20% of the gross profits you derive from
+     the use of Project Gutenberg-tm works calculated using the method
+     you already use to calculate your applicable taxes.  The fee is
+     owed to the owner of the Project Gutenberg-tm trademark, but he
+     has agreed to donate royalties under this paragraph to the
+     Project Gutenberg Literary Archive Foundation.  Royalty payments
+     must be paid within 60 days following each date on which you
+     prepare (or are legally required to prepare) your periodic tax
+     returns.  Royalty payments should be clearly marked as such and
+     sent to the Project Gutenberg Literary Archive Foundation at the
+     address specified in Section 4, "Information about donations to
+     the Project Gutenberg Literary Archive Foundation."
+
+- You provide a full refund of any money paid by a user who notifies
+     you in writing (or by e-mail) within 30 days of receipt that s/he
+     does not agree to the terms of the full Project Gutenberg-tm
+     License.  You must require such a user to return or
+     destroy all copies of the works possessed in a physical medium
+     and discontinue all use of and all access to other copies of
+     Project Gutenberg-tm works.
+
+- You provide, in accordance with paragraph 1.F.3, a full refund of any
+     money paid for a work or a replacement copy, if a defect in the
+     electronic work is discovered and reported to you within 90 days
+     of receipt of the work.
+
+- You comply with all other terms of this agreement for free
+     distribution of Project Gutenberg-tm works.
+
+1.E.9.  If you wish to charge a fee or distribute a Project Gutenberg-tm
+electronic work or group of works on different terms than are set
+forth in this agreement, you must obtain permission in writing from
+both the Project Gutenberg Literary Archive Foundation and Michael
+Hart, the owner of the Project Gutenberg-tm trademark.  Contact the
+Foundation as set forth in Section 3 below.
+
+1.F.
+
+1.F.1.  Project Gutenberg volunteers and employees expend considerable
+effort to identify, do copyright research on, transcribe and proofread
+public domain works in creating the Project Gutenberg-tm
+collection.  Despite these efforts, Project Gutenberg-tm electronic
+works, and the medium on which they may be stored, may contain
+"Defects," such as, but not limited to, incomplete, inaccurate or
+corrupt data, transcription errors, a copyright or other intellectual
+property infringement, a defective or damaged disk or other medium, a
+computer virus, or computer codes that damage or cannot be read by
+your equipment.
+
+1.F.2.  LIMITED WARRANTY, DISCLAIMER OF DAMAGES - Except for the "Right
+of Replacement or Refund" described in paragraph 1.F.3, the Project
+Gutenberg Literary Archive Foundation, the owner of the Project
+Gutenberg-tm trademark, and any other party distributing a Project
+Gutenberg-tm electronic work under this agreement, disclaim all
+liability to you for damages, costs and expenses, including legal
+fees.  YOU AGREE THAT YOU HAVE NO REMEDIES FOR NEGLIGENCE, STRICT
+LIABILITY, BREACH OF WARRANTY OR BREACH OF CONTRACT EXCEPT THOSE
+PROVIDED IN PARAGRAPH F3.  YOU AGREE THAT THE FOUNDATION, THE
+TRADEMARK OWNER, AND ANY DISTRIBUTOR UNDER THIS AGREEMENT WILL NOT BE
+LIABLE TO YOU FOR ACTUAL, DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE OR
+INCIDENTAL DAMAGES EVEN IF YOU GIVE NOTICE OF THE POSSIBILITY OF SUCH
+DAMAGE.
+
+1.F.3.  LIMITED RIGHT OF REPLACEMENT OR REFUND - If you discover a
+defect in this electronic work within 90 days of receiving it, you can
+receive a refund of the money (if any) you paid for it by sending a
+written explanation to the person you received the work from.  If you
+received the work on a physical medium, you must return the medium with
+your written explanation.  The person or entity that provided you with
+the defective work may elect to provide a replacement copy in lieu of a
+refund.  If you received the work electronically, the person or entity
+providing it to you may choose to give you a second opportunity to
+receive the work electronically in lieu of a refund.  If the second copy
+is also defective, you may demand a refund in writing without further
+opportunities to fix the problem.
+
+1.F.4.  Except for the limited right of replacement or refund set forth
+in paragraph 1.F.3, this work is provided to you 'AS-IS' WITH NO OTHER
+WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO
+WARRANTIES OF MERCHANTIBILITY OR FITNESS FOR ANY PURPOSE.
+
+1.F.5.  Some states do not allow disclaimers of certain implied
+warranties or the exclusion or limitation of certain types of damages.
+If any disclaimer or limitation set forth in this agreement violates the
+law of the state applicable to this agreement, the agreement shall be
+interpreted to make the maximum disclaimer or limitation permitted by
+the applicable state law.  The invalidity or unenforceability of any
+provision of this agreement shall not void the remaining provisions.
+
+1.F.6.  INDEMNITY - You agree to indemnify and hold the Foundation, the
+trademark owner, any agent or employee of the Foundation, anyone
+providing copies of Project Gutenberg-tm electronic works in accordance
+with this agreement, and any volunteers associated with the production,
+promotion and distribution of Project Gutenberg-tm electronic works,
+harmless from all liability, costs and expenses, including legal fees,
+that arise directly or indirectly from any of the following which you do
+or cause to occur: (a) distribution of this or any Project Gutenberg-tm
+work, (b) alteration, modification, or additions or deletions to any
+Project Gutenberg-tm work, and (c) any Defect you cause.
+
+
+Section  2.  Information about the Mission of Project Gutenberg-tm
+
+Project Gutenberg-tm is synonymous with the free distribution of
+electronic works in formats readable by the widest variety of computers
+including obsolete, old, middle-aged and new computers.  It exists
+because of the efforts of hundreds of volunteers and donations from
+people in all walks of life.
+
+Volunteers and financial support to provide volunteers with the
+assistance they need, is critical to reaching Project Gutenberg-tm's
+goals and ensuring that the Project Gutenberg-tm collection will
+remain freely available for generations to come.  In 2001, the Project
+Gutenberg Literary Archive Foundation was created to provide a secure
+and permanent future for Project Gutenberg-tm and future generations.
+To learn more about the Project Gutenberg Literary Archive Foundation
+and how your efforts and donations can help, see Sections 3 and 4
+and the Foundation web page at https://www.pglaf.org.
+
+
+Section 3.  Information about the Project Gutenberg Literary Archive
+Foundation
+
+The Project Gutenberg Literary Archive Foundation is a non profit
+501(c)(3) educational corporation organized under the laws of the
+state of Mississippi and granted tax exempt status by the Internal
+Revenue Service.  The Foundation's EIN or federal tax identification
+number is 64-6221541.  Its 501(c)(3) letter is posted at
+https://pglaf.org/fundraising.  Contributions to the Project Gutenberg
+Literary Archive Foundation are tax deductible to the full extent
+permitted by U.S. federal laws and your state's laws.
+
+The Foundation's principal office is located at 4557 Melan Dr. S.
+Fairbanks, AK, 99712., but its volunteers and employees are scattered
+throughout numerous locations.  Its business office is located at
+809 North 1500 West, Salt Lake City, UT 84116, (801) 596-1887, email
+business@pglaf.org.  Email contact links and up to date contact
+information can be found at the Foundation's web site and official
+page at https://pglaf.org
+
+For additional contact information:
+     Dr. Gregory B. Newby
+     Chief Executive and Director
+     gbnewby@pglaf.org
+
+
+Section 4.  Information about Donations to the Project Gutenberg
+Literary Archive Foundation
+
+Project Gutenberg-tm depends upon and cannot survive without wide
+spread public support and donations to carry out its mission of
+increasing the number of public domain and licensed works that can be
+freely distributed in machine readable form accessible by the widest
+array of equipment including outdated equipment.  Many small donations
+($1 to $5,000) are particularly important to maintaining tax exempt
+status with the IRS.
+
+The Foundation is committed to complying with the laws regulating
+charities and charitable donations in all 50 states of the United
+States.  Compliance requirements are not uniform and it takes a
+considerable effort, much paperwork and many fees to meet and keep up
+with these requirements.  We do not solicit donations in locations
+where we have not received written confirmation of compliance.  To
+SEND DONATIONS or determine the status of compliance for any
+particular state visit https://pglaf.org
+
+While we cannot and do not solicit contributions from states where we
+have not met the solicitation requirements, we know of no prohibition
+against accepting unsolicited donations from donors in such states who
+approach us with offers to donate.
+
+International donations are gratefully accepted, but we cannot make
+any statements concerning tax treatment of donations received from
+outside the United States.  U.S. laws alone swamp our small staff.
+
+Please check the Project Gutenberg Web pages for current donation
+methods and addresses.  Donations are accepted in a number of other
+ways including including checks, online payments and credit card
+donations.  To donate, please visit: https://pglaf.org/donate
+
+
+Section 5.  General Information About Project Gutenberg-tm electronic
+works.
+
+Professor Michael S. Hart was the originator of the Project Gutenberg-tm
+concept of a library of electronic works that could be freely shared
+with anyone.  For thirty years, he produced and distributed Project
+Gutenberg-tm eBooks with only a loose network of volunteer support.
+
+
+Project Gutenberg-tm eBooks are often created from several printed
+editions, all of which are confirmed as Public Domain in the U.S.
+unless a copyright notice is included.  Thus, we do not necessarily
+keep eBooks in compliance with any particular paper edition.
+
+
+Most people start at our Web site which has the main PG search facility:
+
+     https://www.gutenberg.org
+
+This Web site includes information about Project Gutenberg-tm,
+including how to make donations to the Project Gutenberg Literary
+Archive Foundation, how to help produce our new eBooks, and how to
+subscribe to our email newsletter to hear about new eBooks.
+
+
+</pre>
+
+</body>
+</html>
diff --git a/16767-h/images/back.jpg b/16767-h/images/back.jpg
new file mode 100644
index 0000000..b3763b6
--- /dev/null
+++ b/16767-h/images/back.jpg
diff --git a/16767-h/images/fig01.jpg b/16767-h/images/fig01.jpg
new file mode 100644
index 0000000..a733911
--- /dev/null
+++ b/16767-h/images/fig01.jpg
diff --git a/16767-h/images/fig02.jpg b/16767-h/images/fig02.jpg
new file mode 100644
index 0000000..092d4d7
--- /dev/null
+++ b/16767-h/images/fig02.jpg
diff --git a/16767-h/images/fig03.jpg b/16767-h/images/fig03.jpg
new file mode 100644
index 0000000..6c94339
--- /dev/null
+++ b/16767-h/images/fig03.jpg
diff --git a/16767-h/images/fig04.jpg b/16767-h/images/fig04.jpg
new file mode 100644
index 0000000..158981a
--- /dev/null
+++ b/16767-h/images/fig04.jpg
diff --git a/16767-h/images/fig05.jpg b/16767-h/images/fig05.jpg
new file mode 100644
index 0000000..bd13793
--- /dev/null
+++ b/16767-h/images/fig05.jpg
diff --git a/16767-h/images/fig06.jpg b/16767-h/images/fig06.jpg
new file mode 100644
index 0000000..42145a4
--- /dev/null
+++ b/16767-h/images/fig06.jpg
diff --git a/16767-h/images/fig07.jpg b/16767-h/images/fig07.jpg
new file mode 100644
index 0000000..9c21bcc
--- /dev/null
+++ b/16767-h/images/fig07.jpg
diff --git a/16767-h/images/fig08.jpg b/16767-h/images/fig08.jpg
new file mode 100644
index 0000000..5c121a6
--- /dev/null
+++ b/16767-h/images/fig08.jpg
diff --git a/16767-h/images/fig09.jpg b/16767-h/images/fig09.jpg
new file mode 100644
index 0000000..47e97fa
--- /dev/null
+++ b/16767-h/images/fig09.jpg
diff --git a/16767-h/images/fig10.jpg b/16767-h/images/fig10.jpg
new file mode 100644
index 0000000..b0afd2e
--- /dev/null
+++ b/16767-h/images/fig10.jpg
diff --git a/16767-h/images/fig11.jpg b/16767-h/images/fig11.jpg
new file mode 100644
index 0000000..d1c2c4e
--- /dev/null
+++ b/16767-h/images/fig11.jpg
diff --git a/16767-h/images/fig12.jpg b/16767-h/images/fig12.jpg
new file mode 100644
index 0000000..4d0e578
--- /dev/null
+++ b/16767-h/images/fig12.jpg
diff --git a/16767-h/images/fig13.jpg b/16767-h/images/fig13.jpg
new file mode 100644
index 0000000..5ed4f25
--- /dev/null
+++ b/16767-h/images/fig13.jpg
diff --git a/16767-h/images/fig14.jpg b/16767-h/images/fig14.jpg
new file mode 100644
index 0000000..d593163
--- /dev/null
+++ b/16767-h/images/fig14.jpg
diff --git a/16767-h/images/fig15.jpg b/16767-h/images/fig15.jpg
new file mode 100644
index 0000000..7333a5f
--- /dev/null
+++ b/16767-h/images/fig15.jpg
diff --git a/16767-h/images/front.jpg b/16767-h/images/front.jpg
new file mode 100644
index 0000000..b9a5428
--- /dev/null
+++ b/16767-h/images/front.jpg
diff --git a/16767-h/images/hr.jpg b/16767-h/images/hr.jpg
new file mode 100644
index 0000000..7773541
--- /dev/null
+++ b/16767-h/images/hr.jpg
diff --git a/16767-h/images/platei1.jpg b/16767-h/images/platei1.jpg
new file mode 100644
index 0000000..2306335
--- /dev/null
+++ b/16767-h/images/platei1.jpg
diff --git a/16767-h/images/platei1_lg.jpg b/16767-h/images/platei1_lg.jpg
new file mode 100644
index 0000000..8b772f5
--- /dev/null
+++ b/16767-h/images/platei1_lg.jpg
diff --git a/16767-h/images/platei2.jpg b/16767-h/images/platei2.jpg
new file mode 100644
index 0000000..62387d6
--- /dev/null
+++ b/16767-h/images/platei2.jpg
diff --git a/16767-h/images/platei2_lg.jpg b/16767-h/images/platei2_lg.jpg
new file mode 100644
index 0000000..b716b8b
--- /dev/null
+++ b/16767-h/images/platei2_lg.jpg
diff --git a/16767-h/images/platei3.jpg b/16767-h/images/platei3.jpg
new file mode 100644
index 0000000..de75a19
--- /dev/null
+++ b/16767-h/images/platei3.jpg
diff --git a/16767-h/images/platei3_lg.jpg b/16767-h/images/platei3_lg.jpg
new file mode 100644
index 0000000..c301e9c
--- /dev/null
+++ b/16767-h/images/platei3_lg.jpg
diff --git a/16767-h/images/platei4.jpg b/16767-h/images/platei4.jpg
new file mode 100644
index 0000000..803b32a
--- /dev/null
+++ b/16767-h/images/platei4.jpg
diff --git a/16767-h/images/platei4_lg.jpg b/16767-h/images/platei4_lg.jpg
new file mode 100644
index 0000000..b819315
--- /dev/null
+++ b/16767-h/images/platei4_lg.jpg
diff --git a/16767-h/images/plateii1.jpg b/16767-h/images/plateii1.jpg
new file mode 100644
index 0000000..a3e2161
--- /dev/null
+++ b/16767-h/images/plateii1.jpg
diff --git a/16767-h/images/plateii1_lg.jpg b/16767-h/images/plateii1_lg.jpg
new file mode 100644
index 0000000..aabe1d7
--- /dev/null
+++ b/16767-h/images/plateii1_lg.jpg
diff --git a/16767-h/images/plateii2.jpg b/16767-h/images/plateii2.jpg
new file mode 100644
index 0000000..19729e9
--- /dev/null
+++ b/16767-h/images/plateii2.jpg
diff --git a/16767-h/images/plateii2_lg.jpg b/16767-h/images/plateii2_lg.jpg
new file mode 100644
index 0000000..c4facc9
--- /dev/null
+++ b/16767-h/images/plateii2_lg.jpg
diff --git a/16767-h/images/plateiii1.jpg b/16767-h/images/plateiii1.jpg
new file mode 100644
index 0000000..2767da9
--- /dev/null
+++ b/16767-h/images/plateiii1.jpg
diff --git a/16767-h/images/plateiii1_lg.jpg b/16767-h/images/plateiii1_lg.jpg
new file mode 100644
index 0000000..6cb4e5e
--- /dev/null
+++ b/16767-h/images/plateiii1_lg.jpg
diff --git a/16767-h/images/plateiii2.jpg b/16767-h/images/plateiii2.jpg
new file mode 100644
index 0000000..0337055
--- /dev/null
+++ b/16767-h/images/plateiii2.jpg
diff --git a/16767-h/images/plateiii2_lg.jpg b/16767-h/images/plateiii2_lg.jpg
new file mode 100644
index 0000000..c9cb82a
--- /dev/null
+++ b/16767-h/images/plateiii2_lg.jpg
diff --git a/16767-h/images/plateiv1.jpg b/16767-h/images/plateiv1.jpg
new file mode 100644
index 0000000..1aec22c
--- /dev/null
+++ b/16767-h/images/plateiv1.jpg
diff --git a/16767-h/images/plateiv1_lg.jpg b/16767-h/images/plateiv1_lg.jpg
new file mode 100644
index 0000000..180883a
--- /dev/null
+++ b/16767-h/images/plateiv1_lg.jpg
diff --git a/16767-h/images/plateiv2.jpg b/16767-h/images/plateiv2.jpg
new file mode 100644
index 0000000..8956ee4
--- /dev/null
+++ b/16767-h/images/plateiv2.jpg
diff --git a/16767-h/images/plateiv2_lg.jpg b/16767-h/images/plateiv2_lg.jpg
new file mode 100644
index 0000000..bb7bad9
--- /dev/null
+++ b/16767-h/images/plateiv2_lg.jpg
diff --git a/16767-h/images/platev1.jpg b/16767-h/images/platev1.jpg
new file mode 100644
index 0000000..cddab52
--- /dev/null
+++ b/16767-h/images/platev1.jpg
diff --git a/16767-h/images/platev1_lg.jpg b/16767-h/images/platev1_lg.jpg
new file mode 100644
index 0000000..ce6ce27
--- /dev/null
+++ b/16767-h/images/platev1_lg.jpg
diff --git a/16767-h/images/platev2.jpg b/16767-h/images/platev2.jpg
new file mode 100644
index 0000000..055afa0
--- /dev/null
+++ b/16767-h/images/platev2.jpg
diff --git a/16767-h/images/platev2_lg.jpg b/16767-h/images/platev2_lg.jpg
new file mode 100644
index 0000000..9a4dc86
--- /dev/null
+++ b/16767-h/images/platev2_lg.jpg
diff --git a/16767-h/images/platevi1.jpg b/16767-h/images/platevi1.jpg
new file mode 100644
index 0000000..345537a
--- /dev/null
+++ b/16767-h/images/platevi1.jpg
diff --git a/16767-h/images/platevi1_lg.jpg b/16767-h/images/platevi1_lg.jpg
new file mode 100644
index 0000000..84bdfae
--- /dev/null
+++ b/16767-h/images/platevi1_lg.jpg
diff --git a/16767-h/images/platevi2.jpg b/16767-h/images/platevi2.jpg
new file mode 100644
index 0000000..780fb7b
--- /dev/null
+++ b/16767-h/images/platevi2.jpg
diff --git a/16767-h/images/platevi2_lg.jpg b/16767-h/images/platevi2_lg.jpg
new file mode 100644
index 0000000..2c276f2
--- /dev/null
+++ b/16767-h/images/platevi2_lg.jpg
diff --git a/16767-h/images/platevi3.jpg b/16767-h/images/platevi3.jpg
new file mode 100644
index 0000000..4421d1a
--- /dev/null
+++ b/16767-h/images/platevi3.jpg
diff --git a/16767-h/images/platevi3_lg.jpg b/16767-h/images/platevi3_lg.jpg
new file mode 100644
index 0000000..48d5661
--- /dev/null
+++ b/16767-h/images/platevi3_lg.jpg
diff --git a/16767-h/images/platevi4.jpg b/16767-h/images/platevi4.jpg
new file mode 100644
index 0000000..9ebc999
--- /dev/null
+++ b/16767-h/images/platevi4.jpg
diff --git a/16767-h/images/platevi4_lg.jpg b/16767-h/images/platevi4_lg.jpg
new file mode 100644
index 0000000..3d9f579
--- /dev/null
+++ b/16767-h/images/platevi4_lg.jpg
diff --git a/16767-h/images/platevi5.jpg b/16767-h/images/platevi5.jpg
new file mode 100644
index 0000000..0831ed4
--- /dev/null
+++ b/16767-h/images/platevi5.jpg
diff --git a/16767-h/images/platevi5_lg.jpg b/16767-h/images/platevi5_lg.jpg
new file mode 100644
index 0000000..2f6eb2a
--- /dev/null
+++ b/16767-h/images/platevi5_lg.jpg
diff --git a/16767-h/images/platevii1.jpg b/16767-h/images/platevii1.jpg
new file mode 100644
index 0000000..c5a45a2
--- /dev/null
+++ b/16767-h/images/platevii1.jpg
diff --git a/16767-h/images/platevii1_lg.jpg b/16767-h/images/platevii1_lg.jpg
new file mode 100644
index 0000000..2f0c172
--- /dev/null
+++ b/16767-h/images/platevii1_lg.jpg
diff --git a/16767-h/images/platevii2.jpg b/16767-h/images/platevii2.jpg
new file mode 100644
index 0000000..d8ae0a5
--- /dev/null
+++ b/16767-h/images/platevii2.jpg
diff --git a/16767-h/images/platevii2_lg.jpg b/16767-h/images/platevii2_lg.jpg
new file mode 100644
index 0000000..6857642
--- /dev/null
+++ b/16767-h/images/platevii2_lg.jpg
diff --git a/16767-h/images/platevii3.jpg b/16767-h/images/platevii3.jpg
new file mode 100644
index 0000000..04d4a5b
--- /dev/null
+++ b/16767-h/images/platevii3.jpg
diff --git a/16767-h/images/platevii3_lg.jpg b/16767-h/images/platevii3_lg.jpg
new file mode 100644
index 0000000..0e11efb
--- /dev/null
+++ b/16767-h/images/platevii3_lg.jpg
diff --git a/16767-h/images/platevii4.jpg b/16767-h/images/platevii4.jpg
new file mode 100644
index 0000000..3bfa0e8
--- /dev/null
+++ b/16767-h/images/platevii4.jpg
diff --git a/16767-h/images/platevii4_lg.jpg b/16767-h/images/platevii4_lg.jpg
new file mode 100644
index 0000000..adba9f8
--- /dev/null
+++ b/16767-h/images/platevii4_lg.jpg
diff --git a/16767-h/images/platevii5.jpg b/16767-h/images/platevii5.jpg
new file mode 100644
index 0000000..9d02b23
--- /dev/null
+++ b/16767-h/images/platevii5.jpg
diff --git a/16767-h/images/platevii5_lg.jpg b/16767-h/images/platevii5_lg.jpg
new file mode 100644
index 0000000..20779d6
--- /dev/null
+++ b/16767-h/images/platevii5_lg.jpg
diff --git a/16767-h/images/platevii6.jpg b/16767-h/images/platevii6.jpg
new file mode 100644
index 0000000..25fda31
--- /dev/null
+++ b/16767-h/images/platevii6.jpg
diff --git a/16767-h/images/platevii6_lg.jpg b/16767-h/images/platevii6_lg.jpg
new file mode 100644
index 0000000..4411f3d
--- /dev/null
+++ b/16767-h/images/platevii6_lg.jpg
diff --git a/16767.txt b/16767.txt
new file mode 100644
index 0000000..033703d
--- /dev/null
+++ b/16767.txt
@@ -0,0 +1,3833 @@
+Project Gutenberg's Half-hours with the Telescope, by Richard A. Proctor
+
+This eBook is for the use of anyone anywhere 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
+
+
+Title: Half-hours with the Telescope
+       Being a Popular Guide to the Use of the Telescope as a
+       Means of Amusement and Instruction.
+
+Author: Richard A. Proctor
+
+Release Date: September 28, 2005 [EBook #16767]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK HALF-HOURS WITH THE TELESCOPE ***
+
+
+
+
+Produced by Jason Isbell and the Online Distributed
+Proofreading Team at https://www.pgdp.net
+
+
+
+
+
+[Illustration: PLATE I. Maps I.-IV.]
+
+
+
+HALF-HOURS
+
+WITH
+
+THE TELESCOPE;
+
+BEING A POPULAR GUIDE TO THE USE OF THE TELESCOPE
+AS A MEANS OF AMUSEMENT AND INSTRUCTION.
+
+BY
+
+RICHARD A. PROCTOR, B.A., F.R.A.S.,
+AUTHOR OF "SATURN AND ITS SYSTEM," ETC.
+
+WITH ILLUSTRATIONS ON STONE AND WOOD.
+
+
+       *       *       *       *       *
+
+    An undevout astronomer is mad:
+    True, all things speak a God; but, in the small
+    Men trace out Him: in great He seizes man.
+                                  YOUNG.
+
+       *       *       *       *       *
+
+NEW YORK:
+
+G.P. PUTNAM'S SONS.
+
+1873.
+
+LONDON:
+
+PRINTED BY WILLIAM CLOWES AND SONS, STAMFORD STREET
+AND CHARING CROSS.
+
+
+
+
+PREFACE.
+
+
+The object which the Author and Publisher of this little work have
+proposed to themselves, has been the production, at a moderate price, of
+a useful and reliable guide to the amateur telescopist.
+
+Among the celestial phenomena described or figured in this treatise, by
+far the larger number may be profitably examined with small telescopes,
+and there are none which are beyond the range of a good 3-inch
+achromatic.
+
+The work also treats of the construction of telescopes, the nature and
+use of star-maps, and other subjects connected with the requirements of
+amateur observers.
+
+R.A.P.
+
+_January_, 1868.
+
+
+
+
+CONTENTS.
+
+
+CHAPTER I.                                          PAGE
+A HALF-HOUR ON THE STRUCTURE OF THE TELESCOPE          1
+
+CHAPTER II.
+A HALF-HOUR WITH ORION, LEPUS, TAURUS, ETC.           33
+
+CHAPTER III.
+A HALF-HOUR WITH LYRA, HERCULES, CORVUS, CRATER, ETC. 47
+
+CHAPTER IV.
+A HALF-HOUR WITH BOOTES, SCORPIO, OPHIUCHUS, ETC.     56
+
+CHAPTER V.
+A HALF-HOUR WITH ANDROMEDA, CYGNUS, ETC.              66
+
+CHAPTER VI.
+HALF-HOURS WITH THE PLANETS                           74
+
+CHAPTER VII.
+HALF-HOURS WITH THE SUN AND MOON                      93
+
+
+
+
+DESCRIPTION OF PLATES.
+
+
+PLATE I.--_Frontispiece._
+
+This plate presents the aspect of the heavens at the four seasons, dealt
+with in Chapters II., III., IV., and V. In each map of this plate the
+central point represents the point vertically over the observer's head,
+and the circumference represents his horizon. The plan of each map is
+such that the direction of a star or constellation, as respects the
+compass-points, and its elevation, also, above the horizon, at the given
+season, can be at once determined. Two illustrations of the use of the
+maps will serve to explain their nature better than any detailed
+description. Suppose first, that--at one of the hours named under Map
+I.--the observer wishes to find Castor and Pollux:--Turning to Map I. he
+sees that these stars lie in the lower left-hand quadrant, and very
+nearly towards the point marked S.E.; that is, they are to be looked for
+on the sky towards the south-east. Also, it is seen that the two stars
+lie about one-fourth of the way from the centre towards the
+circumference. Hence, on the sky, the stars will be found about
+one-fourth of the way from the zenith towards the horizon: Castor will
+be seen immediately above Pollux. Next, suppose that at one of the hours
+named the observer wishes to learn what stars are visible towards the
+west and north-west:--Turning the map until the portion of the
+circumference marked W ... N.W. is lowermost, he sees that in the
+direction named the square of Pegasus lies not very high above the
+horizon, one diagonal of the square being vertical, the other nearly
+horizontal. Above the square is Andromeda, to the right of which lies
+Cassiopeia, the stars [beta] and [epsilon] of this constellation lying
+directly towards the north-west, while the star [alpha] lies almost
+exactly midway between the zenith and the horizon. Above Andromeda, a
+little towards the left, lies Perseus, Algol being almost exactly
+towards the west and one-third of the way from the zenith towards the
+horizon (because one-third of the way from the centre towards the
+circumference of the map). Almost exactly in the zenith is the star
+[delta] Aurigae.
+
+The four maps are miniatures of Maps I., IV., VII., and X. of my
+'Constellation Seasons,' fourth-magnitude stars, however, being omitted.
+
+
+PLATES II., III., IV., and V., illustrating Chapters II., III., IV., and V.
+
+Plates II. and IV. contain four star-maps. They not only serve to
+indicate the configuration of certain important star-groups, but they
+illustrate the construction of maps, such as the observer should make
+for himself when he wishes to obtain an accurate knowledge of particular
+regions of the sky. They are all made to one scale, and on the conical
+projection--the simplest and best of all projections for maps of this
+sort. The way in which the meridians and parallels for this projection
+are laid down is described in my 'Handbook of the Stars.' With a little
+practice a few minutes will suffice for sweeping out the equidistant
+circular arcs which mark the parallels and ruling in the straight
+meridians.
+
+The dotted line across three of the maps represents a portion of the
+horizontal circle midway between the zenith and the horizon at the hour
+at which the map is supposed to be used. At other hours, of course, this
+line would be differently situated.
+
+Plates III. and V. represent fifty-two of the objects mentioned in the
+above-named chapters. As reference is made to these figures in the text,
+little comment is here required. It is to be remarked, however, that the
+circles, and especially the small circles, do not represent the whole
+of the telescope's field of view, only a small portion of it. The object
+of these figures is to enable the observer to know what to expect when
+he turns his telescope towards a difficult double star. Many of the
+objects depicted are very easy doubles: these are given as objects of
+reference. The observer having seen the correspondence between an easy
+double and its picture, as respects the relation between the line
+joining the components and the apparent path of the double across the
+telescope's field of view, will know how to interpret the picture of a
+difficult double in this respect. And as all the small figures are drawn
+to one scale, he will also know how far apart he may expect to find the
+components of a difficult double. Thus he will have an exact conception
+of the sort of duplicity he is to look for, and this is--_crede
+experto_--a great step towards the detection of the star's duplicity.
+
+
+PLATES VI. and VII., illustrating Chapters VI. and VII.
+
+The views of Mercury, Venus, and Mars in these plates (except the
+smaller view of Jupiter in Plate VII.) are supposed to be seen with the
+same "power."
+
+The observer must not expect to see the details presented in the views
+of Mars with anything like the distinctness I have here given to them.
+If he place the plate at a distance of six or seven yards he will see
+the views more nearly as Mars is likely to appear in a good three-inch
+aperture.
+
+The chart of Mars is a reduction of one I have constructed from views by
+Mr. Dawes. I believe that nearly all the features included in the chart
+are permanent, though not always visible. I take this opportunity of
+noting that the eighteen orthographic pictures of Mars presented with my
+shilling chart are to be looked on rather as maps than as representing
+telescopic views. They illustrate usefully the varying presentation of
+Mars towards the earth. The observer can obtain other such illustrations
+for himself by filling in outlines, traced from those given at the foot
+of Plate VI., with details from the chart. It is to be noted that Mars
+varies in presentation, not only as respects the greater or less opening
+out of his equator towards the north or south, but as respects the
+apparent slope of his polar axis to the right or left. The four
+projections as shown, or inverted, or seen from the back of the plate
+(held up to the light) give presentations of Mars towards the sun at
+twelve periods of the Martial year,--viz., at the autumnal and vernal
+equinoxes, at the two solstices, and at intermediate periods
+corresponding to our terrestrial months.
+
+In fact, by means of these projections one might readily form a series
+of sun-views of Mars resembling my 'Sun-views of the Earth.'
+
+In the first view of Jupiter it is to be remarked that the three
+satellites outside the disc are supposed to be moving in directions
+appreciably parallel to the belts on the disc--the upper satellites from
+right to left, the lower one from left to right. In general the
+satellites, when so near to the disc, are not seen in a straight line,
+as the three shown in the figure happen to be. Of the three spots on the
+disc, the faintest is a satellite, the neighbouring dark spot its
+shadow, the other dark spot the shadow of the satellite close to the
+planet's disc.
+
+
+
+
+HALF-HOURS WITH THE TELESCOPE.
+
+
+
+
+CHAPTER I.
+
+A HALF-HOUR ON THE STRUCTURE OF THE TELESCOPE.
+
+
+There are few instruments which yield more pleasure and instruction than
+the Telescope. Even a small telescope--only an inch and a half or two
+inches, perhaps, in aperture--will serve to supply profitable amusement
+to those who know how to apply its powers. I have often seen with
+pleasure the surprise with which the performance even of an opera-glass,
+well steadied, and directed towards certain parts of the heavens, has
+been witnessed by those who have supposed that nothing but an expensive
+and colossal telescope could afford any views of interest. But a
+well-constructed achromatic of two or three inches in aperture will not
+merely supply amusement and instruction,--it may be made to do useful
+work.
+
+The student of astronomy is often deterred from telescopic observation
+by the thought that in a field wherein so many have laboured, with
+abilities and means perhaps far surpassing those he may possess, he is
+little likely to reap results of any utility. He argues that, since the
+planets, stars, and nebulae have been scanned by Herschel and Rosse, with
+their gigantic mirrors, and at Pulkova and Greenwich with refractors
+whose construction has taxed to the utmost the ingenuity of the
+optician and mechanic, it must be utterly useless for an unpractised
+observer to direct a telescope of moderate power to the examination of
+these objects.
+
+Now, passing over the consideration that a small telescope may afford
+its possessor much pleasure of an intellectual and elevated character,
+even if he is never able by its means to effect original discoveries,
+two arguments may be urged in favour of independent telescopic
+observation. In the first place, the student who wishes to appreciate
+the facts and theories of astronomy should familiarize himself with the
+nature of that instrument to which astronomers have been most largely
+indebted. In the second place, some of the most important discoveries in
+astronomy have been effected by means of telescopes of moderate power
+used skilfully and systematically. One instance may suffice to show what
+can be done in this way. The well-known telescopist Goldschmidt (who
+commenced astronomical observation at the age of forty-eight, in 1850)
+added fourteen asteroids to the solar system, not to speak of important
+discoveries of nebulae and variable stars, by means of a telescope only
+five feet in focal length, mounted on a movable tripod stand.
+
+The feeling experienced by those who look through a telescope for the
+first time,--especially if it is directed upon a planet or nebula--is
+commonly one of disappointment. They have been told that such and such
+powers will exhibit Jupiter's belts, Saturn's rings, and the
+continent-outlines on Mars; yet, though perhaps a higher power is
+applied, they fail to detect these appearances, and can hardly believe
+that they are perfectly distinct to the practised eye.
+
+The expectations of the beginner are especially liable to
+disappointment in one particular. He forms an estimate of the view he is
+to obtain of a planet by multiplying the apparent diameter of the planet
+by the magnifying power of his telescope, and comparing the result with
+the apparent diameter of the sun or moon. Let us suppose, for instance,
+that on the day of observation Jupiter's apparent diameter is 45", and
+that the telescopic power applied is 40, then in the telescope Jupiter
+should appear to have a diameter of 1800", or half a degree, which is
+about the same as the moon's apparent diameter. But when the observer
+looks through the telescope he obtains a view--interesting, indeed, and
+instructive--but very different from what the above calculation would
+lead him to expect. He sees a disc apparently much smaller than the
+moon's, and not nearly so well-defined in outline; in a line with the
+disc's centre there appear three or four minute dots of light, the
+satellites of the planet; and, perhaps, if the weather is favourable and
+the observer watchful, he will be able to detect faint traces of belts
+across the planet's disc.
+
+Yet in such a case the telescope is not in fault. The planet really
+appears of the estimated size. In fact, it is often possible to prove
+this in a very simple manner. If the observer wait until the planet and
+the moon are pretty near together, he will find that it is possible to
+view the planet with one eye through the telescope and the moon with the
+unaided eye, in such a manner that the two discs may coincide, and thus
+their relative apparent dimensions be at once recognised. Nor should the
+indistinctness and incompleteness of the view be attributed to
+imperfection of the telescope; they are partly due to the nature of the
+observation and the low power employed, and partly to the inexperience
+of the beginner.
+
+It is to such a beginner that the following pages are specially
+addressed, with the hope of affording him aid and encouragement in the
+use of one of the most enchanting of scientific instruments,--an
+instrument that has created for astronomers a new sense, so to speak, by
+which, in the words of the ancient poet:
+
+    Subjecere oculis distantia sidera nostris,
+      AEtheraque ingenio supposuere suo.
+
+In the first place, it is necessary that the beginner should rightly
+know what is the nature of the instrument he is to use. And this is the
+more necessary because, while it is perfectly easy to obtain such
+knowledge without any profound acquaintance with the science of optics,
+yet in many popular works on this subject the really important points
+are omitted, and even in scientific works such points are too often left
+to be gathered from a formula. When the observer has learnt what it is
+that his instrument is actually to do for him, he will know how to
+estimate its performance, and how to vary the application of its
+powers--whether illuminating or magnifying--according to the nature of
+the object to be observed.
+
+Let us consider what it is that limits the range of _natural_ vision
+applied to distant objects. What causes an object to become invisible as
+its distance increases? Two things are necessary that an object should
+be visible. It must be _large_ enough to be appreciated by the eye, and
+it must _send light_ enough. Thus increase of distance may render an
+object invisible, either through diminution of its apparent size, or
+through diminution in the quantity of light it sends to the eye, or
+through both these causes combined. A telescope, therefore, or (as its
+name implies) an instrument to render distant objects visible, must be
+both a magnifying and an illuminating instrument.
+
+[Illustration: _Fig. 1._]
+
+Let EF, fig. 1, be an object, not near to AB as in the figure, but so
+far off that the bounding lines from A and B would meet at the point
+corresponding to the point P. Then if a large convex glass AB (called an
+_object-glass_) be interposed between the object and the eye, all those
+rays which, proceeding from P, fall on AB, will be caused to converge
+nearly to a point _p_. The same is true for every point of the object
+EMF, and thus a small image, _emf_, will be formed. This image will not
+lie exactly on a flat surface, but will be curved about the point midway
+between A and B as a centre. Now if the lens AB is removed, and an eye
+is placed at _m_ to view the distant object EMF, those rays only from
+each point of the object which fall on the pupil of the eye (whose
+diameter is about equal to _mp_ suppose) will serve to render the object
+visible. On the other hand, every point of the image _emf_ has received
+the whole of the light gathered up by the large glass AB. If then we can
+only make this light _available_, it is clear that we shall have
+acquired a large increase of _light_ from the distant object. Now it
+will be noticed that the light which has converged to _p_, diverges from
+_p_ so that an eye, placed that this diverging pencil of rays may fall
+upon it, would be too small to receive the whole of the pencil. Or, if
+it did receive the whole of this pencil, it clearly could not receive
+the whole of the pencils proceeding from other parts of the image _emf_.
+_Something_ would be gained, though, even in this case, since it is
+clear that an eye thus placed at a distance of ten inches from _emf_
+(which is about the average distance of distinct vision) would not only
+receive much more light from the image _emf_, than it would from the
+object EMF, but see the image much larger than the object. It is in this
+way that a simple object-glass forms a telescope, a circumstance we
+shall presently have to notice more at length. But we want to gain the
+full benefit of the light which has been gathered up for us by our
+object-glass. We therefore interpose a small convex glass _ab_ (called
+an eye-glass) between the image and the eye, at such a distance from the
+image that the divergent pencil of rays is converted into a pencil of
+parallel or nearly parallel rays. Call this an emergent pencil. Then all
+the emergent pencils now converge to a point on the axial line _m_M
+(produced beyond _m_), and an eye suitably placed can take in all of
+them at once. Thus the whole, or a large part, of the image is seen at
+once. But the image is seen inverted as shown. This is the Telescope, as
+it was first discovered, and such an arrangement would now be called a
+_simple astronomical Telescope_.
+
+Let us clearly understand what each part of the astronomical telescope
+does for us:--
+
+The object-glass AB gives us an illuminated image, the amount of
+illumination depending on the size of the object-glass. The eye-glass
+enables us to examine the image microscopically.
+
+We may apply eye-glasses of different focal length. It is clear that the
+shorter the focal length of _ab_, the nearer must _ab_ be placed to the
+image, and the smaller will the emergent pencils be, but the greater the
+magnifying power of the eye-glass. If the emergent pencils are severally
+larger than the pupil of the eye, light is wasted at the expense of
+magnifying power. Therefore the eye-glass should never be of greater
+focal length than that which makes the emergent pencils about equal in
+diameter to the pupil of the eye. On the other hand, the eye-glass must
+not be of such small focal length that the image appears indistinct and
+contorted, or dull for want of light.
+
+[Illustration: _Fig. 2._]
+
+Let us compare with the arrangement exhibited in fig. 1 that adopted by
+Galileo. Surprise is sometimes expressed that this instrument, which in
+the hands of the great Florentine astronomer effected so much, should
+now be known as the _non-astronomical Telescope_. I think this will be
+readily understood when we compare the two arrangements.
+
+In the Galilean Telescope a small concave eye-glass, _ab_ (fig. 2), is
+placed between the object-glass and the image. In fact, no image is
+allowed to be formed in this arrangement, but the convergent pencils are
+intercepted by the concave eye-glass, and converted into parallel
+emergent pencils. Now in fig. 2 the concave eye-glass is so placed as to
+receive only a part of the convergent pencil A _p_ B, and this is the
+arrangement usually adopted. By using a concave glass of shorter focus,
+which would therefore be placed nearer to _m p_, the whole of the
+convergent pencil might be received in this as in the former case. But
+then the axis of the emergent pencil, instead of returning (as we see it
+in fig. 1) _towards_ the axis of the telescope, would depart as much
+_from_ that axis. Thus there would be no point on the axis at which the
+eye could be so placed as to receive emergent pencils showing any
+considerable part of the object. The difference may be compared to that
+between looking through the small end of a cone-shaped roll of paper and
+looking through the large end; in the former case the eye sees at once
+all that is to be seen through the roll (supposed fixed in position), in
+the latter the eye may be moved about so as to command the same range of
+view, but _at any instant_ sees over a much smaller range.
+
+To return to the arrangement actually employed, which is illustrated by
+the common opera-glass. We see that the full illuminating power of the
+telescope is not brought into play. But this is not the only objection
+to the Galilean Telescope. It is obvious that if the part C D of the
+object-glass were covered, the point P would not be visible, whereas, in
+the astronomical arrangement no other effect is produced on the
+visibility of an object, by covering part of the object-glass, than a
+small loss of illumination. In other words, the dimensions of the field
+of view of a Galilean Telescope depend on the size of the object-glass,
+whereas in the astronomical Telescope the field of view is independent
+of the size of the object-glass. The difference may be readily tested.
+If we direct an opera-glass upon any object, we shall find that any
+covering placed over a part of the object-glass _becomes visible_ when
+we look through the instrument, interfering therefore _pro tanto_ with
+the range of view. A covering similarly placed on any part of the
+object-glass of an astronomical telescope does not become visible when
+we look through the instrument. The distinction has a very important
+bearing on the theory of telescopic vision.
+
+In considering the application of the telescope to practical
+observation, the circumstance that in the Galilean Telescope no real
+image is formed, is yet more important. A real image admits of
+measurement, linear or angular, while to a _virtual_ image (such an
+image, for instance, as is formed by a common looking-glass) no such
+process can be applied. In simple observation the only noticeable effect
+of this difference is that, whereas in the astronomical Telescope a
+_stop_ or diaphragm can be inserted in the tube so as to cut off what is
+called the _ragged edge_ of the field of view (which includes all the
+part not reached by _full pencils of light_ from the object-glass),
+there is no means of remedying the corresponding defect in the Galilean
+Telescope. It would be a very annoying defect in a telescope intended
+for astronomical observation, since in general the edge of the field of
+view is not perceptible at night. The unpleasant nature of the defect
+may be seen by looking through an opera-glass, and noticing the gradual
+fading away of light round the circumference of the field of view.
+
+The properties of reflection as well as of refraction have been enlisted
+into the service of the astronomical observer. The formation of an image
+by means of a concave mirror is exhibited in fig. 3. As the observer's
+head would be placed between the object and the mirror, if the image,
+formed as in fig. 3, were to be microscopically examined, various
+devices are employed in the construction of reflecting telescopes to
+avoid the loss of light which would result--a loss which would be
+important even with the largest mirrors yet constructed. Thus, in
+Gregory's Telescope, a small mirror, having its concavity towards the
+great one, is placed in the axis of the tube and forms an image which is
+viewed through an aperture in the middle of the great mirror. A similar
+plan is adopted in Cassegrain's Telescope, a small convex mirror
+replacing the concave one. In Newton's Telescope a small inclined-plane
+reflector is used, which sends the pencil of light off at right-angles
+to the axis of the tube. In Herschel's Telescope the great mirror is
+inclined so that the image is formed at a slight distance from the axis
+of the telescope. In the two first cases the object is viewed in the
+usual or direct way, the image being erect in Gregory's and inverted in
+Cassegrain's. In the third the observer looks through the side of the
+telescope, seeing an inverted image of the object. In the last the
+observer sees the object inverted, but not altered as respects right and
+left. The last-mentioned method of viewing objects is the only one in
+which the observer's back is turned towards the object, yet this method
+is called the _front view_--apparently _quasi lucus a non lucendo_.
+
+[Illustration: _Fig. 3._]
+
+It appears, then, that in all astronomical Telescopes, reflecting or
+refracting, a _real image_ of an object is submitted to microscopical
+examination.
+
+Of this fact the possessor of a telescope may easily assure himself;
+for if the eye-glass be removed, and a small screen be placed at the
+focus of the object-glass, there will appear upon the screen a small
+picture of any object towards which the tube is turned. But the image
+may be viewed in another way which requires to be noticed. If the eye,
+placed at a distance of five or six inches from the image, be directed
+down the tube, the image will be seen as before; in fact, just as a
+single convex lens of short focus is the simplest microscope, so a
+simple convex lens of long focus is the simplest telescope.[1] But a
+singular circumstance will immediately attract the observer's notice. A
+real picture, or the image formed on the screen as in the former case,
+can be viewed at varying distances; but when we view the image directly,
+it will be found that for distinct vision the eye must be placed almost
+exactly at a fixed distance from the image. This peculiarity is more
+important than it might be thought at first sight. In fact, it is
+essential that the observer who would rightly apply the powers of his
+telescope, or fairly test its performance, should understand in what
+respect an image formed by an object-glass or object-mirror differs from
+a real object.
+
+The peculiarities to be noted are the _curvature_, _indistinctness_, and
+_false colouring_ of the image.
+
+The curvature of the image is the least important of the three defects
+named--a fortunate circumstance, since this defect admits neither of
+remedy nor modification. The image of a distant object, instead of lying
+in a plane, that is, forming what is technically called a _flat field_,
+forms part of a spherical surface whose centre is at the centre of the
+object-glass. Hence the centre of the field of view is somewhat nearer
+to the eye than are the outer parts of the field. The amount of
+curvature clearly depends on the extent of the field of view, and
+therefore is not great in powerful telescopes. Thus, if we suppose that
+the angular extent of the field is about half a degree (a large or
+low-power field), the centre is nearer than the boundary of the field by
+about 1-320th part only of the field's diameter.
+
+The indistinctness of the image is partly due to the obliquity of the
+pencils which form parts of the image, and partly to what is termed
+_spherical aberration_. The first cause cannot be modified by the
+optician's skill, and is not important when the field of view is small.
+Spherical aberration causes those parts of a pencil which fall near the
+boundary of a convex lens to converge to a nearer (_i.e._ shorter) focus
+than those which fall near the centre. This may be corrected by a proper
+selection of the forms of the two lenses which replace, in all modern
+telescopes, the single lens hitherto considered.
+
+The false colouring of the image is due to _chromatic aberration_. The
+pencil of light proceeding from a point, converges, not to one point,
+but to a short line of varying colour. Thus a series of coloured images
+is formed, at different distances from the object-glass. So that, if a
+screen were placed to receive the mean image _in focus_, a coloured
+fringe due to the other images (_out of focus, and therefore too large_)
+would surround the mean image.
+
+Newton supposed that it was impossible to get rid of this defect, and
+therefore turned his attention to the construction of reflectors. But
+the discovery that the _dispersive_ powers of different glasses are not
+proportional to their reflective powers, supplied opticians with the
+means of remedying the defect. Let us clearly understand what is the
+discovery referred to. If with a glass prism of a certain form we
+produce a spectrum of the sun, this spectrum will be thrown a certain
+distance away from the point on which the sun's rays would fall if not
+interfered with. This distance depends on the _refractive_ power of the
+glass. The spectrum will have a certain length, depending on the
+_dispersive_ power of the glass. Now, if we change our prism for another
+of exactly the same shape, but made of a different kind of glass, we
+shall find the spectrum thrown to a different spot. If it appeared that
+the length of the new spectrum was increased or diminished in exactly
+the same proportion as its distance from the line of the sun's direct
+light, it would have been hopeless to attempt to remedy chromatic
+aberration. Newton took it for granted that this was so. But the
+experiments of Hall and the Dollonds showed that there is no such strict
+proportionality between the dispersive and refractive powers of
+different kinds of glass. It accordingly becomes possible to correct the
+chromatic aberration of one glass by superadding that of another.
+
+[Illustration: _Fig. 4._]
+
+This is effected by combining, as shown in fig. 4, a convex lens of
+_crown_ glass with a concave lens of _flint_ glass, the convex lens
+being placed nearest to the object. A little colour still remains, but
+not enough to interfere seriously with the distinctness of the image.
+
+But even if the image formed by the object-glass were perfect, yet this
+image, viewed through a single convex lens of short focus placed as in
+fig. 1, would appear curved, indistinct, coloured, and also _distorted_,
+because viewed by pencils of light which do not pass through the centre
+of the eye-glass. These effects can be diminished (but not entirely
+removed _together_) by using an _eye-piece_ consisting of two lenses
+instead of a single eye-glass. The two forms of eye-piece most commonly
+employed are exhibited in figs. 5 and 6. Fig. 5 is Huyghens' eye-piece,
+called also the _negative_ eye-piece, because a real image is formed
+_behind_ the _field-glass_ (the lens which lies nearest to the
+object-glass). Fig. 6 represents Ramsden's eye-piece, called also the
+_positive_ eye-piece, because the real image formed by the object-glass
+lies _in front of_ the field-glass.
+
+[Illustration: _Fig. 5._]
+
+[Illustration: _Fig. 6._]
+
+The course of a slightly oblique pencil through either eye-piece is
+exhibited in the figures. The lenses are usually plano-convex, the
+convexities being turned towards the object-glass in the negative
+eye-piece, and towards each other in the positive eye-piece. Coddington
+has shown, however, that the best forms for the lenses of the negative
+eye-piece are those shown in fig. 5.
+
+The negative eye-piece, being achromatic, is commonly employed in all
+observations requiring distinct vision only. But as it is clearly unfit
+for observations requiring micrometrical measurement, or reference to
+fixed lines at the focus of the object-glass, the positive eye-piece is
+used for these purposes.
+
+For observing objects at great elevations the diagonal eye-tube is
+often convenient. Its construction is shown in fig. 7. ABC is a totally
+reflecting prism of glass. The rays from the object-glass fall on the
+face AB, are totally reflected on the face BC, and emerge through the
+face AC. In using this eye-piece, it must be remembered that it
+lengthens the sliding eye-tube, which must therefore be thrust further
+in, or the object will not be seen in focus. There is an arrangement by
+which the change of direction is made to take place between the two
+glasses of the eye-piece. With this arrangement (known as the _diagonal
+eye-piece_) no adjustment of the eye-tube is required. However, for
+amateurs' telescopes the more convenient arrangement is the diagonal
+eye-tube, since it enables the observer to apply any eye-piece he
+chooses, just as with the simple sliding eye-tube.
+
+[Illustration: _Fig. 7._]
+
+We come next to the important question of the _mounting_ of our
+telescope.
+
+The best known, and, in some respects, the simplest method of
+mounting a telescope for general observation is that known as the
+_altitude-and-azimuth_ mounting. In this method the telescope is
+pointed towards an object by two motions,--one giving the tube the
+required _altitude_ (or elevation), the other giving it the required
+_azimuth_ (or direction as respects the compass points).
+
+For small alt-azimuths the ordinary pillar-and-claw stand is
+sufficiently steady. For larger instruments other arrangements are
+needed, both to give the telescope steadiness, and to supply slow
+movements in altitude and azimuth. The student will find no difficulty
+in understanding the arrangement of sliding-tubes and rack-work commonly
+adopted. This arrangement seems to me to be in many respects defective,
+however. The slow movement in altitude is not uniform, but varies in
+effect according to the elevation of the object observed. It is also
+limited in range; and quite a little series of operations has to be gone
+through when it is required to direct the telescope towards a new
+quarter of the heavens. However expert the observer may become by
+practice in effecting these operations, they necessarily take up some
+time (performed as they must be in the dark, or by the light of a small
+lantern), and during this time it often happens that a favourable
+opportunity for observation is lost.
+
+These disadvantages are obviated when the telescope is mounted in such a
+manner as is exhibited in fig. 8, which represents a telescope of my own
+construction. The slow movement in altitude is given by rotating the rod
+_he_, the endless screw in which turns the small wheel at _b_, whose
+axle in turn bears a pinion-wheel working in the teeth of the quadrant
+_a_. The slow movement in azimuth is given in like manner by rotating
+the rod _h'e'_, the lantern-wheel at the end of which turns a
+crown-wheel on whose axle is a pinion-wheel working in the teeth of the
+circle _c_. The casings at _e_ and _e'_, in which the rods _he_ and
+_h'e'_ respectively work, are so fastened by elastic cords that an
+upward pressure on the handle _h_, or a downward pressure on the handle
+_h'_, at once releases the endless screw or the crown-wheel
+respectively, so that the telescope can be swept at once through any
+desired angle in altitude or azimuth. This method of mounting has other
+advantages; the handles are conveniently situated and constant in
+position; also, as they do not work directly on the telescope, they can
+be turned without setting the tube in vibration.
+
+[Illustration: _Fig. 8._]
+
+I do not recommend the mounting to be exactly as shown in fig. 8. That
+method is much too expensive for an alt-azimuth. But a simple
+arrangement of belted wheels in place of the toothed wheels _a_ and _c_
+might very readily be prepared by the ingenious amateur telescopist; and
+I feel certain that the comfort and convenience of the arrangement would
+amply repay him for the labour it would cost him. My own
+telescope--though the large toothed-wheel and the quadrant were made
+inconveniently heavy (through a mistake of the workman who constructed
+the instrument)--worked as easily and almost as conveniently as an
+equatorial.
+
+Still, it is well for the observer who wishes systematically to survey
+the heavens--and who can afford the expense--to obtain a well-mounted
+_equatorial_. In this method of mounting, the main axis is directed to
+the pole of the heavens; the other axis, at right angles to the first,
+carries the telescope-tube. One of the many methods adopted for mounting
+equatorials is that exhibited--with the omission of some minor
+details--in fig. 9. _a_ is the polar axis, _b_ is the axis (called the
+declination axis) which bears the telescope. The circles _c_ and _d_
+serve to indicate, by means of verniers revolving with the axes, the
+motion of the telescope in right ascension and declination,
+respectively. The weight _w_ serves to counterpoise the telescope, and
+the screws _s_, _s_, _s_, _s_, serve to adjust the instrument so that
+the polar axis shall be in its proper position. The advantage gained by
+the equatorial method of mounting is that only one motion is required to
+follow a star. Owing to the diurnal rotation of the earth, the stars
+appear to move uniformly in circles parallel to the celestial equator;
+and it is clear that a star so moving will be kept in the field of
+view, if the telescope, once directed to the star, be made to revolve
+uniformly and at a proper rate round the polar axis.
+
+[Illustration: _Fig. 9._]
+
+The equatorial can be directed by means of the circles _c_ and _d_ to
+any celestial object whose right ascension and declination are known. On
+the other hand, to bring an object into the field of view of an
+alt-azimuth, it is necessary, either that the object itself should be
+visible to the naked eye, or else that the position of the object should
+be pretty accurately learned from star-maps, so that it may be picked up
+by the alt-azimuth after a little searching. A small telescope called a
+_finder_ is usually attached to all powerful telescopes intended for
+general observation. The finder has a large field of view, and is
+adjusted so as to have its axis parallel to that of the large telescope.
+Thus a star brought to the centre of the large field of the finder
+(indicated by the intersection of two lines placed at the focus of the
+eye-glass) is at, or very near, the centre of the small field of the
+large telescope.
+
+If a telescope has no finder, it will be easy for the student to
+construct one for himself, and will be a useful exercise in optics. Two
+convex lenses not very different in size from those shown in fig. 1, and
+placed as there shown--the distance between them being the sum of the
+focal lengths of the two glasses--in a small tube of card, wood, or tin,
+will serve the purpose of a finder for a small telescope. It can be
+attached by wires to the telescope-tube, and adjusted each night before
+commencing observation. The adjustment is thus managed:--a low power
+being applied to the telescope, the tube is turned towards a bright
+star; this is easily effected with a low power; then the finder is to be
+fixed, by means of its wires, in such a position that the star shall be
+in the centre of the field of the finder when also in the centre of the
+telescope's field. When this has been done, the finder will greatly help
+the observations of the evening; since with high powers much time would
+be wasted in bringing an object into the field of view of the telescope
+without the aid of a finder. Yet more time would be wasted in the case
+of an object not visible to the naked eye, but whose position with
+reference to several visible stars is known; since, while it is easy to
+bring the point required to the centre of the _finder's_ field, in which
+the guiding stars are visible, it is very difficult to direct the
+_telescope's_ tube on a point of this sort. A card tube with wire
+fastenings, such as we have described, may appear a very insignificant
+contrivance to the regular observer, with his well-mounted equatorial
+and carefully-adjusted finder. But to the first attempts of the amateur
+observer it affords no insignificant assistance, as I can aver from my
+own experience. Without it--a superior finder being wanting--our
+"half-hours" would soon be wasted away in that most wearisome and
+annoying of all employments, trying to "pick up" celestial objects.
+
+It behoves me at this point to speak of star-maps. Such maps are of many
+different kinds. There are the Observatory maps, in which the places of
+thousands of stars are recorded with an amazing accuracy. Our beginner
+is not likely to make use of, or to want, such maps as these. Then there
+are maps merely intended to give a good general idea of the appearance
+of the heavens at different hours and seasons. Plate I. presents four
+maps of this sort; but a more complete series of eight maps has been
+published by Messrs. Walton and Maberly in an octavo work; and my own
+'Constellation-Seasons' give, at the same price, twelve quarto maps (of
+four of which those in Plate I. are miniatures), showing the appearance
+of the sky at any hour from month to month, or on any night, at
+successive intervals of two hours. But maps intermediate in character to
+these and to Observatory maps are required by the amateur observer.
+Such are the Society's six gnomonic maps, the set of six gnomonic maps
+in Johnstone's 'Atlas of Astronomy,' and my own set of twelve gnomonic
+maps. The Society's maps are a remarkably good set, containing on the
+scale of a ten-inch globe all the stars in the Catalogue of the
+Astronomical Society (down to the fifth magnitude). The distortion,
+however, is necessarily enormous when the celestial sphere is presented
+in only six gnomonic maps. In my maps all the stars of the British
+Association Catalogue down to the fifth magnitude are included on the
+scale of a six-inch globe. The distortion is scarcely a fourth of that
+in the Society's maps. The maps are so arranged that the relative
+positions of all the stars in each hemisphere can be readily gathered
+from a single view; and black duplicate-maps serve to show the
+appearance of the constellations.
+
+It is often convenient to make small maps of a part of the heavens we
+may wish to study closely. My 'Handbook of the Stars' has been prepared
+to aid the student in the construction of such maps.
+
+In selecting maps it is well to be able to recognise the amount of
+distortion and scale-variation. This may be done by examining the spaces
+included between successive parallels and meridians, near the edges and
+angles of the maps, and comparing these either with those in the centre
+of the map, or with the known figures and dimensions of the
+corresponding spaces on a globe.
+
+We may now proceed to discuss the different tests which the intending
+purchaser of a telescope should apply to the instrument.
+
+The excellence of an object-glass can be satisfactorily determined only
+by testing the performance of the telescope in the manner presently to
+be described. But it is well to examine the quality of the glass as
+respects transparency and uniformity of texture. Bubbles, scratches, and
+other such defects, are not very important, since they do not affect the
+distinctness of the field as they would in a Galilean Telescope,--a
+little light is lost, and that is all. The same remark applies to dust
+upon the glass. The glass should be kept as free as possible from dirt,
+damp, or dust, but it is not advisable to remove every speck which,
+despite such precaution, may accidentally fall upon the object-glass.
+When it becomes necessary to clean the glass, it is to be noted that the
+substance used should be soft, perfectly dry, and free from dust. Silk
+is often recommended, but some silk is exceedingly objectionable in
+texture,--old silk, perfectly soft to the touch, is perhaps as good as
+anything. If the dust which has fallen on the glass is at all gritty,
+the glass will suffer by the method of cleaning commonly adopted, in
+which the dust is _gathered up_ by pressure. The proper method is to
+clean a small space near the edge of the glass, and to _sweep_ from that
+space as centre. In this way the dust is _pushed before_ the silk or
+wash-leather, and does not cut the glass. It is well always to suspect
+the presence of gritty dust, and adopt this cautious method of cleaning.
+
+The two glasses should on no account be separated.
+
+In examining an eye-piece, the quality of the glass should be noted, and
+care taken that both glasses (but especially the field-glass) are free
+from the least speck, scratch, or blemish of any kind, for these defects
+will be exhibited in a magnified state in the field of view. Hence the
+eye-pieces require to be as carefully preserved from damp and dust as
+the object-glass, and to be more frequently cleaned.
+
+The tube of the telescope should be light, but strong, and free from
+vibration. Its quality in the last respect can be tested by lightly
+striking it when mounted; the sound given out should be dead or
+non-resonant. The inside of the tube must absorb extraneous light, and
+should therefore be coloured a dull black; and stops of varying radius
+should be placed along its length with the same object. Sliding tubes,
+rack-work, etc., should work closely, yet easily.
+
+The telescope should be well balanced for vision with the small
+astronomical eye-pieces. But as there is often occasion to use
+appliances which disturb the balance, it is well to have the means of at
+once restoring equilibrium. A cord ring running round the tube (pretty
+tightly, so as to rest still when the tube is inclined), and bearing a
+small weight, will be all that is required for this purpose; it must be
+slipped along the tube until the tube is found to be perfectly balanced.
+Nothing is more annoying than, after getting a star well in the field,
+to see the tube shift its position through defective balance, and thus
+to have to search again for the star. Even with such an arrangement as
+is shown in fig. 8, though the tube cannot readily shift its position,
+it is better to have it well balanced.
+
+The quality of the stand has a very important influence on the
+performance of a telescope. In fact, a moderately good telescope,
+mounted on a steady stand, working easily and conveniently, will not
+only enable the observer to pass his time much more pleasantly, but will
+absolutely exhibit more difficult objects than a finer instrument on a
+rickety, ill-arranged stand. A good observing-chair is also a matter of
+some importance, the least constraint or awkwardness of position
+detracting considerably from the power of distinct vision. Such, at
+least, is my own experience.
+
+But the mere examination of the glasses, tube, mounting, &c., is only
+the first step in the series of tests which should be applied to a
+telescope, since the excellence of the instrument depends, not on its
+size, the beauty of its mounting, or any extraneous circumstances, but
+on its performance.
+
+The observer should first determine whether the chromatic aberration is
+corrected. To ascertain this the telescope should be directed to the
+moon, or (better) to Jupiter, and accurately focussed for distinct
+vision. If, then, on moving the eye-piece towards the object-glass, a
+ring of purple appears round the margin of the object, and on moving the
+eye-glass in the contrary direction a ring of green, the chromatic
+aberration is corrected, since these are the colours of the secondary
+spectrum.
+
+To determine whether the spherical aberration is corrected, the
+telescope should be directed towards a star of the third or fourth
+magnitude, and focussed for distinct vision. A cap with an aperture of
+about one-half its diameter should then be placed over the object-glass.
+If no new adjustment is required for distinct vision, the spherical
+aberration is corrected, since the mean focal length and the focal
+length of the central rays are equal. If, when the cap is on, the
+eye-piece has to be pulled out for distinct vision, the spherical
+aberration has not been fully corrected; if the eye-piece has to be
+pushed in, the aberration has been over-corrected. As a further test, we
+may cut off the central rays, by means of a circular card covering the
+middle of the object-glass, and compare the focal length for distinct
+vision with the focal length when the cap is applied. The extent of the
+spherical aberration may be thus determined; but if the first experiment
+gives a satisfactory result, no other is required.
+
+A star of the first magnitude should next be brought into the field of
+view. If an irradiation from one side is perceived, part of the
+object-glass has not the same refractive power as the rest; and the
+part which is defective can be determined by applying in different
+positions a cap which hides half the object-glass. If the irradiation is
+double, it will probably be found that the object-glass has been too
+tightly screwed, and the defect will disappear when the glass is freed
+from such undue pressure.
+
+If the object-glass is not quite at right angles to the axis of the
+tube, or if the eye-tube is at all inclined, a like irradiation will
+appear when a bright star is in the field. The former defect is not
+easily detected or remedied; nor is it commonly met with in the work of
+a careful optician. The latter defect may be detected by cutting out
+three circular cards of suitable size with a small aperture at the
+centre of each, and inserting one at each end of the eye-tube, and one
+over the object-glass. If the tube is rightly placed the apertures will
+of course lie in a right line, so that it will be possible to look
+through all three at once. If not, it will be easy to determine towards
+what part of the object-glass the eye-tube is directed, and to correct
+the position of the tube accordingly.
+
+The best tests for determining the defining power of a telescope are
+close double or multiple stars, the components of which are not very
+unequal. The illuminating power should be tested by directing the
+telescope towards double or multiple stars having one or more minute
+components. Many of the nebulae serve as tests both for illumination and
+defining power. As we proceed we shall meet with proper objects for
+testing different telescopes. For the present, let the following list
+suffice. It is selected from Admiral Smyth's tests, obtained by
+diminishing the aperture of a 6-in. telescope having a focal length of
+8-1/2 feet:
+
+A two-inch aperture, with powers of from 60 to 100, should exhibit
+
+[alpha] Piscium (3".5).   | [delta] Cassiopeiae (9".5),
+                          |    mag. (4 and 7-1/2)
+[gamma] Leonis  (3".2).   | Polaris (18".6), mag. (2-1/2
+                          |    and 9-1/2)
+
+A four-inch, powers 80 to 120, should exhibit
+
+[xi] Ursae Majoris (2".4). | [sigma] Cassiopeiae (3".1),
+                          |    mag. (6 and 8).
+[gamma] Ceti (2".6).      | [delta] Geminorum (7".1),
+                          |    mag. (4 and 9).
+
+The tests in the first column are for definition, those in the second
+for illumination. It will be noticed that, though in the case of Polaris
+the smaller aperture may be expected to show the small star of less than
+the 9th magnitude, a larger aperture is required to show the 8th
+magnitude component of [sigma] Cassiopeiae, on account of the greater
+closeness of this double.
+
+In favourable weather the following is a good general test of the
+performance of a telescope:--A star of the 3rd or 4th magnitude at a
+considerable elevation above the horizon should exhibit a small well
+defined disc, surrounded by two or three fine rings of light.
+
+A telescope should not be mounted within doors, if it can be
+conveniently erected on solid ground, as every movement in the house
+will cause the instrument to vibrate unpleasantly. Further, if the
+telescope is placed in a warm room, currents of cold air from without
+will render observed objects hazy and indistinct. In fact, Sir W.
+Herschel considered that a telescope should not even be erected near a
+house or elevation of any kind round which currents of air are likely to
+be produced. If a telescope is used in a room, the temperature of the
+room should be made as nearly equal as possible to that of the outer
+air.
+
+When a telescope is used out of doors a 'dew-cap,' that is, a tube of
+tin or pasteboard, some ten or twelve inches long, should be placed on
+the end of the instrument, so as to project beyond the object-glass. For
+glass is a good radiator of heat, so that dew falls heavily upon it,
+unless the radiation is in some way checked. The dew-cap does this
+effectually. It should be blackened within, especially if made of metal.
+"After use," says old Kitchener, "the telescope should be kept in a warm
+place long enough for any moisture on the object-glass to evaporate." If
+damp gets between the glasses it produces a fog (which opticians call a
+sweat) or even a seaweed-like vegetation, by which a valuable glass may
+be completely ruined.
+
+The observer should not leave to the precious hours of the night the
+study of the bearing and position of the objects he proposes to examine.
+This should be done by day--an arrangement which has a twofold
+advantage,--the time available for observation is lengthened, and the
+eyes are spared sudden changes from darkness to light, and _vice versa_.
+Besides, the eye is ill-fitted to examine difficult objects, after
+searching by candle-light amongst the minute details recorded in maps or
+globes. Of the effect of rest to the eye we have an instance in Sir J.
+Herschel's rediscovery of the satellites of Uranus, which he effected
+after keeping his eyes in darkness for a quarter of an hour. Kitchener,
+indeed, goes so far as to recommend (with a _crede experto_) an
+_interval of sleep_ in the darkness of the observing-room before
+commencing operations. I have never tried the experiment, but I should
+expect it to have a bad rather than a good effect on the eyesight, as
+one commonly sees the eyes of a person who has been sleeping in his
+day-clothes look heavy and bloodshot.
+
+The object or the part of an object to be observed should be brought as
+nearly as possible to the centre of the field of view. When there is no
+apparatus for keeping the telescope pointed upon an object, the best
+plan is so to direct the telescope by means of the finder, that the
+object shall be just out of the field of view, and be brought (by the
+earth's motion) across the centre of the field. Thus the vibrations
+which always follow the adjustment of the tube will have subsided before
+the object appears. The object should then be intently watched during
+the whole interval of its passage across the field of view.
+
+It is important that the student should recognise the fact that the
+highest powers do not necessarily give the best views of celestial
+objects. High powers in all cases increase the difficulty of
+observation, since they diminish the field of view and the illumination
+of the object, increase the motion with which (owing to the earth's
+motion) the image moves across the field, and magnify all defects due to
+instability of the stand, imperfection of the object-glass, or
+undulation of the atmosphere. A good object-glass of three inches
+aperture will in very favourable weather bear a power of about 300, when
+applied to the observation of close double or multiple stars, but for
+all other observations much lower powers should be used. Nothing but
+failure and annoyance can follow the attempt to employ the highest
+powers on unsuitable objects or in unfavourable weather.
+
+The greatest care should be taken in focussing the telescope. When high
+powers are used this is a matter of some delicacy. It would be well if
+the eye-pieces intended for a telescope were so constructed that when
+the telescope is focussed for one, this might be replaced by any other
+without necessitating any use of the focussing rack-work. This could be
+readily effected by suitably placing the shoulder which limits the
+insertion of the eye-piece.
+
+It will be found that, even in the worst weather for observation, there
+are instants of distinct vision (with moderate powers) during which the
+careful observer may catch sight of important details; and, similarly,
+in the best observing weather, there are moments of unusually distinct
+vision well worth patient waiting for, since in such weather alone the
+full powers of the telescope can be employed.
+
+The telescopist should not be deterred from observation by the presence
+of fog or haze, since with a hazy sky definition is often singularly
+good.
+
+The observer must not expect distinct vision of objects near the
+horizon. Objects near the eastern horizon during the time of morning
+twilight are especially confused by atmospheric undulations; in fact,
+early morning is a very unfavourable time for the observation of all
+objects.
+
+The same rules which we have been applying to refractors, serve for
+reflectors. The performance of a reflector will be found to differ in
+some respects, however, from that of a refractor. Mr. Dawes is, we
+believe, now engaged in testing reflectors, and his unequalled
+experience of refractors will enable him to pronounce decisively on the
+relative merits of the two classes of telescopes.
+
+We have little to say respecting the construction of telescopes. Whether
+it is advisable or not for an amateur observer to attempt the
+construction of his own telescope is a question depending entirely on
+his mechanical ability and ingenuity. My own experience of telescope
+construction is confined to the conversion of a 3-feet into a 5-1/2-feet
+telescope. This operation involved some difficulties, since the aperture
+had to be increased by about an inch. I found a tubing made of alternate
+layers of card and calico well pasted together, to be both light and
+strong. But for the full length of tube I think a core of metal is
+wanted. A learned and ingenious friend, Mr. Sharp, Fellow of St. John's
+College, informs me that a tube of tin, covered with layers of brown
+paper, well pasted and thicker near the middle of the tube, forms a
+light and strong telescope-tube, almost wholly free from vibration.
+
+Suffer no inexperienced person to deal with your object-glass. I knew a
+valuable glass ruined by the proceedings of a workman who had been told
+to attach three pieces of brass round the cell of the double lens. What
+he had done remained unknown, but ever after a wretched glare of light
+surrounded all objects of any brilliancy.
+
+One word about the inversion of objects by the astronomical telescope.
+It is singular that any difficulty should be felt about so simple a
+matter, yet I have seen in the writings of more than one distinguished
+astronomer, wholly incorrect views as to the nature of the inversion.
+One tells us that to obtain the correct presentation from a picture
+taken with a telescope, the view should be inverted, held up to the
+light, and looked at from the back of the paper. Another tells us to
+invert the picture and hold it opposite a looking-glass. Neither method
+is correct. The simple correction wanted is to hold the picture upside
+down--the same change which brings the top to the bottom brings the
+right to the left, _i.e._, fully corrects the inversion.
+
+In the case, however, of a picture taken by an Herschelian reflector,
+the inversion not being complete, a different method must be adopted. In
+fact, either of the above-named processes, incorrect for the ordinary
+astronomical, would be correct for the Herschelian Telescope. The latter
+inverts but does not reverse right and left; therefore after inverting
+our picture we must interchange right and left because they have been
+reversed by the inversion. This is effected either by looking at the
+picture from behind, or by holding it up to a mirror.
+
+[Illustration: PLATE II.]
+
+
+
+
+CHAPTER II.
+
+A HALF-HOUR WITH ORION, LEPUS TAURUS, ETC.
+
+
+Any of the half-hours here assigned to the constellation-seasons may be
+taken first, and the rest in seasonal or cyclic order. The following
+introductory remarks are applicable to each:--
+
+If we stand on an open space, on any clear night, we see above us the
+celestial dome spangled with stars, apparently fixed in position. But
+after a little time it becomes clear that these orbs are slowly shifting
+their position. Those near the eastern horizon are rising, those near
+the western setting. Careful and continuous observation would show that
+the stars are all moving in the same way, precisely, as they would if
+they were fixed to the concave surface of a vast hollow sphere, and this
+sphere rotated about an axis. This axis, in our latitude, is inclined
+about 51-1/2 deg. to the horizon. Of course only one end of this imaginary
+axis can be above our horizon. This end lies very near a star which it
+will be well for us to become acquainted with at the beginning of our
+operations. It lies almost exactly towards the north, and is raised from
+50 deg. to 53 deg. (according to the season and hour) above the horizon. There
+is an easy method of finding it.
+
+We must first find the Greater Bear. It will be seen from Plate 1, that
+on a spring evening the seven conspicuous stars of this constellation
+are to be looked for towards the north-east, about half way between the
+horizon and the point overhead (or _zenith_), the length of the set of
+stars being vertical. On a summer's evening the Great Bear is nearly
+overhead. On an autumn evening he is towards the north-west, the length
+of the set of seven being somewhat inclined to the horizon. Finally, on
+a winter's evening, he is low down towards the north, the length of the
+set of seven stars being nearly in a horizontal direction.
+
+Having found the seven stars, we make use of the pointers [alpha] and
+[beta] (shown in Plate 1) to indicate the place of the Pole-star, whose
+distance from the pointer [alpha] is rather more than three times the
+distance of [alpha] from [beta].
+
+Now stand facing the Pole-star. Then all the stars are travelling round
+that star _in a direction contrary to that in which the hands of a watch
+move_. Thus the stars below the pole are moving _towards the right_,
+those above the pole _towards the left_, those to the right of the pole
+_upwards_, those to the left of the pole _downwards_.
+
+Next face the south. Then all the stars on our left, that is, towards
+the east, are rising slantingly towards the south; those due south are
+moving horizontally to the right, that is, towards the west; and those
+on our right are passing slantingly downwards towards the west.
+
+It is important to familiarise ourselves with these motions, because it
+is through them that objects pass out of the field of view of the
+telescope, and by moving the tube in a proper direction we can easily
+pick up an object that has thus passed away, whereas if we are not
+familiar with the varying motions in different parts of the celestial
+sphere, we may fail in the attempt to immediately recover an object, and
+waste time in the search for it.
+
+The consideration of the celestial motions shows how advantageous it is,
+when using an alt-azimuth, to observe objects as nearly as possible due
+south. Of course in many cases this is impracticable, because a
+phenomenon we wish to watch may occur when an object is not situated
+near the meridian. But in examining double stars there is in general no
+reason for selecting objects inconveniently situated. We can wait till
+they come round to the meridian, and then observe them more comfortably.
+Besides, most objects are higher, and therefore better seen, when due
+south.
+
+Northern objects, and especially those within the circle of perpetual
+apparition, often culminate (that is, cross the meridian, or north and
+south line) at too great a height for comfortable vision. In this case
+we should observe them towards the east or west, and remember that in
+the first case they are rising, and in the latter they are setting, and
+that in both cases they have also a motion from left to right.
+
+If we allow an object to pass right across the field of view (the
+telescope being fixed), the apparent direction of its motion is the
+exact reverse of the true direction of the star's motion. This will
+serve as a guide in shifting the alt-azimuth after a star has passed out
+of the field of view.
+
+The following technical terms must be explained. That part of the field
+of view towards which the star appears to move is called the _preceding_
+part of the field, the opposite being termed the _following_ part. The
+motion for all stars, except those lying in an oval space extending from
+the zenith to the pole of the heavens, is more or less from right to
+left (in the inverted field). Now, if we suppose a star to move along a
+diameter of the field so as to divide the field into two semicircles,
+then in all cases in which this motion takes places from right to left,
+that semicircle which contains the lowest point (apparently) of the
+field is the _northern_ half, the other is the _southern_ half. Over the
+oval space just mentioned the reverse holds.
+
+Thus the field is divided into four quadrants, and these are termed
+_north following_ (_n.f._) and _south following_ (_s.f._); _north
+preceding_ (_n.p._), and _south preceding_ (_s.p._). The student can
+have no difficulty in interpreting these terms, since he knows which is
+the following and which the preceding _semicircle_, which the northern
+and which the southern. In the figures of plates 3 and 5, the letters
+_n.f._, _n.p._, &c., are affixed to the proper quadrants. It is to be
+remembered that the quadrants thus indicated are measured either way
+from the point and feather of the diametral arrows.
+
+Next, of the apparent annual motion of the stars. This takes place in
+exactly the same manner as the daily motion. If we view the sky at eight
+o'clock on any day, and again at the same hour one month later, we shall
+find that at the latter observation (as compared with the former) the
+heavens appear to have rotated by the _twelfth part_ of a complete
+circumference, and the appearance presented is precisely the same as we
+should have observed had we waited for two hours (the _twelfth part_ of
+a day) on the day of the first observation.
+
+       *       *       *       *       *
+
+Our survey of the heavens is supposed to be commenced during the first
+quarter of the year, at ten o'clock on the 20th of January, or at nine
+on the 5th of February, or at eight on the 19th of February, or at seven
+on the 6th of March, or at hours intermediate to these on intermediate
+days.
+
+We look first for the Great Bear towards the north-east, as already
+described, and thence find the Pole-star; turning towards which we see,
+towards the right and downwards, the two guardians of the pole ([beta]
+and [gamma] Ursae Minoris). Immediately under the Pole-star is the
+Dragon's Head, a conspicuous diamond of stars. Just on the horizon is
+Vega, scintillating brilliantly. Overhead is the brilliant Capella, near
+which the Milky Way is seen passing down to the horizon on either side
+towards the quarters S.S.E. and N.N.W.
+
+For the present our business is with the southern heavens, however.
+
+Facing the south, we see a brilliant array of stars, Sirius
+unmistakeably overshining the rest. Orion is shining in full glory, his
+leading brilliant, Betelgeuse[2] being almost exactly on the meridian,
+and also almost exactly half way between the horizon and the zenith. In
+Plate 2 is given a map of this constellation and its neighbourhood.
+
+Let us first turn the tube on Sirius. It is easy to get him in the field
+without the aid of a finder. The search will serve to illustrate a
+method often useful when a telescope has no finder. Having taking out
+the eye-piece--a low-power one, suppose--direct the tube nearly towards
+Sirius. On looking through it, a glare of light will be seen within the
+tube. Now, if the tube be slightly moved about, the light will be seen
+to wax and wane, according as the tube is more or less accurately
+directed. Following these indications, it will be found easy to direct
+the tube, so that the object-glass shall appear _full of light_. When
+this is done, insert the eye-piece, and the star will be seen in the
+field.
+
+But the telescope is out of focus, therefore we must turn the small
+focussing screw. Observe the charming chromatic changes--green, and
+red, and blue light, purer than the hues of the rainbow, scintillating
+and coruscating with wonderful brilliancy. As we get the focus, the
+excursions of these light flashes diminish until--if the weather is
+favourable--the star is seen, still scintillating, and much brighter
+than to the naked eye, but reduced to a small disc of light, surrounded
+(in the case of so bright a star as Sirius) with a slight glare. If
+after obtaining the focus the focussing rack work be still turned, we
+see a coruscating image as before. In the case of a very brilliant star
+these coruscations are so charming that we may be excused for calling
+the observer's attention to them. The subject is not without interest
+and difficulty as an optical one. But the astronomer's object is to get
+rid of all these flames and sprays of coloured light, so that he has
+very little sympathy with the admiration which Wordsworth is said to
+have expressed for out-of-focus views of the stars.
+
+We pass to more legitimate observations, noticing in passing that Sirius
+is a double star, the companion being of the tenth magnitude, and
+distant about ten seconds from the primary. But our beginner is not
+likely to see the companion, which is a very difficult object, vowing to
+the overpowering brilliancy of the primary.
+
+Orion affords the observer a splendid field of research. It is a
+constellation rich in double and multiple stars, clusters, and nebulae.
+We will begin with an easy object.
+
+The star [delta] (Plate 3), or _Mintaka_, the uppermost of the three
+stars forming the belt, is a wide double. The primary is of the second
+magnitude, the secondary of the seventh, both being white.
+
+The star [alpha] (_Betelgeuse_) is an interesting object, on account of
+its colour and brilliance, and as one of the most remarkable variables
+in the heavens. It was first observed to be variable by Sir John
+Herschel in 1836. At this period its variations were "most marked and
+striking." This continued until 1840, when the changes became "much less
+conspicuous. In January, 1849, they had recommenced, and on December
+5th, 1852, Mr. Fletcher observed [alpha] Orionis brighter than Capella,
+and actually the largest star in the northern hemisphere." That a star
+so conspicuous, and presumably so large, should present such remarkable
+variations, is a circumstance which adds an additional interest to the
+results which have rewarded the spectrum-analysis of this star by Mr.
+Huggins and Professor Miller. It appears that there is decisive evidence
+of the presence in this luminary of many elements known to exist in our
+own sun; amongst others are found sodium, magnesium, calcium, iron, and
+bismuth. Hydrogen appears to be absent, or, more correctly, there are no
+lines in the star's spectrum corresponding to those of hydrogen in the
+solar spectrum. Secchi considers that there is evidence of an actual
+change in the spectrum of the star, an opinion in which Mr. Huggins does
+not coincide. In the telescope Betelgeuse appears as "a rich and
+brilliant gem," says Lassell, "a rich topaz, in hue and brilliancy
+differing from any that I have seen."
+
+Turn next to [beta] (Rigel), the brightest star below the belt. This is
+a very noted double, and will severely test our observer's telescope, if
+small. The components are well separated (see Plate 3), compared with
+many easier doubles; the secondary is also of the seventh magnitude, so
+that neither as respects closeness nor smallness of the secondary, is
+Rigel a difficult object. It is the combination of the two features
+which makes it a test-object. Kitchener says a 1-3/4-inch object-glass
+should show Rigel double; in earlier editions of his work he gave
+2-3/4-inches as the necessary aperture. Smyth mentions Rigel as a test
+for a 4-inch aperture, with powers of from 80 to 120. A 3-inch aperture,
+however, will certainly show the companion. Rigel is an orange star, the
+companion blue.
+
+Turn next to [lambda] the northernmost of the set of three stars in the
+head of Orion. This is a triple star, though an aperture of 3 inches
+will show it only as a double. The components are 5" apart, the colours
+pale white and violet. With the full powers of a 3-1/2-inch glass a
+faint companion may be seen above [lambda].
+
+The star [zeta], the lowest in the belt, may be tried with a 3-1/2-inch
+glass. It is a close double, the components being nearly equal, and
+about 2-1/2" apart (see Plate 3).
+
+For a change we will now try our telescope on a nebula, selecting the
+great nebula in the Sword. The place of this object is indicated in
+Plate 2. There can be no difficulty in finding it since it is clearly
+visible to the naked eye on a moonless night--the only sort of night on
+which an observer would care to look at nebulae. A low power should be
+employed.
+
+The nebula is shown in Plate 3 as I have seen it with a 3-inch aperture.
+We see nothing of those complex streams of light which are portrayed in
+the drawings of Herschel, Bond, and Lassell, but enough to excite our
+interest and wonder. What is this marvellous light-cloud? One could
+almost imagine that there was a strange prophetic meaning in the words
+which have been translated "Canst thou loose the bands of Orion?"
+Telescope after telescope had been turned on this wonderful object with
+the hope of resolving its light into stars. But it proved intractable to
+Herschel's great reflector, to Lassell's 2-feet reflector, to Lord
+Rosse's 3-feet reflector, and even partially to the great 6-feet
+reflector. Then we hear of its supposed resolution into stars, Lord
+Rosse himself writing to Professor Nichol, in 1846, "I may safely say
+there can be little, if any, doubt as to the resolvability of the
+nebula;--all about the trapezium is a mass of stars, the rest of the
+nebula also abounding with stars, and exhibiting the characteristics of
+resolvability strongly marked."
+
+It was decided, therefore, that assuredly the great nebula is a
+congeries of stars, and not a mass of nebulous matter as had been
+surmised by Sir W. Herschel. And therefore astronomers were not a little
+surprised when it was proved by Mr. Huggins' spectrum-analysis that the
+nebula consists of gaseous matter. How widely extended this gaseous
+universe may be we cannot say. The general opinion is that the nebulae
+are removed far beyond the fixed stars. If this were so, the dimensions
+of the Orion nebula would be indeed enormous, far larger probably than
+those of the whole system whereof our sun is a member. I believe this
+view is founded on insufficient evidence, but this would not be the
+place to discuss the subject. I shall merely point out that the nebula
+occurs in a region rich in stars, and if it is not, like the great
+nebula in Argo, clustered around a remarkable star, it is found
+associated in a manner which I cannot look upon as accidental with a set
+of small-magnitude stars, and notably with the trapezium which surrounds
+that very remarkable black gap within the nebula. The fact that the
+nebula shares the proper motion of the trapezium appears inexplicable if
+the nebula is really far out in space beyond the trapezium. A very small
+proper motion of the trapezium (alone) would long since have destroyed
+the remarkable agreement in the position of the dark gap and the
+trapezium which has been noticed for so many years.
+
+But whether belonging to our system or far beyond it, the great nebula
+must have enormous dimensions. A vast gaseous system it is, sustained by
+what arrangements or forces we cannot tell, nor can we know what
+purposes it subserves. Mr. Huggins' discovery that comets have gaseous
+nuclei, (so far as the two he has yet examined show) may suggest the
+speculation that in the Orion nebula we see a vast system of comets
+travelling in extensive orbits around nuclear stars, and so slowly as to
+exhibit for long intervals of time an unchanged figure. "But of such
+speculations" we may say with Sir J. Herschel "there is no end."
+
+To return to our telescopic observations:--The trapezium affords a
+useful test for the light-gathering power of the telescope. Large
+instruments exhibit nine stars. But our observer may be well satisfied
+with his instrument and his eye-sight if he can see five with a
+3-1/2-inch aperture.[3] A good 3-inch glass shows four distinctly. But
+with smaller apertures only three are visible.
+
+The whole neighbourhood of the great nebula will well repay research.
+The observer may sweep over it carefully on any dark night with profit.
+Above the nebula is the star-cluster 362 H. The star [iota] (double as
+shown in Plate 3) below the nebula is involved in a strong nebulosity.
+And in searching over this region we meet with delicate double, triple,
+and multiple stars, which make the survey interesting with almost any
+power that may be applied.
+
+Above the nebula is the star [sigma], a multiple star. To an observer
+with a good 3-1/2-inch glass [sigma] appears as an octuple star. It is
+well seen, however, as a fine multiple star with a smaller aperture.
+Some of the stars of this group appear to be variable.
+
+The star [rho] Orionis is an unequal, easy double, the components being
+separated by nearly seven seconds. The primary is orange, the smaller
+star smalt-blue (see Plate 3).
+
+The middle star of the belt ([epsilon]) has a distant blue companion.
+This star, like [iota], is nebulous. In fact, the whole region within
+the triangle formed by stars [gamma], [kappa] and [beta] is full of
+nebulous double and multiple stars, whose aggregation in this region I
+do not consider wholly accidental.
+
+We have not explored half the wealth of Orion, but leave much for future
+observation. We must turn, however, to other constellations.
+
+Below Orion is Lepus, the Hare, a small constellation containing some
+remarkable doubles. Among these we may note [xi], a white star with a
+scarlet companion; [gamma], a yellow and garnet double; and [iota], a
+double star, white and pale violet, with a distant red companion. The
+star [kappa] Leporis is a rather close double, white with a small green
+companion. The intensely red star R Leporis (a variable) will be found
+in the position indicated in the map.
+
+Still keeping within the boundary of our map, we may next turn to the
+fine cluster 2 H (vii.) in Monoceros. This cluster is visible to the
+naked eye, and will be easily found. The nebula 2 H (iv.) is a
+remarkable one with a powerful telescope.
+
+The star 11 Monocerotis is a fine triple star described by the elder
+Herschel as one of the finest sights in the heavens. Our observer,
+however, will see it as a double (see Plate 3). [delta] Monocerotis is
+an easy double, yellow and lavender.
+
+We may now leave the region covered by the map and take a survey of the
+heavens for some objects well seen at this season.
+
+Towards the south-east, high up above the horizon, we see the twin-stars
+Castor and Pollux. The upper is Castor, the finest double star visible
+in the northern heavens. The components are nearly equal and rather more
+than 5" apart (see Plate 3). Both are white according to the best
+observers, but the smaller is thought by some to be slightly greenish.
+
+Pollux is a coarse but fine triple star (in large instruments multiple).
+The components orange, grey, and lilac.
+
+There are many other fine objects in Gemini, but we pass to Cancer.
+
+The fine cluster Praesepe in Cancer may easily be found as it is
+distinctly visible to the naked eye in the position shown in Plate 1,
+Map I. In the telescope it is seen as shown in Plate 3.
+
+The star [iota] Cancri is a wide double, the colours orange and blue.
+
+Procyon, the first-magnitude star between Praesepe and Sirius, is finely
+coloured--yellow with a distant orange companion, which appears to be
+variable.
+
+Below the Twins, almost in a line with them, is the star [alpha] Hydrae,
+called Al Fard, or "the Solitary One." It is a 2nd magnitude variable. I
+mention it, however, not on its own account, but as a guide to the fine
+double [epsilon] Hydrae. This star is the middle one of a group of three,
+lying between Pollux and Al Fard rather nearer the latter. The
+components of [epsilon] Hydrae are separated by about 3-1/2" (see Plate
+3). The primary is of the fourth, the companion of the eighth magnitude;
+the former is yellow, the latter a ruddy purple. The period of [epsilon]
+Hydrae is about 450 years.
+
+The constellation Leo Minor, now due east and about midway between the
+horizon and the zenith, is well worth sweeping over. It contains several
+fine fields.
+
+Let us next turn to the western heavens. Here there are some noteworthy
+objects.
+
+To begin with, there are the Pleiades, showing to the naked eye only six
+or seven stars. In the telescope the Pleiades appear as shown in Plate
+3.
+
+The Hyades also show some fine fields with low powers.
+
+Aldebaran, the principal star of the Hyades, as also of the
+constellation Taurus, is a noted red star. It is chiefly remarkable for
+the close spectroscopic analysis to which it has been subjected by
+Messrs. Huggins and Miller. Unlike Betelgeuse, the spectrum of Aldebaran
+exhibits the lines corresponding to hydrogen, and no less than eight
+metals--sodium, magnesium, calcium, iron, bismuth, tellurium, antimony,
+and mercury, are proved to exist in the constitution of this brilliant
+red star.
+
+On the right of Aldebaran, in the position indicated in Plate 1, Map I.,
+are the stars [zeta] and [beta] Tauri. If with a low power the observer
+sweep from [zeta] towards [beta], he will soon find--not far from [zeta]
+(at a distance of about one-sixth of the distance separating [beta] from
+[zeta]), the celebrated Crab nebula, known as 1 M. This was the first
+nebula discovered by Messier, and its discovery led to the formation of
+his catalogue of 103 nebulae. In a small telescope this object appears as
+a nebulous light of oval form, no traces being seen of the wisps and
+sprays of light presented in Lord Rosse's well known picture of the
+nebula.
+
+Here I shall conclude the labours of our first half-hour among the
+stars, noticing that the examination of Plate 1 will show what other
+constellations besides those here considered are well situated for
+observation at this season. It will be remarked that many constellations
+well seen in the third half-hour (Chapter IV.) are favourably seen in
+the first also, and _vice versa_. For instance, the constellation Ursa
+Major well-placed towards the north-east in the first quarter of the
+year, is equally well-placed towards the north-west in the third, and
+similarly of the constellation Cassiopeia. The same relation connects
+the second and fourth quarters of the year.
+
+[Illustration: PLATE III.]
+
+
+
+
+CHAPTER III.
+
+A HALF-HOUR WITH LYRA, HERCULES, CORVUS, CRATER, ETC.
+
+
+The observations now to be commenced are supposed to take place during
+the second quarter of the year,--at ten o'clock on the 20th of April, or
+at nine on the 5th of May, or at eight on the 21st of May, or at seven
+on the 5th of June, or at hours intermediate to these on intermediate
+days.
+
+We again look first for the Great Bear, now near the zenith, and thence
+find the Pole-star. Turning towards the north, we see Cassiopeia between
+the Pole-star and the horizon. Towards the north-west is the brilliant
+Capella, and towards the north-east the equally brilliant Vega, beneath
+which, and somewhat northerly, is the cross in Cygnus. The Milky Way
+passes from the eastern horizon towards the north (low down), and so
+round to the western horizon.
+
+In selecting a region for special observation, we shall adopt a
+different plan from that used in the preceding "half-hour." The region
+on the equator and towards the south is indeed particularly interesting,
+since it includes the nebular region in Virgo. Within this space nebulae
+are clustered more closely than over any corresponding space in the
+heavens, save only the greater Magellanic cloud. But to the observer
+with telescopes of moderate power these nebulae present few features of
+special interest; and there are regions of the sky now well situated for
+observation, which, at most other epochs are either low down towards
+the horizon or inconveniently near to the zenith. We shall therefore
+select one of these, the region included in the second map of Plate 2,
+and the neighbouring part of the celestial sphere.
+
+At any of the hours above named, the constellation Hercules lies towards
+the east. A quadrant taken from the zenith to the eastern horizon passes
+close to the last star ([eta]) of the Great Bear's tail, through [beta],
+a star in Bootes' head, near [beta] Herculis, between the two "Alphas"
+which mark the heads of Hercules and Ophiuchus, and so past [beta]
+Ophiuchi, a third-magnitude star near the horizon. And here we may turn
+aside for a moment to notice the remarkable vertical row of six
+conspicuous stars towards the east-south-east; these are, counting them
+in order from the horizon, [zeta], [epsilon], and [delta] Ophiuchi,
+[epsilon], [alpha], and [delta] Serpentis.
+
+Let the telescope first be directed towards Vega. This orb presents a
+brilliant appearance in the telescope. Its colour is a bluish-white. In
+an ordinary telescope Vega appears as a single star, but with a large
+object-glass two distant small companions are seen. A nine-inch glass
+shows also two small companions within a few seconds of Vega. In the
+great Harvard refractor Vega is seen with no less than thirty-five
+companions. I imagine that all these stars, and others which can be seen
+in neighbouring fields, indicate the association of Vega with the
+neighbouring stream of the Milky Way.
+
+Let our observer now direct his telescope to the star [epsilon] Lyrae. Or
+rather, let him first closely examine this star with the naked eye. The
+star is easily identified, since it lies to the left of Vega, forming
+with [zeta] a small equilateral triangle. A careful scrutiny suffices to
+indicate a peculiarity in this star. If our observer possesses very
+good eye-sight, he will distinctly recognise it as a "naked-eye double";
+but more probably he will only notice that it appears lengthened in a
+north and south direction.[4] In the finder the star is easily divided.
+Applying a low power to the telescope itself, we see [epsilon] Lyrae as a
+wide double, the line joining the components lying nearly north and
+south. The southernmost component (the upper in the figure) is called
+[epsilon]^{1}, the other [epsilon]^{2}. Seen as a double, both
+components appear white.
+
+Now, if the observer's telescope is sufficiently powerful, each of the
+components may be seen to be itself double. First try [epsilon]^{1}, the
+northern pair. The line joining the components is directed as shown in
+Plate 3. The distance between them is 3".2, their magnitudes 5 and
+6-1/2, and their colours yellow and ruddy. If the observer succeeds in
+seeing [epsilon]^{1} fairly divided, he will probably not fail in
+detecting the duplicity of [epsilon]^{2}, though this is a rather closer
+pair, the distance between the components being only 2".6. The
+magnitudes are 5 and 5-1/2, both being white. Between [epsilon]^{1} and
+[epsilon]^{2} are three faint stars, possibly forming with the quadruple
+a single system.
+
+Let us next turn to the third star of the equilateral triangle mentioned
+above--viz. to the star [zeta] Lyrae. This is a splendid but easy double.
+It is figured in Plate 3, but it must be noticed that the figure of
+[zeta] and the other nine small figures are not drawn on the same scale
+as [epsilon] Lyrae. The actual distance between the components of [zeta]
+Lyra is 44", or about one-fourth of the distance separating
+[epsilon]^{1} from [epsilon]^{2}. The components of [zeta] are very
+nearly equal in magnitude, in colour topaz and green, the topaz
+component being estimated as of the fifth magnitude, the green component
+intermediate between the fifth and sixth magnitudes.
+
+We may now turn to a star not figured in the map, but readily found. It
+will be noticed that the stars [epsilon], [alpha], [beta], and [gamma]
+form, with two small stars towards the left, a somewhat regular
+hexagonal figure--a hexagon, in fact, having three equal long sides and
+three nearly equal short sides alternating with the others. The star
+[eta] Lyrae forms the angle next to [epsilon]. It is a wide and unequal
+double, as figured in Plate 3. The larger component is azure blue; the
+smaller is violet, and, being only of the ninth magnitude, is somewhat
+difficult to catch with apertures under 3 inches.
+
+The star [delta]^{2} Lyrae is orange, [delta]^{1} blue. In the same field
+with these are seen many other stars.
+
+The stars [gamma]^{1} and [gamma]^{2} may also be seen in a single
+field, the distance between them being about half the moon's mean
+diameter. The former is quadruple, the components being yellow, bluish,
+pale blue, and blue.
+
+Turn next to the stars [beta] and [gamma] Lyrae, the former a multiple,
+the latter an unequal double star. It is not, however, in these respects
+that these stars are chiefly interesting, but for their variability. The
+variability of [gamma] has not indeed been fully established, though it
+is certain that, having once been less bright, [gamma] is now
+considerably brighter than [beta]. The change, however, may be due to
+the variation of [beta] alone. This star is one of the most remarkable
+variables known. Its period is 12d. 21h. 53m. 10s. In this time it
+passes from a maximum brilliancy--that of a star of the 3.4
+magnitude--to a minimum lustre equal to that of a star of the 4.3
+magnitude, thence to the same maximum brilliancy as before, thence to
+another minimum of lustre--that of a star of the 4.5 magnitude--and so
+to its maximum lustre again, when the cycle of changes recommences.
+These remarkable changes seem to point to the existence of two unequal
+dark satellites, whose dimensions bear a much greater proportion to
+those of the bright components of [beta] Lyrae than the dimensions of the
+members of the Solar System bear to those of the sun. In this case, at
+any rate, the conjecture hazarded about Algol, that the star revolves
+around a dark central orb, would be insufficient to account for the
+observed variation.
+
+Nearly midway between [beta] and [gamma] lies the wonderful ring-nebula
+57 M, of which an imperfect idea will be conveyed by the last figure of
+Plate 3. This nebula was discovered in 1772, by Darquier, at Toulouse.
+It is seen as a ring of light with very moderate telescopic power. In a
+good 3-1/2-inch telescope the nebula exhibits a mottled appearance and a
+sparkling light. Larger instruments exhibit a faint light within the
+ring; and in Lord Rosse's great Telescope "wisps of stars" are seen
+within, and faint streaks of light stream from the outer border of the
+ring. This nebula has been subjected to spectrum-analysis by Mr.
+Huggins. It turns out to be a gaseous nebula! In fact, ring-nebulae--of
+which only seven have been detected--seem to belong to the same class as
+the planetary nebulae, all of which exhibit the line-spectrum indicative
+of gaseity. The brightest of the three lines seen in the spectrum of the
+ring-nebula in Lyra presents a rather peculiar appearance, "since it
+consists," says Mr. Huggins, "of two bright dots, corresponding to
+sections of the ring, and between these there is not darkness, but an
+excessively faint line joining them. This observation makes it probable
+that the faint nebulous matter occupying the central portion is similar
+in constitution to that of the ring."
+
+The constellation Hercules also contains many very interesting objects.
+Let us first inspect a nebula presenting a remarkable contrast with that
+just described. I refer to the nebula 13 M, known as Halley's nebula
+(Plate 3). This nebula is visible to the naked eye, and in a good
+telescope it is a most wonderful object: "perhaps no one ever saw it for
+the first time without uttering a shout of wonder." It requires a very
+powerful telescope completely to resolve this fine nebula, but the
+outlying streamers may be resolved with a good 3-inch telescope. Sir W.
+Herschel considered that the number of the stars composing this
+wonderful object was at least 14,000. The accepted views respecting
+nebulae would place this and other clusters far beyond the limits of our
+sidereal system, and would make the component stars not very unequal (on
+the average) to our own sun. It seems to me far more probable, on the
+contrary, that the cluster belongs to our own system, and that its
+components are very much smaller than the average of separate stars.
+Perhaps the whole mass of the cluster does not exceed that of an average
+first-magnitude star.
+
+The nebulae 92 M and 50 H may be found, after a little searching, from
+the positions indicated in the map. They are both well worthy of study,
+the former being a very bright globular cluster, the latter a bright and
+large round nebula. The spectra of these, as of the great cluster,
+resemble the solar spectrum, being continuous, though, of course, very
+much fainter.
+
+The star [delta] Herculis (seen at the bottom of the map) is a wide and
+easy double--a beautiful object. The components, situated as shown in
+Plate 3, are of the fourth and eighth magnitude, and coloured
+respectively greenish-white and grape-red.
+
+The star [kappa] Herculis is not shown in the map, but may be very
+readily found, lying between the two gammas, [gamma] Herculis and
+[gamma] Serpentis (_see_ Frontispiece, Map 2), rather nearer the latter.
+It is a wide double, the components of fifth and seventh magnitude, the
+larger yellowish-white, the smaller ruddy yellow.[5]
+
+Ras Algethi, or [alpha] Herculis, is also beyond the limits of the map,
+but may be easily found by means of Map 2, Frontispiece. It is, properly
+speaking, a multiple star. Considered as a double, the arrangement of
+the components is that shown in Plate 3. The larger is of magnitude
+3-1/2, the smaller of magnitude 5-1/2; the former orange, the latter
+emerald. The companion stars are small, and require a good telescope to
+be well seen. Ras Algethi is a variable, changing from magnitude 3 to
+magnitude 3-1/2 in a period of 66-1/3 days.
+
+The star [rho] Herculis is a closer double. The components are 3".7
+apart, and situated as shown in Plate 3. The larger is of magnitude 4,
+the smaller 5-1/2; the former bluish-white, the latter pale emerald.
+
+There are other objects within the range of our map which are well
+worthy of study. Such are [mu] Draconis, a beautiful miniature of
+Castor; [gamma]^{1} and [gamma]^{2} Draconis, a wide double, the
+distance between the components being nearly 62" (both grey); and
+[gamma]^{1} and [gamma]^{2} Coronae, a naked-eye double, the components
+being 6' apart, and each double with a good 3-inch telescope.
+
+We turn, however, to another region of the sky. Low down, towards the
+south is seen the small constellation Corvus, recognised by its
+irregular quadrilateral of stars. Of the two upper stars, the left-hand
+one is Algorab, a wide double, the components placed as in Plate 3,
+23".5 apart, the larger of magnitude 3, the smaller 8-1/2, the colours
+pale yellow and purple.
+
+There is a red star in this neighbourhood which is well worth looking
+for. To the right of Corvus is the constellation Crater, easily
+recognised as forming a tolerably well-marked small group. The star
+Alkes, or [alpha] Crateris, must first be found. It is far from being
+the brightest star in the constellation, and may be assumed to have
+diminished considerably in brilliancy since it was entitled [alpha] by
+Bayer. It will easily be found, however, by means of the observer's star
+maps. If now the telescope be directed to Alkes, there will be found,
+following him at a distance of 42.5 s, and about one minute southerly, a
+small red star, R. Crateris. Like most red stars, this one is a
+variable. A somewhat smaller blue star may be seen in the same field.
+
+There is another red star which may be found pretty easily at this
+season. First find the stars [eta] and [omicron] Leonis, the former
+forming with Regulus (now lying towards the south-west, and almost
+exactly midway between the zenith and the horizon) the handle of the
+Sickle in Leo, the other farther off from Regulus towards the right, but
+lower down. Now sweep from [omicron] towards [eta] with a low power.[6]
+There will be found a sixth-magnitude star about one-fourth of the way
+from [omicron] to [eta]. South, following this, will be found a group of
+four stars, of which one is crimson. This is the star R Leonis. Like R
+Crateris and R Leporis it is variable.
+
+Next, let the observer turn towards the south again. Above Corvus, in
+the position shown in the Frontispiece, there are to be seen five stars,
+forming a sort of wide V with somewhat bowed legs. At the angle is the
+star [gamma] Virginis, a noted double. In 1756 the components were 6-1/2
+seconds apart. They gradually approached till, in 1836, they could not
+be separated by the largest telescopes. Since then they have been
+separating, and they are now 4-1/2 seconds apart, situated as shown in
+Plate 3. They are nearly equal in magnitude (4), and both pale yellow.
+
+The star [gamma] Leonis is a closer and more beautiful double. It will
+be found above Regulus, and is the brightest star on the blade of the
+Sickle. The components are separated by about 3-1/5 seconds, the larger
+of the second, the smaller of the fourth magnitude; the former
+yellow-orange, the latter greenish-yellow.
+
+Lastly, the star [iota] Leonis may be tried. It will be a pretty severe
+test for our observer's telescope, the components being only 2".4 apart,
+and the smaller scarcely exceeding the eighth magnitude. The brighter
+(fourth magnitude) is pale yellow, the other light blue.
+
+
+
+
+CHAPTER IV.
+
+A HALF-HOUR WITH BOOTES, SCORPIO, OPHIUCHUS, ETC.
+
+
+We now commence a series of observations suited to the third quarter of
+the year, and to the following hours:--Ten o'clock on the 22nd of July;
+nine on the 8th of August; eight on the 23rd of August; seven on the 8th
+of October; and intermediate hours on days intermediate to these.
+
+We look first for the Great Bear towards the north-west, and thence find
+the Pole-star. Turning towards the north we see Capella and [beta]
+Aurigae low down and slightly towards the left of the exact north point.
+The Milky Way crosses the horizon towards the north-north-east and
+passes to the opposite point of the compass, attaining its highest point
+above the horizon towards east-south-east. This part of the Milky Way is
+well worth observing, being marked by singular variations of brilliancy.
+Near Arided (the principal star of Cygnus, and now lying due east--some
+twenty-five degrees from the zenith) there is seen a straight dark rift,
+and near this space is another larger cavity, which has been termed the
+northern Coal-sack. The space between [gamma], [delta], and [beta] Cygni
+is covered by a large oval mass, exceedingly rich and brilliant. The
+neighbouring branch, extending from [epsilon] Cygni, is far less
+conspicuous here, but near Sagitta becomes brighter than the other,
+which in this neighbourhood suddenly loses its brilliancy and fading
+gradually beyond this point becomes invisible near [beta] Ophiuchi.
+The continuous stream becomes patchy--in parts very brilliant--where it
+crosses Aquila and Clypeus. In this neighbourhood the other stream
+reappears, passing over a region very rich in stars. We see now the
+greatest extent of the Milky Way, towards this part of its length, ever
+visible in our latitudes--just as in spring we see its greatest extent
+towards Monoceros and Argo.
+
+[Illustration: PLATE IV.]
+
+I may note here in passing that Sir John Herschel's delineation of the
+northern portion of the Milky Way, though a great improvement on the
+views given in former works, seems to require revision, and especially
+as respects the very remarkable patches and streaks which characterise
+the portion extending over Cepheus and Cygnus. It seems to me, also,
+that the evidence on which it has been urged that the stars composing
+the Milky Way are (on an average) comparable in magnitude to our own
+sun, or to stars of the leading magnitudes, is imperfect. I believe, for
+instance, that the brilliant oval of milky light in Cygnus comes from
+stars intimately associated with the leading stars in that
+constellation, and not far removed in space (proportionately) beyond
+them. Of course, if this be the case, the stars, whose combined light
+forms the patch of milky light, must be far smaller than the leading
+brilliants of Cygnus. However, this is not the place to enter on
+speculations of this sort; I return therefore to the business we have
+more immediately in hand.
+
+Towards the east is the square of Pegasus low down towards the horizon.
+Towards the south is Scorpio, distinguished by the red and brilliant
+Antares, and by a train of conspicuous stars. Towards the west is
+Bootes, his leading brilliant--the ruddy Arcturus--lying somewhat nearer
+the horizon than the zenith, and slightly south of west. Bootes as a
+constellation is easily found if we remember that he is delineated as
+chasing away the Greater Bear. Thus at present he is seen in a slightly
+inclined position, his head (marked by the third-magnitude star [beta])
+lying due west, some thirty degrees from the zenith. It has always
+appeared to me, by the way, that Bootes originally had nobler
+proportions than astronomers now assign to him. It is known that Canes
+Venatici now occupy the place of an upraised arm of Bootes, and I
+imagine that Corona Borealis, though undoubtedly a very ancient
+constellation, occupies the place of his other arm. Giving to the
+constellation the extent thus implied, it exhibits (better than most
+constellations) the character assigned to it. One can readily picture to
+oneself the figure of a Herdsman with upraised arms driving Ursa Major
+before him. This view is confirmed, I think, by the fact that the Arabs
+called this constellation the Vociferator.
+
+Bootes contains many beautiful objects. Partly on this account, and
+partly because this is a constellation with which the observer should be
+specially familiar, a map of it is given in Plate 4.
+
+Arcturus has a distant pale lilac companion, and is in other respects a
+remarkable and interesting object. It is of a ruddy yellow colour.
+Schmidt, indeed, considers that the star has changed colour of late
+years, and that whereas it was once very red it is now a yellow star.
+This opinion does not seem well grounded, however. The star _may_ have
+been more ruddy once than now, though no other observer has noticed such
+a peculiarity; but it is certainly not a pure yellow star at present (at
+any rate as seen in our latitude). Owing probably to the difference of
+colour between Vega, Capella and Arcturus, photometricians have not been
+perfectly agreed as to the relative brilliancy of these objects. Some
+consider Vega the most brilliant star in the northern heavens, while
+others assign the superiority to Capella. The majority, however,
+consider Arcturus the leading northern brilliant, and in the whole
+heavens place three only before him, viz., Sirius, Canopus, and [alpha]
+Centauri. Arcturus is remarkable in other respects. His proper motion is
+very considerable, so great in fact that since the time of Ptolemy the
+southerly motion (alone) of Arcturus has carried him over a space nearly
+half as great again as the moon's apparent diameter. One might expect
+that so brilliant a star, apparently travelling at a rate so great
+compared with the average proper motions of the stars, must be
+comparatively near to us. This, however, has not been found to be the
+case. Arcturus is, indeed, one of the stars whose distance it has been
+found possible to estimate roughly. But he is found to be some three
+times as far from us as the small star 61 Cygni, and more than seven
+times as far from us as [alpha] Centauri.
+
+The star [delta] Bootis is a wide and unequal double, the smaller
+component being only of the ninth magnitude.
+
+Above Alkaid the last star in the tail of the Greater Bear, there will
+be noticed three small stars. These are [theta], [iota], and [kappa]
+Bootis, and are usually placed in star-maps near the upraised hand of
+the Herdsman. The two which lie next to Alkaid, [iota] and [kappa], are
+interesting doubles. The former is a wide double (see Plate 5), the
+magnitudes of components 4 and 8, their colours yellow and white. The
+larger star of this pair is itself double. The star [kappa] Bootis is
+not so wide a double (see Plate 5), the magnitudes of the components 5
+and 8, their colours white and faint blue--a beautiful object.
+
+The star [xi] Bootis is an exceedingly interesting object. It is
+double, the colours of the components being orange-yellow and ruddy
+purple, their magnitudes 3-1/2 and 6-1/2. When this star was first
+observed by Herschel in 1780 the position of the components was quite
+different from that presented in Plate 5. They were also much closer,
+being separated by a distance of less than 3-1/2 seconds. Since that
+time the smaller component has traversed nearly a full quadrant, its
+distance from its primary first increasing, till in 1831 the stars were
+nearly 7-1/2 seconds apart, and thence slowly diminishing, so that at
+present the stars are less than 5 seconds apart. The period usually
+assigned to the revolution of this binary system is 117 years, and the
+period of peri-astral passage is said to be 1779. It appears to me,
+however, that the period should be about 108 years, the epoch of last
+peri-astral passage 1777 and of next peri-astral passage, therefore,
+1885. The angular motion of the secondary round the primary is now
+rapidly increasing, and the distance between the components is rapidly
+diminishing, so that in a few years a powerful telescope will be
+required to separate the pair.
+
+Not far from [xi] is [pi] Bootis, represented in Plate 5 as a somewhat
+closer double, but in reality--now at any rate--a slightly wider pair,
+since the distance between the components of [xi] has greatly diminished
+of late. Both the components of [pi] are white, and their magnitudes are
+3-1/2 and 6.
+
+We shall next turn to an exceedingly beautiful and delicate object, the
+double star [epsilon] Bootis, known also as Mirac and, on account of its
+extreme beauty, called Pulcherrima by Admiral Smyth. The components of
+this beautiful double are of the third and seventh magnitude, the
+primary orange, the secondary sea-green. The distance separating the
+components is about 3 seconds, perhaps more; it appears to have been
+slowly increasing during the past ten or twelve years. Smyth assigns to
+this system a period of revolution of 980 years, but there can be little
+doubt that the true period is largely in excess of this estimate.
+Observers in southern latitudes consider that the colours of the
+components are yellow and blue, not orange and green as most of our
+northern observers have estimated them.
+
+A little beyond the lower left-hand corner of the map is the star
+[delta] Serpentis, in the position shown in the Frontispiece, Map 3.
+This is the star which at the hour and season depicted in Map 2 formed
+the uppermost of a vertical row of stars, which has now assumed an
+almost horizontal position. The components of [delta] Serpentis are
+about 3-1/2 seconds apart, their magnitudes 3 and 5, both white.
+
+The stars [theta]^{1} and [theta]^{2} Serpentis form a wide double, the
+distance between the components being 21-1/2 seconds. They are nearly
+equal in magnitude, the primary being 4-1/2, the secondary 5. Both are
+yellow, the primary being of a paler yellow colour than the smaller
+star. But the observer may not know where to look for [theta] Serpentis,
+since it falls in a part of the constellation quite separated from that
+part in which [delta] Serpentis lies. In fact [theta] lies on the
+extreme easterly verge of the eastern half of the constellation. It is
+to be looked for at about the same elevation as the brilliant Altair,
+and (as to azimuth) about midway between Altair and the south.
+
+The stars [alpha]^{1} and [alpha]^{2} Librae form a wide double, perhaps
+just separable by the naked eye in very favourable weather. The larger
+component is of the third, the smaller of the sixth magnitude, the
+former yellow the latter light grey.
+
+The star [beta] Librae is a beautiful light-green star to the naked eye;
+in the telescope a wide double, pale emerald and light blue.
+
+In Scorpio there are several very beautiful objects:--
+
+The star Antares or Cor Scorpionis is one of the most beautiful of the
+red stars. It has been termed the Sirius of red stars, a term better
+merited perhaps by Aldebaran, save for this that, in our latitude,
+Antares is, like Sirius, always seen as a brilliant "scintillator"
+(because always low down), whereas Aldebaran rises high above the
+horizon. Antares is a double star, its companion being a minute green
+star. In southern latitudes the companion of Antares may be seen with a
+good 4-inch, but in our latitudes a larger opening is wanted. Mr. Dawes
+once saw the companion of Antares shining alone for seven seconds, the
+primary being hidden by the moon. He found that the colour of the
+secondary is not merely the effect of contrast, but that this small star
+is really a green sun.
+
+The star [beta] Scorpionis is a fine double, the components 13".1 apart,
+their magnitudes 2 and 5-1/2, colours white and lilac. It has been
+supposed that this pair is only an optical double, but a long time must
+elapse before a decisive opinion can be pronounced on such a point.
+
+The star [sigma] Scorpionis is a wider but much more difficult double,
+the smaller component being below the 9th magnitude. The colour of the
+primary (4) is white, that of the secondary maroon.
+
+The star [xi] Scorpionis is a neat double, the components 7".2 apart,
+their magnitudes 4-1/2 and 7-1/2, their colours white and grey. This
+star is really triple, a fifth-magnitude star lying close to the
+primary.
+
+In Ophiuchus, a constellation covering a wide space immediately above
+Scorpio, there are several fine doubles. Among others--
+
+39 Ophiuchi, distance between components 12".1, their magnitudes 5-1/2
+and 7-1/2, their colours orange and blue.
+
+The star 70 Ophiuchi, a fourth-magnitude star on the right shoulder of
+Ophiuchus, is a noted double. The distance between the components about
+5-1/2", their magnitudes 4-1/2 and 7, the colours yellow and red. The
+pair form a system whose period of revolution is about 95 years.
+
+36 Ophiuchi (variable), distance 5".2, magnitudes 4-1/2 and 6-1/2,
+colours red and yellow.
+
+[rho] Opiuchi, distance 4", colours yellow and blue, magnitudes 5 and 7.
+
+Between [alpha] and [beta] Scorpionis the fine nebula 80 M may be looked
+for. (Or more closely thus:--below [beta] is the wide Double [omega]^{1}
+and [omega]^{2} Scorpionis; about as far to the right of Antares is the
+star [sigma] Scorpionis, and immediately above this is the
+fifth-magnitude star 19.) The nebula we seek lies between 19 and
+[omega], nearer to 19 (about two-fifths of the way towards [omega]).
+This nebula is described by Sir W. Herschel as "the richest and most
+condensed mass of stars which the firmament offers to the contemplation
+of astronomers."
+
+There are two other objects conveniently situated for observation, which
+the observer may now turn to. The first is the great cluster in the
+sword-hand of Perseus (see Plate 4), now lying about 28 deg. above the
+horizon between N.E. and N.N.E. The stars [gamma] and [delta] Cassiopeiae
+(see Map 3 of Frontispiece) point towards this cluster, which is rather
+farther from [delta] than [delta] from [gamma], and a little south of
+the produced line from these stars. The cluster is well seen with the
+naked eye, even in nearly full moonlight. In a telescope of moderate
+power this cluster is a magnificent object, and no telescope has yet
+revealed its full glory. The view in Plate 5 gives but the faintest
+conception of the glories of [chi] Persei. Sir W. Herschel tried in
+vain to gauge the depths of this cluster with his most powerful
+telescope. He spoke of the most distant parts as sending light to us
+which must have started 4000 or 5000 years ago. But it appears
+improbable that the cluster has in reality so enormous a longitudinal
+extension compared with its transverse section as this view would imply.
+On the contrary, I think we may gather from the appearance of this
+cluster, that stars are far less uniform in size than has been commonly
+supposed, and that the mere irresolvability of a cluster is no proof of
+excessive distance. It is unlikely that the faintest component of the
+cluster is farther off than the brightest (a seventh-magnitude star) in
+the proportion of more than about 20 to 19, while the ordinary estimate
+of star magnitudes, applied by Herschel, gave a proportion of 20 or 30
+to 1 at least. I can no more believe that the components of this cluster
+are stars greatly varying in distance, but accidentally seen in nearly
+the same direction, (or that they form an _enormously long system_
+turned by accident directly towards the earth), than I could look on the
+association of several thousand persons in the form of a procession as a
+fortuitous arrangement.
+
+Next there is the great nebula in Andromeda--known as "the
+transcendantly beautiful queen of the nebulae." It will not be difficult
+to find this object. The stars [epsilon] and [delta] Cassiopeiae (Map 3,
+Frontispiece) point to the star [beta] Andromedae. Almost in a vertical
+line above this star are two fourth-magnitude stars [mu] and [gamma],
+and close above [nu], a little to the right, is the object we
+seek--visible to the naked eye as a faint misty spot. To tell the truth,
+the transcendantly beautiful queen of the nebulae is rather a
+disappointing object in an ordinary telescope. There is seen a long
+oval or lenticular spot of light, very bright near the centre,
+especially with low powers. But there is a want of the interest
+attaching to the strange figure of the Great Orion nebula. The Andromeda
+nebula has been partially resolved by Lord Rosse's great reflector, and
+(it is said) more satisfactorily by the great refractor of Harvard
+College. In the spectroscope, Mr. Huggins informs us, the spectrum is
+peculiar. Continuous from the blue to the orange, the light there
+"appears to cease very abruptly;" there is no indication of gaseity.
+
+Lastly, the observer may turn to the pair Mizar and Alcor, the former
+the middle star in the Great Bear's tail, the latter 15' off. It seems
+quite clear, by the way, that Alcor has increased in brilliancy of late,
+since among the Arabians it was considered an evidence of very good
+eyesight to detect Alcor, whereas this star may now be easily seen even
+in nearly full moonlight. Mizar is a double star, and a fourth star is
+seen in the same field of view with the others (see Plate 5). The
+distance between Mizar and its companion is 14".4; the magnitude of
+Mizar 3, of the companion 5; their colours white and pale green,
+respectively.
+
+
+
+
+CHAPTER V.
+
+A HALF-HOUR WITH ANDROMEDA, CYGNUS, ETC.
+
+
+Our last half-hour with the double stars, &c., must be a short one, as
+we have already nearly filled the space allotted to these objects. The
+observations now to be made are supposed to take place during the fourth
+quarter of the year,--at ten o'clock on October 23rd; or at nine on
+November 7th; or at eight on November 22nd; or at seven on December 6th;
+or at hours intermediate to these on intermediate days.
+
+We look first, as in former cases, for the Great Bear, now lying low
+down towards the north. Towards the north-east, a few degrees easterly,
+are the twin-stars Castor and Pollux, in a vertical position, Castor
+uppermost. Above these, a little towards the right, we see the brilliant
+Capella; and between Capella and the zenith is seen the festoon of
+Perseus. Cassiopeia lies near the zenith, towards the north, and the
+Milky Way extends from the eastern horizon across the zenith to the
+western horizon. Low down in the east is Orion, half risen above
+horizon. Turning to the south, we see high up above the horizon the
+square of Pegasus. Low down towards the south-south-west is Fomalhaut,
+pointed to by [beta] and [alpha] Pegasi. Towards the west, about
+half-way between the zenith and the horizon, is the noble cross in
+Cygnus; below which, towards the left, we see Altair, and his companions
+[beta] and [gamma] Aquilae: while towards the right we see the brilliant
+Vega.
+
+During this half-hour we shall not confine ourselves to any particular
+region of the heavens, but sweep the most conveniently situated
+constellations.
+
+[Illustration: PLATE V.]
+
+First, however, we should recommend the observer to try and get a good
+view of the great nebula in Andromeda, which is _not_ conveniently
+situated for observation, but is so high that after a little trouble the
+observer may expect a more distinct view than in the previous quarter.
+He will see [beta] Andromedae towards the south-east, about 18 deg. from the
+zenith, [mu] and [nu] nearly in a line towards the zenith, and the
+nebula about half-way between [beta] and the zenith.
+
+With a similar object it will be well to take another view of the great
+cluster in Perseus, about 18 deg. from the zenith towards the
+east-north-east (_see_ the pointers [gamma] and [delta] Cassiopeiae in
+Map 4, Frontispiece), the cluster being between [delta] Cassiopeiae and
+[alpha] Persei.
+
+Not very far off is the wonderful variable Algol, now due east, and
+about 58 deg. above the horizon. The variability of this celebrated object
+was doubtless discovered in very ancient times, since the name Al-gol,
+or "the Demon" seems to point to a knowledge of the peculiarity of this
+"slowly winking eye." To Goodricke, however, is due the rediscovery of
+Algol's variability. The period of variation is 2d. 20h. 48m.; during
+2h. 14m. Algol appears of the second magnitude; the remaining 6-3/4
+hours are occupied by the gradual decline of the star to the fourth
+magnitude, and its equally gradual return to the second. It will be
+found easy to watch the variations of this singular object, though, of
+course, many of the minima are attained in the daytime. The following
+may help the observer:--
+
+On October 8th, 1867, at about half-past eleven in the evening, I
+noticed that Algol had reached its minimum of brilliancy. Hence the next
+minimum was attained at about a quarter-past eight on the evening of
+October 11th; the next at about five on the evening of October 14th,
+and so on. Now, if this process be carried on, it will be found that the
+next evening minimum occurred at about 10h. (_circiter_) on the evening
+of October 31st, the next at about 11h. 30m. on the evening of November
+20th. Thus at whatever hour any minimum occurs, another occurs _six
+weeks and a day later_, at about the same hour. This would be exact
+enough if the period of variation were _exactly_ 2d. 20m. 48s., but the
+period is nearly a minute greater, and as there are fifteen periods in
+six weeks and a day, it results that there is a difference of about 13m.
+in the time at which the successive recurrences at nearly the same hour
+take place. Hence we are able to draw up the two following Tables, which
+will suffice to give all the minima conveniently observable during the
+next two years. Starting from a minimum at about 11h. 45m. on November
+20th, 1867, and noticing that the next 43-day period (with the 13m.
+added) gives us an observation at midnight on January 2nd, 1868, and
+that successive periods would make the hour later yet, we take the
+minimum next after that of January 2nd, viz. that of January 5th, 1868,
+8h. 48m., and taking 43-day periods (with 13m. added to each), we get
+the series--
+
+                 h.  m.
+Jan.   5, 1868,   8  45 P.M.
+Feb.  17, ----,   8  58 ----
+Mar.  31, ----,   9  11 ----
+May   13, ----,   9  24 ----
+June  25, ----,   9  37 ----
+Aug.   7, ----,   9  50 ----
+Sept. 19, ----,  10   3 ----
+Nov.   1  ----,  10  16 ----
+Dec.  14, ----,  10  29 ----
+Jan.  26, 1869,  10  42 ----
+Mar.  10, ----,  10  25 ----
+Mar.  13, ----,   7  43 ----[7]
+Apr.  25, ----,   7  56 ----
+June   7, ----,   8   9 ----
+July  20, ----,   8  22 ----
+Sept.  1, ----,   8  35 ----
+Oct.  14, ----,   8  48 ----
+Nov.  26, ----,   9   1 ----
+Jan.   8, 1870,   9  14 ----
+Feb.  20, ----,   9  27 ----
+
+From the minimum at about 10 P.M. on October 31st, 1867, we get in like
+manner the series--
+
+                 h.  m.
+Dec.  13, 1867,  10  13 P.M.
+Jan.  25, 1868,  10  26 ----
+Mar.   8, ----,  10  39 ----
+Apr.  20, ----,  10  52 ----
+June   2, ----,  11   5 ----
+June   5, ----,   7  53 ----[8]
+July  18, ----,   8   6 ----
+Aug.  30, ----,   8  19 ----
+Oct.  12, ----,   8  32 ----
+Nov.  24, ----,   8  45 ----
+Jan.   6, 1869,   8  58 ----
+Feb.  18, ----,   9  11 ----
+Apr.   2, ----,   9  24 ----
+May   15, ----,   9  37 ----
+June  27, ----,   9  50 ----
+Aug.   9, ----,  10   3 ----
+Sept. 21, ----,  10  16 ----
+Nov.   3, ----,  10  29 ----
+Dec.  16, ----,  10  42 ----
+Jan.  28, 1870,  10  55 ----
+
+From one or other of these tables every observable minimum can be
+obtained. Thus, suppose the observer wants to look for a minimum during
+the last fortnight in August, 1868. The first table gives him no
+information, the latter gives him a minimum at 8h. 19m. P.M. on August
+30; hence of course there is a minimum at 11h. 31m. P.M. on August 27;
+and there are no other conveniently observable minima during the
+fortnight in question.
+
+The cause of the remarkable variation in this star's brilliancy has been
+assigned by some astronomers to the presence of an opaque secondary,
+which transits Algol at regular intervals; others have adopted the view
+that Algol is a luminous secondary, revolving around an opaque primary.
+Of these views the former seems the most natural and satisfactory. It
+points to a secondary whose mass bears a far greater proportion to that
+of the primary, than the mass even of Jupiter bears to the sun; the
+shortness of the period is also remarkable. It may be noticed that
+observation points to a gradual diminution in the period of Algol's
+variation, and the diminution seems to be proceeding more and more
+rapidly. Hence (assuming the existence of a dark secondary) we must
+suppose that either it travels in a resisting medium which is gradually
+destroying its motion, or that there are other dependent orbs whose
+attractions affect the period of this secondary. In the latter case the
+decrease in the period will attain a limit and be followed by an
+increase.
+
+However, interesting as the subject may be, it is a digression from
+telescopic work, to which we now return.
+
+Within the confines of the second map in Plate 4 is seen the fine star
+[gamma] Andromedae. At the hour of our observations it lies high up
+towards E.S.E. It is seen as a double star with very moderate telescopic
+power, the distance between the components being upwards of 10"; their
+magnitudes 3 and 5-1/2, their colours orange and green. Perhaps there is
+no more interesting double visible with low powers. The smaller star is
+again double in first-class telescopes, the components being yellow and
+blue according to some observers, but according to others, both green.
+
+Below [gamma] Andromedae lie the stars [beta] and [gamma] Triangulorum,
+[gamma] a fine naked-eye triple (the companions being [delta] and [eta]
+Triangulorum), a fine object with a very low power. To the right is
+[alpha] Triangulorum, certainly less brilliant than [beta]. Below
+[alpha] are the three stars [alpha], [beta], and [gamma] Arietis, the
+first an unequal and difficult double, the companion being purple, and
+only just visible (under favourable circumstances) with a good 3-inch
+telescope; the last an easy double, interesting as being the first ever
+discovered (by Hook, in 1664), the colours of components white and grey.
+
+Immediately below [alpha] Arietis is the star [alpha] Ceti, towards the
+right of which (a little lower) is Mira, a wonderful variable. This star
+has a period of 331-1/3 days; during a fortnight it appears as a star of
+the 2nd magnitude,--on each side of this fortnight there is a period of
+three months during one of which the star is increasing, while during
+the other it is diminishing in brightness: during the remaining five
+months of the period the star is invisible to the naked eye. There are
+many peculiarities and changes in the variation of this star, into which
+space will not permit me to enter.
+
+Immediately above Mira is the star [alpha] Piscium at the knot of the
+Fishes' connecting band. This is a fine double, the distance between the
+components being about 3-1/2", their magnitudes 5 and 6, their colours
+pale green and blue (see Plate 5).
+
+Close to [gamma] Aquarii (see Frontispiece, Map 4), above and to the
+left of it, is the interesting double [zeta] Aquarii; the distance
+between the components is about 3-1/2", their magnitudes 4 and 4-1/2,
+both whitish yellow. The period of this binary seems to be about 750
+years.
+
+Turning next towards the south-west we see the second-magnitude star
+[epsilon] Pegasi, some 40 deg. above the horizon. This star is a wide but
+not easy double, the secondary being only of the ninth magnitude; its
+colour is lilac, that of the primary being yellow.
+
+Towards the right of [epsilon] Pegasi and lower down are seen the three
+fourth-magnitude stars which mark the constellation Equuleus. Of these
+the lowest is [alpha], to the right of which lies [epsilon] Equulei, a
+fifth-magnitude star, really triple, but seen as a double star with
+ordinary telescopes (Plate 5). The distance between the components is
+nearly 11", their colours white and blue, their magnitudes 5-1/2 and
+7-1/2. The primary is a very close double, which appears, however, to be
+opening out rather rapidly.
+
+Immediately below Equuleus are the stars [alpha]^{1} and [alpha]^2
+Capricorni, seen as a naked-eye double to the right of and above [beta].
+Both [alpha]^1 and [alpha]^2 are yellow; [alpha]^2 is of the 3rd,
+[alpha]^1 of the 4th magnitude; in a good telescope five stars are seen,
+the other three being blue, ash-coloured, and lilac. The star [beta]
+Capricorni is also a wide double, the components yellow and blue, with
+many telescopic companions.
+
+To the right of Equuleus, towards the west-south-west is the
+constellation Delphinus. The upper left-hand star of the rhombus of
+stars forming the head of the Delphinus is the star [gamma] Delphini, a
+rather easy double (see Plate 5), the components being nearly 12" apart,
+their magnitudes 4 and 7, their colours golden yellow and flushed grey.
+
+Turn we next to the charming double Albireo, on the beak of Cygnus,
+about 36 deg. above the horizon towards the west. The components are 34-1/2"
+apart, their magnitudes 3 and 6, their colours orange-yellow, and blue.
+It has been supposed (perhaps on insufficient evidence) that this star
+is merely an optical double. It must always be remembered that a certain
+proportion of stars (amongst those separated by so considerable a
+distance) _must_ be optically combined only.
+
+The star [chi] Cygni is a wide double (variable) star. The components
+are separated by nearly 26", their magnitudes 5 and 9, their colours
+yellow and light blue. [chi] may be found by noticing that there is a
+cluster of small stars in the middle of the triangle formed by the stars
+[gamma], [delta], and [beta] Cygni (see Map 4, Frontispiece), and that
+[chi] is the nearest star _of the cluster_ to [beta]. The star [phi]
+Cygni, which is just above and very close to [beta] (Albireo), does not
+belong to the cluster. [chi] is about half as far again from [phi] as
+[phi] from Albireo. But as [chi] descends to the 11th magnitude at its
+minimum the observer must not always expect to find it very easily. It
+has been known to be invisible at the epoch when it should have been
+most conspicuous. The period of this variable is 406 days.
+
+The star 61 Cygni is an interesting one. So far as observation has yet
+extended, it would seem to be the nearest to us of all stars visible in
+the northern hemisphere. It is a fine double, the components nearly
+equal (5-1/2 and 6), both yellow, and nearly 19" apart. The period of
+this binary appears to be about 540 years. To find 61 Cygni note that
+[epsilon] and [delta] Cygni form the diameter of a semicircle divided
+into two quadrants by [alpha] Cygni (Arided). On this semicircle, on
+either side of [alpha], lie the stars [nu] and [alpha] Cygni, [nu]
+towards [epsilon]. Now a line from [alpha] to [nu] produced passes very
+near to 61 Cygni at a distance from [nu] somewhat greater than half the
+distance of [nu] from [alpha].
+
+The star [mu] Cygni lies in a corner of the constellation, rather
+farther from [zeta] than [zeta] from [epsilon] Cygni. A line from
+[epsilon] to [zeta] produced meets [kappa] Pegasi, a fourth-magnitude
+star; and [mu] Cygni, a fifth-magnitude star, lies close above [kappa]
+Pegasi. The distance between the components is about 5-1/2", their
+magnitudes 5 and 6, their colours white and pale blue.
+
+The star [psi] Cygni may next be looked for, but for this a good map of
+Cygnus will be wanted, as [psi] is not pointed to by any well-marked
+stars. A line from [alpha], parallel to the line joining [gamma] and
+[delta], and about one-third longer than that line, would about mark the
+position of [psi] Cygni. The distance between the components of this
+double is about 3-1/2", their magnitudes 5-1/2 and 8, their colours
+white and lilac.
+
+Lastly, the observer may turn to the stars [gamma]_{1} and [gamma]_{2}
+Draconis towards the north-west about 40 deg. above the horizon (they are
+included in the second map of Plate 2). They form a wide double, having
+equal (fifth-magnitude) components, both grey. (See Plate 5.)
+
+
+
+
+CHAPTER VI.
+
+HALF-HOURS WITH THE PLANETS.
+
+
+In observing the stars, we can select a part of the heavens which may be
+conveniently observed; and in this way in the course of a year we can
+observe every part of the heavens visible in our northern hemisphere.
+But with the planets the case is not quite so simple. They come into
+view at no fixed season of the year: some of them can never be seen _by
+night_ on the meridian; and they all shift their place among the stars,
+so that we require some method of determining where to look for them on
+any particular night, and of recognising them from neighbouring fixed
+stars.
+
+The regular observer will of course make use of the 'Nautical Almanac';
+but 'Dietrichsen and Hannay's Almanac' will serve every purpose of the
+amateur telescopist. I will briefly describe those parts of the almanac
+which are useful to the observer.
+
+It will be found that three pages are assigned to each month, each page
+giving different information. If we call these pages I. II. III., then
+in order that page I. for each month may fall to the left of the open
+double page, and also that I. and II. may be open together, the pages
+are arranged in the following order: I. II. III.; III. I. II.; I. II.
+III.; and so on.
+
+Now page III. for any month does not concern the amateur observer. It
+gives information concerning the moon's motions, which is valuable to
+the sailor, and interesting to the student of astronomy, but not
+applicable to amateur observation.
+
+[Illustration: PLATE VI.]
+
+We have then only pages I. and II. to consider:--
+
+Across the top of both pages the right ascension and declination of the
+planets Venus, Jupiter, Mars, Saturn, Mercury, and Uranus are given,
+accompanied by those of two conspicuous stars. This information is very
+valuable to the telescopist. In the first place, as we shall presently
+see, it shows him what planets are well situated for observation, and
+secondly it enables him to map down the path of any planet from day to
+day among the fixed stars. This is a very useful exercise, by the way,
+and also a very instructive one. The student may either make use of the
+regular maps and mark down the planet's path in pencil, taking a light
+curve through the points given by the data in his almanac, or he may lay
+down a set of meridians suited to the part of the heavens traversed by
+the planet, and then proceed to mark in the planet's path and the stars,
+taking the latter either from his maps or from a convenient list of
+stars.[9] My 'Handbook of the Stars' has been constructed to aid the
+student in these processes. It must be noticed that old maps are not
+suited for the work, because, through precession, the stars are all out
+of place as respects R.A. and Dec. Even the Society's maps, constructed
+so as to be right for 1830, are beginning to be out of date. But a
+matter of 20 or 30 years either way is not important.[10] My Maps,
+Handbook and Zodiac-chart have been constructed for the year 1880, so as
+to be serviceable for the next fifty years or so.
+
+Next, below the table of the planets, we have a set of vertical
+columns. These are, in order, the days of the month, the calendar--in
+which are included some astronomical notices, amongst others the
+diameter of Saturn on different dates, the hours at which the sun rises
+and sets, the sun's right ascension, declination, diameter, and
+longitude; then eight columns which do not concern the observer; after
+which come the hours at which the moon rises and sets, the moon's age;
+and lastly (so far as the observer is concerned) an important column
+about Jupiter's system of satellites.
+
+Next, we have, at the foot of the first page, the hours at which the
+planets rise, south, and set; and at the foot of the second page we have
+the dates of conjunctions, oppositions, and of other phenomena, the
+diameters of Venus, Jupiter, Mars, and Mercury, and finally a few words
+respecting the visibility of these four planets.
+
+After the thirty-six pages assigned to the months follow four (pp.
+42-46) in which much important astronomical information is contained;
+but the points which most concern our observer are (i.) a small table
+showing the appearance of Saturn's rings, and (ii.) a table giving the
+hours at which Jupiter's satellites are occulted or eclipsed, re-appear,
+&c.
+
+We will now take the planets in the order of their distance from the
+sun: we shall see that the information given by the almanac is very
+important to the observer.
+
+Mercury is so close to the sun as to be rarely seen with the naked eye,
+since he never sets much more than two hours and a few minutes after the
+sun, or rises by more than that interval before the sun. It must not be
+supposed that at each successive epoch of most favourable appearance
+Mercury sets so long after the sun or rises so long before him. It would
+occupy too much of our space to enter into the circumstances which
+affect the length of these intervals. The question, in fact, is not a
+very simple one. All the necessary information is given in the almanac.
+We merely notice that the planet is most favourably seen as an evening
+star in spring, and as a morning star in autumn.[11]
+
+The observer with an equatorial has of course no difficulty in finding
+Mercury, since he can at once direct his telescope to the proper point
+of the heavens. But the observer with an alt-azimuth might fail for
+years together in obtaining a sight of this interesting planet, if he
+trusted to unaided naked-eye observations in looking for him. Copernicus
+never saw Mercury, though he often looked for him; and Mr. Hind tells me
+he has seen the planet but once with the naked eye--though this perhaps
+is not a very remarkable circumstance, since the systematic worker in an
+observatory seldom has occasion to observe objects with the unaided eye.
+
+By the following method the observer can easily pick up the planet.
+
+Across two uprights (Fig. 10) nail a straight rod, so that when looked
+at from some fixed point of view the rod may correspond to the sun's
+path near the time of observation. The rod should be at right-angles to
+the line of sight to its centre. Fasten another rod at right angles to
+the first. From the point at which the rods cross measure off and mark
+on both rods spaces each subtending a degree as seen from the point of
+view. Thus, if the point of view is 9-1/2 feet off, these spaces must
+each be 2 inches long, and they must be proportionately less or greater
+as the eye is nearer or farther.
+
+[Illustration: _Fig. 10._]
+
+Now suppose the observer wishes to view Mercury on some day, whereon
+Mercury is an evening star. Take, for instance, June 9th, 1868. We find
+from 'Dietrichsen' that on this day (at noon) Mercury's R.A. is 6h. 53m.
+23s.: and the sun's 5h. 11m. 31s. We need not trouble ourselves about
+the odd hours after noon, and thus we have Mercury's R.A. greater than
+the sun's by 1h. 41m. 52s. Now we will suppose that the observer has so
+fixed his uprights and the two rods, that the sun, seen from the fixed
+point of view, appears to pass the point of crossing of the rods at
+half-past seven, then Mercury will pass the cross-rod at 11m. 52s. past
+nine. But where? To learn this we must take out Mercury's declination,
+which is 24 deg. 43' 18" N., and the sun's, which is 22 deg. 59' 10" N. The
+difference, 1 deg. 44' 8" N. gives us Mercury's place, which it appears is
+rather less than 1-3/4 degree north of the sun. Thus, about 1h. 42m.
+after the sun has passed the cross-rod, Mercury will pass it between the
+first and second divisions above the point of fastening. The sun will
+have set about an hour, and Mercury will be easily found when the
+telescope is directed towards the place indicated.
+
+It will be noticed that this method does not require the time to be
+exactly known. All we have to do is to note the moment at which the sun
+passes the point of fastening of the two rods, and to take our 1h. 42m.
+from that moment.
+
+This method, it may be noticed in passing, may be applied to give
+naked-eye observations of Mercury at proper seasons (given in the
+almanac). By a little ingenuity it may be applied as well to morning as
+to evening observations, the sun's passage of the cross-rod being taken
+on one morning and Mercury's on the next, so many minutes _before_ the
+hour of the first observation. In this way several views of Mercury may
+be obtained during the year.
+
+Such methods may appear very insignificant to the systematic observer
+with the equatorial, but that they are effective I can assert from my
+own experience. Similar methods may be applied to determine from the
+position of a known object, that of any neighbouring unknown object even
+at night. The cross-rod must be shifted (or else two cross-rods used)
+when the unknown _precedes_ the known object. If two cross-rods are
+used, account must be taken of the gradual diminution in the length of a
+degree of right ascension as we leave the equator.
+
+Even simpler methods carefully applied may serve to give a view of
+Mercury. To show this, I may describe how I obtained my first view of
+this planet. On June 1st, 1863, I noticed, that at five minutes past
+seven the sun, as seen from my study window, appeared from behind the
+gable-end of Mr. St. Aubyn's house at Stoke, Devon. I estimated the
+effect of Mercury's northerly declination (different of course for a
+vertical wall, than for the cross-rod in fig. 8, which, in fact, agrees
+with a declination-circle), and found that he would pass out opposite a
+particular point of the wall a certain time after the sun. I then turned
+the telescope towards that point, and focussed for distinct vision of
+distant objects, so that the outline of the house was seen out of focus.
+As the calculated time of apparition approached, I moved the telescope
+up and down so that the field swept the neighbourhood of the estimated
+point of apparition. I need hardly say that Mercury did not appear
+exactly at the assigned point, nor did I see him make his first
+appearance; but I picked him up so soon after emergence that the outline
+of the house was in the field of view with him. He appeared as a
+half-disc. I followed him with the telescope until the sun had set, and
+soon after I was able to see him very distinctly with the naked eye. He
+shone with a peculiar brilliance on the still bright sky; but although
+perfectly distinct to the view when his place was indicated, he escaped
+detection by the undirected eye.[12]
+
+Mercury does not present any features of great interest in ordinary
+telescopes; though he usually appears better defined than Venus, at
+least as the latter is seen on a dark sky. The phases are pleasingly
+seen (as shown in Plate 6) with a telescope of moderate power. For their
+proper observation, however, the planet must be looked for with the
+telescope in the manner above indicated, as he always shows a nearly
+semi-circular disc when he is visible to the naked eye.
+
+We come next to Venus, the most splendid of all the planets to the eye.
+In the telescope Venus disappoints the observer, however. Her intense
+lustre brings out every defect of the instrument, and especially the
+chromatic aberration. A dark glass often improves the view, but not
+always. Besides, an interposed glass has an unpleasant effect on the
+field of view.
+
+Perhaps the best method of observing Venus is to search for her when she
+is still high above the horizon, and when therefore the background of
+the sky is bright enough to take off the planet's glare. The method I
+have described for the observation of Mercury will prove very useful in
+the search for Venus when the sun is above the horizon or but just set.
+Of course, when an object is to be looked for high above the horizon,
+the two rods which support the cross-rods must not be upright, but
+square to the line of view to that part of the sky.
+
+But the observer must not expect to see much during his observation of
+Venus. In fact, he can scarcely do more than note her varying phases
+(see Plate 6) and the somewhat uneven boundary of the terminator. Our
+leading observers have done so little with this fascinating but
+disappointing planet, that amateurs must not be surprised at their own
+failure.
+
+I suppose the question whether Venus has a satellite, or at any rate
+whether the object supposed to have been seen by Cassini and other old
+observers were a satellite, must be considered as decided in the
+negative. That Cassini should have seen an object which Dawes and Webb
+have failed to see must be considered utterly improbable.
+
+Leaving the inferior planets, we come to a series of important and
+interesting objects.
+
+First we have the planet Mars, nearly the last in the scale of planetary
+magnitude, but far from being the least interesting of the planets. It
+is in fact quite certain that we obtain a better view of Mars than of
+any object in the heavens, save the Moon alone. He may present a less
+distinguished appearance than Jupiter or Saturn, but we see his surface
+on a larger scale than that of either of those giant orbs, even if we
+assume that we ever obtain a fair view of their real surface.
+
+Nor need the moderately armed observer despair of obtaining interesting
+views of Mars. The telescope with which Beer and Maedler made their
+celebrated series of views was only a 4-inch one, so that with a 3-inch
+or even a 2-inch aperture the attentive observer may expect interesting
+views. In fact, more depends on the observer than on the instrument. A
+patient and attentive scrutiny will reveal features which at the first
+view wholly escape notice.
+
+In Plate 6 I have given a series of views of Mars much more distinct
+than an observer may expect to obtain with moderate powers. I add a
+chart of Mars, a miniature of one I have prepared from a charming
+series of tracings supplied me by Mr. Dawes. The views taken by this
+celebrated observer in 1852, 1856, 1860, 1862, and 1864, are far better
+than any others I have seen. The views by Beer and Maedler are good, as
+are some of Secchi's (though they appear badly drawn), Nasmyth's and
+Phillips'; Delarue's two views are also admirable; and Lockyer has given
+a better set of views than any of the others. But there is an amount of
+detail in Mr. Dawes' views which renders them superior to any yet taken.
+I must confess I failed at a first view to see the full value of Mr.
+Dawes' tracings. Faint marks appeared, which I supposed to be merely
+intended to represent shadings scarcely seen. A more careful study
+shewed me that every mark is to be taken as the representative of what
+Mr. Dawes actually saw. The consistency of the views is perfectly
+wonderful, when compared with the vagueness and inconsistency observable
+in nearly all other views. And this consistency is not shown by mere
+resemblance, which might have been an effect rather of memory
+(unconsciously exerted) than observation. The same feature changes so
+much in figure, as it appears on different parts of the disc, that it
+was sometimes only on a careful projection of different views that I
+could determine what certain features near the limb represented. But
+when this had been done, and the distortion through the effect of
+foreshortening corrected, the feature was found to be as true in shape
+as if it had been seen in the centre of the planet's disc.
+
+In examining Mr. Dawes' drawings it was necessary that the position of
+Mars' axis should be known. The data for determining this were taken
+from Dr. Oudemann's determinations given in a valuable paper on Mars
+issued from Mr. Bishop's observatory. But instead of calculating Mars'
+presentation by the formulae there given, I found it convenient rather to
+make use of geometrical constructions applied to my 'Charts of the
+Terrestrial Planets.' Taking Maedler's start-point for Martial
+longitudes, that is the longitude-line passing near Dawes' forked bay, I
+found that my results agreed pretty fairly with those in Prof. Phillips'
+map, so far as the latter went; but there are many details in my charts
+not found in Prof. Phillips' nor in Maedler's earlier charts.
+
+I have applied to the different features the names of those observers
+who have studied the physical peculiarities presented by Mars. Mr.
+Dawes' name naturally occurs more frequently than others. Indeed, if I
+had followed the rule of giving to each feature the name of its
+discoverer, Mr. Dawes' name would have occurred much more frequently
+than it actually does.
+
+On account of the eccentricity of his orbit, Mars is seen much better in
+some oppositions than in others. When best seen the southern hemisphere
+is brought more into view than the northern because the summer of his
+northern hemisphere occurs when he is nearly in aphelion (as is the case
+with the Earth by the way).
+
+The relative dimensions and presentation of Mars, as seen in opposition
+in perihelion, and in opposition in aphelion, are shown in the two rows
+of figures.
+
+In and near quadrature Mars is perceptibly gibbous. He is seen thus
+about two months before or after opposition. In the former case, he
+rises late and comes to the meridian six hours or so after midnight. In
+the latter case, he is well seen in the evening, coming to the meridian
+at six. His appearance and relative dimensions as he passes from
+opposition to quadrature are shown in the last three figures of the
+upper row.
+
+Mars' polar caps may be seen with very moderate powers.
+
+I add four sets of meridians (Plate 6), by filling in which from the
+charts the observer may obtain any number of views of the planet as it
+appears at different times.
+
+Passing over the asteroids, which are not very interesting objects to
+the amateur telescopist, we come to Jupiter, the giant of the solar
+system, surpassing our Earth more than 1400 times in volume, and
+overweighing all the planets taken together twice over.
+
+Jupiter is one of the easiest of all objects of telescopic observation.
+No one can mistake this orb when it shines on a dark sky, and only Venus
+can be mistaken for it when seen as a morning or evening star. Sometimes
+both are seen together on the twilight sky, and then Venus is generally
+the brighter. Seen, however, at her brightest and at her greatest
+elongation from the sun, her splendour scarcely exceeds that with which
+Jupiter shines when high above the southern horizon at midnight.
+
+Jupiter's satellites may be seen with very low powers; indeed the outer
+ones have been seen with the naked eye, and all are visible in a good
+opera-glass. Their dimensions relatively to the disc are shown in Plate
+7. Their greatest elongations are compared with the disc in the
+low-power view.
+
+Jupiter's belts may also be well seen with moderate telescopic power.
+The outer parts of his disc are perceptibly less bright than the centre.
+
+More difficult of observation are the transits of the satellites and of
+their shadows. Still the attentive observer can see the shadows with an
+aperture of two inches, and the satellites themselves with an aperture
+of three inches.
+
+The minute at which the satellites enter on the disc, or pass off, is
+given in 'Dietrichsen's Almanac.' The 'Nautical Almanac' also gives the
+corresponding data for the shadows.
+
+The eclipses of the satellites in Jupiter's shadow, and their
+occultations by his disc, are also given in 'Dietrichsen's Almanac.'
+
+In the inverting telescope the satellites move from right to left in the
+nearer parts of their orbit, and therefore transit Jupiter's disc in
+that direction, and from left to right in the farther parts. Also note
+that _before_ opposition, (i.) the shadows travel in front of the
+satellites in transiting the disc; (ii.) the satellites are eclipsed in
+Jupiter's _shadow_; (iii.) they reappear from behind his _disc_. On the
+other hand, _after_ opposition, (i.) the shadows travel _behind_ the
+satellites in transiting the disc; (ii.) the satellites are occulted by
+the _disc_; (iii.) they reappear from eclipse in Jupiter's _shadow_.
+
+Conjunctions of the satellites are common phenomena, and may be waited
+for by the observer who sees the chance. An eclipse of one satellite by
+the shadow of another is not a common phenomenon; in fact, I have never
+heard of such an eclipse being seen. That a satellite should be quite
+extinguished by another's shadow is a phenomenon not absolutely
+impossible, but which cannot happen save at long intervals.
+
+The shadows are not _black spots_ as is erroneously stated in nearly all
+popular works on astronomy. The shadow of the fourth, for instance, is
+nearly all penumbra, the really black part being quite minute by
+comparison. The shadow of the third has a considerable penumbra, and
+even that of the first is not wholly black. These penumbras may not be
+perceptible, but they affect the appearance of the shadows. For
+instance, the shadow of the fourth is perceptibly larger but less black
+than that of the third, though the third is the larger satellite.
+
+In transit the first satellite moves fastest, the fourth slowest, the
+others in their order. The shadow moves just as fast (appreciably) as
+the satellite it belongs to. Sometimes the shadow of the satellite may
+be seen to overtake (apparently) the disc of another. In such a case the
+shadow does not pass over the disc, but the disc conceals the shadow.
+This is explained by the fact that the shadow, if visible throughout its
+length, would be a line reaching slantwise from the satellite it belongs
+to, and the end of the shadow (that is, the point where it meets the
+disc) is _not_ the point where the shadow crosses the orbit of any inner
+satellite. Thus the latter may be interposed between the end of the
+shadow--the only part of the shadow really visible--and the eye; but the
+end of the shadow _cannot_ be interposed between the satellite and the
+eye. If a satellite _on the disc_ were eclipsed by another satellite,
+the black spot thus formed would be in another place from the black spot
+on the planet's body. I mention all this because, simple as the question
+may seem, I have known careful observers to make mistakes on this
+subject. A shadow is seen crossing the disc and overtaking, apparently,
+a satellite in transit. It seems therefore, on a first view, that the
+shadow will hide the satellite, and observers have even said that they
+have _seen_ this happen. But they are deceived. It is obvious that _if
+one satellite eclipse another, the shadows of both must occupy the same
+point on Jupiter's body_. Thus it is the overtaking of one _shadow_ by
+another on the disc, and not the overtaking of a _satellite_ by a
+shadow, which determines the occurrence of that as yet unrecorded
+phenomenon, the eclipse of one satellite by another.[13]
+
+The satellites when far from Jupiter seem to lie in a straight line
+through his centre. But as a matter of fact they do not in general lie
+in an exact straight line. If their orbits could be seen as lines of
+light, they would appear, in general, as very long ellipses. The orbit
+of the fourth would frequently be seen to be _quite clear_ of Jupiter's
+disc, and the orbit of the third might in some very exceptional
+instances pass _just_ clear of the disc. The satellites move most nearly
+in a straight line (apparently) when Jupiter comes to opposition in the
+beginning of February or August, and they appear to depart most from
+rectilinear motion when opposition occurs in the beginning of May and
+November. At these epochs the fourth satellite may be seen to pass above
+and below Jupiter's disc at a distance equal to about one-sixth of the
+disc's radius.
+
+The shadows do not travel in the same apparent paths as the satellites
+themselves across the disc, but (in an inverting telescope) _below_ from
+August to January, and _above_ from February to July.
+
+We come now to the most charming telescopic object in the heavens--the
+planet Saturn. Inferior only to Jupiter in mass and volume, this planet
+surpasses him in the magnificence of his system. Seen in a telescope of
+adequate power, Saturn is an object of surpassing loveliness. He must be
+an unimaginative man who can see Saturn for the first time in such a
+telescope, without a feeling of awe and amazement. If there is any
+object in the heavens--I except not even the Sun--calculated to impress
+one with a sense of the wisdom and omnipotence of the Creator it is
+this. "His fashioning hand" is indeed visible throughout space, but in
+Saturn's system it is most impressively manifest.
+
+Saturn, to be satisfactorily seen, requires a much more powerful
+telescope than Jupiter. A good 2-inch telescope will do much, however,
+in exhibiting his rings and belts. I have never seen him satisfactorily
+myself with such an aperture, but Mr. Grover has not only seen the
+above-named features, but even a penumbra to the shadow on the rings
+with a 2-inch telescope.
+
+Saturn revolving round the sun in a long period--nearly thirty
+years--presents slowly varying changes of appearance (see Plate 7). At
+one time the edge of his ring is turned nearly towards the earth; seven
+or eight years later his rings are as much open as they can ever be;
+then they gradually close up during a corresponding interval; open out
+again, exhibiting a different face; and finally close up as first seen.
+The last epoch of greatest opening occurred in 1856, the next occurs in
+1870: the last epoch of disappearance occurred in 1862-63, the next
+occurs in 1879. The successive views obtained are as in Plate 7 in order
+from right to left, then back to the right-hand figure (but sloped the
+other way); inverting the page we have this figure thus sloped, and the
+following changes are now indicated by the other figures in order back
+to the first (but sloped the other way and still inverted), thus
+returning to the right-hand figure as seen without inversion.
+
+The division in the ring can be seen in a good 2-inch aperture in
+favourable weather. The dark ring requires a good 4-inch and good
+weather.
+
+Saturn's satellites do not, like Jupiter's, form a system of nearly
+equal bodies. Titan, the sixth, is probably larger than any of
+Jupiter's satellites. The eighth also (Japetus) is a large body,
+probably at least equal to Jupiter's third satellite. But Rhea, Dione,
+and Tethys are much less conspicuous, and the other three cannot be seen
+without more powerful telescopes than those we are here dealing with.
+
+So far as my own experience goes, I consider that the five larger
+satellites may be seen distinctly in good weather with a good 3-1/2-inch
+aperture. I have never seen them with such an aperture, but I judge from
+the distinctness with which these satellites may be seen with a 4-inch
+aperture. Titan is generally to be looked for at a considerable distance
+from Saturn--_always_ when the ring is widely open. Japetus is to be
+looked for yet farther from the disc. In fact, when Saturn comes to
+opposition in perihelion (in winter only this can happen) Japetus may be
+as far from Saturn as one-third of the apparent diameter of the moon. I
+believe that under these circumstances, or even under less favourable
+circumstances, Japetus could be seen with a good opera-glass. So also
+might Titan.
+
+Transits, eclipses, and occulations of Saturn's satellites can only be
+seen when the ring is turned nearly edgewise towards the earth. For the
+orbits of the seven inner satellites lying nearly in the plane of the
+rings would (if visible throughout their extent) then only appear as
+straight lines, or as long ellipses cutting the planet's disc.
+
+The belts on Saturn are not very conspicuous. A good 3-1/2-inch is
+required (so far as my experience extends) to show them satisfactorily.
+
+The rings when turned edgewise either towards the earth or sun, are not
+visible in ordinary telescopes, neither can they be seen when the earth
+and sun are on opposite sides of the rings. In powerful telescopes the
+rings seem never entirely to disappear.
+
+The shadow of the planet on the rings may be well seen with a good
+2-inch telescope, which will also show Ball's division in the rings. The
+shadow of the rings on the planet is a somewhat more difficult feature.
+The shadow of the planet on the rings is best seen when the rings are
+well open and the planet is in or near quadrature. It is to be looked
+for to the left of the ball (in an inverting telescope) at quadrature
+preceding opposition, and to the right at quadrature following
+opposition. Saturn is more likely to be studied at the latter than at
+the former quadrature, as in quadrature preceding opposition he is a
+morning star. The shadow of the rings on the planet is best seen when
+the rings are but moderately open, and Saturn is in or near quadrature.
+When the shadow lies outside the rings it is best seen, as the dark ring
+takes off from the sharpness of the contrast when the shadow lies within
+the ring. It would take more space than I can spare here to show how it
+is to be determined (independently) whether the shadow lies within or
+without the ring. But the 'Nautical Almanac' gives the means of
+determining this point. When, in the table for assigning the appearance
+of the rings, _l_ is less than _l'_ the shadow lies outside the ring,
+when _l_ is greater than _l'_ the shadow lies within the ring.
+
+Uranus is just visible to the naked eye when he is in opposition, and
+his place accurately known. But he presents no phenomena of interest. I
+have seen him under powers which made his disc nearly equal to that of
+the moon, yet could see nothing but a faint bluish disc.
+
+Neptune also is easily found if his place be accurately noted on a map,
+and a good finder used. We have only to turn the telescope to a few
+stars seen in the finder nearly in the place marked in our map, and
+presently we shall recognise the one we want by the peculiarity of its
+light. What is the lowest power which will exhibit Neptune as a disc I
+do not know, but I am certain no observer can mistake him for a fixed
+star with a 2-inch aperture and a few minutes' patient scrutiny in
+favourable weather.
+
+[Illustration: PLATE VII.]
+
+
+
+
+CHAPTER VII.
+
+HALF-HOURS WITH THE SUN AND MOON.
+
+
+The moon perhaps is the easiest of all objects of telescopic
+observation. A very moderate telescope will show her most striking
+features, while each increase of power is repaid by a view of new
+details. Yet in one sense the moon is a disappointing object even to the
+possessor of a first-class instrument. For the most careful and
+persistent scrutiny, carried on for a long series of years, too often
+fails to reward the observer by any new discoveries of interest. Our
+observer must therefore rather be prepared to enjoy the observation of
+recognised features than expect to add by his labours to our knowledge
+of the earth's nearest neighbour.
+
+Although the moon is a pleasing and surprising telescopic object when
+full, the most interesting views of her features are obtained at other
+seasons. If we follow the moon as she waxes or wanes, we see the true
+nature of that rough and bleak mountain scenery, which when the moon is
+full is partially softened through the want of sharp contrasts of light
+and shadow. If we watch, even for half an hour only, the changing form
+of the ragged line separating light from darkness on the moon's disc, we
+cannot fail to be interested. "The outlying and isolated peak of some
+great mountain-chain becomes gradually larger, and is finally merged in
+the general luminous surface; great circular spaces, enclosed with rough
+and rocky walls many miles in diameter, become apparent; some with flat
+and perfectly smooth floors, variegated with streaks; others in which
+the flat floor is dotted with numerous pits or covered with broken
+fragments of rock. Occasionally a regularly-formed and unusually
+symmetrical circular formation makes its appearance; the exterior
+surface of the wall bristling with terraces rising gradually from the
+plain, the interior one much more steep, and instead of a flat floor,
+the inner space is concave or cup-shaped, with a solitary peak rising in
+the centre. Solitary peaks rise from the level plains and cast their
+long narrow shadows athwart the smooth surface. Vast plains of a dusky
+tint become visible, not perfectly level, but covered with ripples,
+pits, and projections. Circular wells, which have no surrounding wall
+dip below the plain, and are met with even in the interior of the
+circular mountains and on the tops of their walls. From some of the
+mountains great streams of a brilliant white radiate in all directions
+and can be traced for hundreds of miles. We see, again, great fissures,
+almost perfectly straight and of great length, although very narrow,
+which appear like the cracks in moist clayey soil when dried by the
+sun."[14]
+
+But interesting as these views may be, it was not for such discoveries
+as these that astronomers examined the surface of the moon. The
+examination of mere peculiarities of physical condition is, after all,
+but barren labour, if it lead to no discovery of physical variation. The
+principal charm of astronomy, as indeed of all observational science,
+lies in the study of change--of progress, development, and decay, and
+specially of systematic variations taking place in regularly-recurring
+cycles. And it is in this relation that the moon has been so
+disappointing an object of astronomical observation. For two centuries
+and a half her face has been scanned with the closest possible scrutiny;
+her features have been portrayed in elaborate maps; many an astronomer
+has given a large portion of his life to the work of examining craters,
+plains, mountains, and valleys, for the signs of change; but until
+lately no certain evidence--or rather, no evidence save of the most
+doubtful character--has been afforded that the moon is other than "a
+dead and useless waste of extinct volcanoes." Whether the examination of
+the remarkable spot called Linne--where lately signs were supposed to
+have been seen of a process of volcanic eruption--will prove an
+exception to this rule, remains to be seen. The evidence seems to me
+strongly to favour the supposition of a change of some sort having taken
+place in this neighbourhood.
+
+The sort of scrutiny required for the discovery of changes, or for the
+determination of their extent, is far too close and laborious to be
+attractive to the general observer. Yet the kind of observation which
+avails best for the purpose is perhaps also the most interesting which he
+can apply to the lunar details. The peculiarities presented by a spot upon
+the moon are to be observed from hour to hour (or from day to day,
+according to the size of the spot) as the sun's light gradually sweeps
+across it, until the spot is fully lighted; then as the moon wanes and the
+sun's light gradually passes from the spot, the series of observations is
+to be renewed. A comparison of them is likely--especially if the observer
+is a good artist and has executed several faithful delineations of the
+region under observation, to throw much light upon the real contour of the
+moon's surface at this point.
+
+In the two lunar views in Plate 7 some of the peculiarities I have
+described are illustrated. But the patient observer will easily be able
+to construct for himself a set of interesting views of different
+regions.
+
+It may be noticed that for observation of the waning moon there is no
+occasion to wait for those hours in which only the waning moon is
+visible _during the night_. Of course for the observation of a
+particular region under a particular illumination, the observer has no
+choice as to hour. But for generally interesting observations of the
+waning moon he can wait till morning and observe by daylight. The moon
+is, of course, very easily found by the unaided eye (in the day time)
+when not very near to the sun; and the methods described in Chapter V.
+will enable the observer to find the moon when she is so near to the sun
+as to present the narrowest possible sickle of light.
+
+One of the most interesting features of the moon, when she is observed
+with a good telescope, is the variety of colour presented by different
+parts of her surface. We see regions of the purest white--regions which
+one would be apt to speak of as _snow-covered_, if one could conceive
+the possibility that snow should have fallen where (now, at least) there
+is neither air nor water. Then there are the so-called seas, large grey
+or neutral-tinted regions, differing from the former not merely in
+colour and in tone, but in the photographic quality of the light they
+reflect towards the earth. Some of the seas exhibit a greenish tint, as
+the Sea of Serenity and the Sea of Humours. Where there is a central
+mountain within a circular depression, the surrounding plain is
+generally of a bluish steel-grey colour. There is a region called the
+Marsh of Sleep, which exhibits a pale red tint, a colour seen also near
+the Hyrcinian mountains, within a circumvallation called Lichtenburg.
+The brightest portion of the whole lunar disc is Aristarchus, the peaks
+of which shine often like stars, when the mountain is within the
+unillumined portion of the moon. The darkest regions are Grimaldi and
+Endymion and the great plain called Plato by modern astronomers--but, by
+Hevelius, the Greater Black Lake.
+
+The Sun.--Observation of the sun is perhaps on the whole the most
+interesting work to which the possessor of a moderately good telescope
+can apply his instrument. Those wonderful varieties in the appearance of
+the solar surface which have so long perplexed astronomers, not only
+supply in themselves interesting subjects of observation and
+examination, but gain an enhanced meaning from the consideration that
+they speak meaningly to us of the structure of an orb which is the
+source of light and heat enjoyed by a series of dependent worlds whereof
+our earth is--in size at least--a comparatively insignificant member.
+Swayed by the attraction of this giant globe, Jupiter and Saturn, Uranus
+and Neptune, as well as the four minor planets, and the host of
+asteroids, sweep continuously in their appointed orbits, in ever new but
+ever safe and orderly relations amongst each other. If the sun's light
+and heat were lost, all life and work among the denizens of these orbs
+would at once cease; if his attractive energy were destroyed, these orbs
+would cease to form a _system_.
+
+The sun may be observed conveniently in many ways, some more suited to
+the general observer who has not time or opportunity for systematic
+observation; others more instructive, though involving more of
+preparation and arrangement.
+
+The simplest method of observing the sun is to use the telescope in the
+ordinary manner, protecting the eye by means of dark-green or
+neutral-tinted glasses. Some of the most interesting views I have ever
+obtained of the sun, have resulted from the use of the ordinary
+terrestrial or erecting eye-piece, capped with a dark glass. The
+magnifying power of such an eye-piece is, in general, much lower than
+that available with astronomical eye-pieces. But vision is very pleasant
+and distinct when the sun is thus observed, and a patient scrutiny
+reveals almost every feature which the highest astronomical power
+applicable could exhibit. Then, owing to the greater number of
+intervening lenses, there is not the same necessity for great darkness
+or thickness in the coloured glass, so that the colours of the solar
+features are seen much more satisfactorily than when astronomical
+eye-pieces are employed.
+
+In using astronomical eye-pieces it is convenient to have a rotating
+wheel attached, by which darkening glasses of different power may be
+brought into use as the varying illumination may require.
+
+Those who wish to observe carefully and closely a minute portion of the
+solar disc, should employ Dawes' eye-piece: in this a metallic screen
+placed in the focus keeps away all light but such as passes through a
+minute hole in the diaphragm.
+
+Another convenient method of diminishing the light is to use a glass
+prism, light being partially reflected from one of the exterior
+surfaces, while the refracted portion is thrown out at another.
+
+Very beautiful and interesting views may be obtained by using such a
+pyramidal box as is depicted in fig. 11.
+
+[Illustration: _Fig. 11._]
+
+This box should be made of black cloth or calico fastened over a light
+framework of wire or cane. The base of the pyramid should be covered on
+the inside with a sheet of white glazed paper, or with some other
+uniform white surface. Captain Noble, I believe, makes use of a surface
+of plaster of Paris, smoothed while wet with plate-glass. The door _b
+c_ enables the observer to "change power" without removing the box,
+while larger doors, _d e_ and _g f_, enable him to examine the image; a
+dark cloth, such as photographers use, being employed, if necessary, to
+keep out extraneous light. The image may also be examined from without,
+if the bottom of the pyramid be formed of a sheet of cut-glass or oiled
+tissue-paper.
+
+When making use of the method just described, it is very necessary that
+the telescope-tube should be well balanced. A method by which this may
+be conveniently accomplished has been already described in Chapter I.
+
+But, undoubtedly, for the possessor of a moderately good telescope there
+is no way of viewing the sun's features comparable to that now to be
+described, which has been systematically and successfully applied for a
+long series of years by the Rev. F. Howlett. To use his own words: "Any
+one possessing a good achromatic of not more than three inches'
+aperture, who has a little dexterity with his pencil, and a little time
+at his disposal (all the better if it be at a somewhat early hour of the
+morning)" may by this method "deliberately and satisfactorily view,
+measure, and (if skill suffice) delineate most of those interesting and
+grand solar phenomena of which he may have read, or which he may have
+seen depicted, in various works on physical astronomy."[15]
+
+The method in question depends on the same property which is involved in
+the use of the pyramidal box just described, supplemented (where exact
+and systematic observation is required) by the fact that objects lying
+on or between the lenses of the eye-piece are to be seen faithfully
+projected on the white surface on which the sun's image is received. In
+place, however, of a box carried upon the telescope-tube, a darkened
+room (or true _camera obscura_) contains the receiving sheet.
+
+A chamber is to be selected, having a window looking towards the
+south--a little easterly, if possible, so as to admit of morning
+observation. All windows are to be completely darkened save one, through
+which the telescope is directed towards the sun. An arrangement is to be
+adopted for preventing all light from entering by this window except
+such light as passes down the tube of the telescope. This can readily be
+managed with a little ingenuity. Mr. Howlett describes an excellent
+method. The following, perhaps, will sufficiently serve the purposes of
+the general observer:--A plain frame (portable) is to be constructed to
+fit into the window: to the four sides of this frame triangular pieces
+of cloth (impervious to light) are to be attached, their shape being
+such that when their adjacent edges are sewn together and the flaps
+stretched out, they form a rectangular pyramid of which the frame is the
+base. Through the vertex of this pyramid (near which, of course, the
+cloth flaps are not sewn together) the telescope tube is to be passed,
+and an elastic cord so placed round the ends of the flaps as to prevent
+any light from penetrating between them and the telescope. It will now
+be possible, without disturbing the screen (fixed in the window), to
+move the telescope so as to follow the sun during the time of
+observation. And the same arrangement will serve for all seasons, if so
+managed that the elastic cord is not far from the middle of the
+telescope-tube; for in this case the range of motion is small compared
+to the range of the tube's extremity.
+
+A large screen of good drawing-paper should next be prepared. This
+should be stretched on a light frame of wood, and placed on an easel,
+the legs of which should be furnished with holes and pegs that the
+screen may be set at any required height, and be brought square to the
+tube's axis. A large T-square of light wood will be useful to enable the
+observer to judge whether the screen is properly situated in the last
+respect.
+
+We wish now to direct the tube towards the sun, and this "without
+dazzling the eyes as by the ordinary method." This may be done in two
+ways. We may either, before commencing work--that is, before fastening
+our elastic cord so as to exclude all light--direct the tube so that its
+shadow shall be a perfect circle (when of course it is truly directed),
+then fasten the cord and afterwards we can easily keep the sun in the
+field by slightly shifting the tube as occasion requires. Or (if the
+elastic cord has already been fastened) we may remove the eye-tube and
+shift the telescope-tube about--the direction in which the sun lies
+being roughly known--until we see the spot of light received down the
+telescope's axis grow brighter and brighter and finally become a _spot
+of sun-light_. If a card be held near the focus of the telescope there
+will be seen in fact an image of the sun. The telescope being now
+properly directed, the eye-tube may be slipped in again, and the sun may
+be kept in the field as before.
+
+There will now be seen upon the screen a picture of the sun very
+brilliant and pleasing, but perhaps a little out of focus. The focusing
+should therefore next be attended to, the increase of clearness in the
+image being the test of approach to the true focus. And again, it will
+be well to try the effect of slight changes of distance between the
+screen and the telescope's eye-piece. Mr. Howlett considers one yard as
+a convenient distance for producing an excellent effect with almost any
+eye-piece that the state of the atmosphere will admit of. Of course, the
+image becomes more sharply defined if we bring the screen nearer to the
+telescope, while all the details are enlarged when we move the screen
+away. The enlargement has no limits save those depending on the amount
+of light in the image. But, of course, the observer must not expect
+enlargement to bring with it a view of new details, after a certain
+magnitude of image has been attained. Still there is something
+instructive, I think, in occasionally getting a very magnified view of
+some remarkable spot. I have often looked with enhanced feelings of awe
+and wonder on the gigantic image of a solar spot thrown by means of the
+diagonal eye-piece upon the ceiling of the observing-room. Blurred and
+indistinct through over-magnifying, yet with a new meaning to me,
+_there_ the vast abysm lies pictured; vague imaginings of the vast and
+incomprehensible agencies at work in the great centre of our system
+crowd unbidden into my mind; and I seem to _feel_--not merely think
+about--the stupendous grandeur of that life-emitting orb.
+
+To return, however, to observation:--By slightly shifting the tube,
+different parts of the solar disc can be brought successively upon the
+screen and scrutinized as readily as if they were drawn upon a chart.
+"With a power of--say about 60 or 80 linear--the most minute solar spot,
+properly so called, that is capable of formation" (Mr. Howlett believes
+"they are never less than three seconds in length or breadth) will be
+more readily detected than by any other method," see Plate 7; "as also
+will any faculae, mottling, or in short, any other phenomena that may
+then be existing on the disc." "Drifting clouds frequently sweep by, to
+vary the scene, and occasionally an aerial hail- or snow-storm." Mr.
+Howlett has more than once seen a distant flight of rooks pass slowly
+across the disc with wonderful distinctness, when the sun has been at a
+low altitude, and likewise, much more frequently, the rapid dash of
+starlings, which, very much closer at hand, frequent his church-tower."
+
+An eclipse of the sun, or a transit of an inferior planet, is also much
+better seen in this way than by any other method of observing the solar
+disc. In Plate 7 are presented several solar spots as they have appeared
+to Mr. Howlett, with an instrument of moderate power. The grotesque
+forms of some of these are remarkable; and the variations the spots
+undergo from day to day are particularly interesting to the thoughtful
+observer.
+
+A method of measuring the spots may now be described. It is not likely
+indeed that the ordinary observer will care to enter upon any systematic
+series of measurements. But even in his case, the means of forming a
+general comparison between the spots he sees at different times cannot
+fail to be valuable. Also the knowledge--which a simple method of
+measurement supplies--of the actual dimensions of a spot in miles
+(roughly) is calculated to enhance our estimate of the importance of
+these features of the solar disc. I give Mr. Howlett's method in his own
+words:--
+
+"Cause your optician to rule for you on a circular piece of glass a
+number of fine graduations, the 200th part of an inch apart, each fifth
+and tenth line being of a different length in order to assist the eye in
+their enumeration. Insert this between the anterior and posterior lenses
+of a Huygenian eye-piece of moderate power, say 80 linear. Direct your
+telescope upon the sun, and having so arranged it that the whole disc of
+the sun may be projected on the screen, count carefully the number of
+graduations that are seen to exactly occupy the solar diameter.... It
+matters not in which direction you measure your diameter, provided only
+the sun has risen some 18 deg. or 20 deg. above the horizon, and so escaped the
+distortion occasioned by refraction.[16]
+
+"Next let us suppose that our observer has been observing the sun on any
+day of the year, say, if you choose, at the time of its mean apparent
+diameter, namely about the first of April or first of October, and has
+ascertained that" (as is the case with Mr. Howlett's instrument)
+"sixty-four graduations occupy the diameter of the projected image. Now
+the semi-diameter of the sun, at the epochs above mentioned, according
+to the tables given for every day of the year in the 'Nautical Almanac'
+(the same as in Dietrichsen and Hannay's very useful compilation) is
+16' 2", and consequently his mean total diameter is 32' 4" or 1924". If
+now we divide 1924" by 64" this will, of course, award as nearly as
+possible 30" as the value in celestial arc of each graduation, either as
+seen on the screen, or as applied directly to the sun or any heavenly
+body large enough to be measured by it."
+
+Since the sun's diameter is about 850,000 miles, each graduation (in the
+case above specified) corresponds to one-64th part of 850,000
+miles--that is, to a length of 13,256 miles on the sun's surface. Any
+other case can be treated in precisely the same manner.
+
+It will be found easy so to place the screen that the distance between
+successive graduations (as seen projected upon the screen) may
+correspond to any desired unit of linear measurement--say an inch. Then
+if the observer use transparent tracing-paper ruled with faint lines
+forming squares half-an-inch in size, he can comfortably copy directly
+from the screen any solar phenomena he may be struck with. A variety of
+methods of drawing will suggest themselves. Mr. Howlett, in the paper I
+have quoted from above, describes a very satisfactory method, which
+those who are anxious to devote themselves seriously to solar
+observation will do well to study.
+
+It is necessary that the observer should be able to determine
+approximately where the sun's equator is situated at the time of any
+observation, in order that he may assign to any spot or set of spots its
+true position in relation to solar longitude and latitude. Mr. Howlett
+shows how this may be done by three observations of the sun made at any
+fixed hour on successive days. Perhaps the following method will serve
+the purpose of the general observer sufficiently well:--
+
+The hour at which the sun crosses the meridian must be taken for the
+special observation now to be described. This hour can always be learnt
+from 'Dietrichsen's Almanac'; but noon, civil time, is near enough for
+practical purposes. Now it is necessary first to know the position of
+the ecliptic with reference to the celestial equator. Of course, at noon
+a horizontal line across the sun's disc is parallel to the equator, but
+the position of that diameter of the sun which coincides with the
+ecliptic is not constant: at the summer and winter solstices this
+diameter coincides with the other, or is horizontal at noon; at the
+spring equinox the sun (which travels on the ecliptic) is passing
+towards the north of the equator, crossing that curve at an angle of
+23-1/2 deg., so that the ecliptic coincides with that diameter of the sun
+which cuts the horizontal one at an angle of 23-1/2 deg. and has its _left_
+end above the horizontal diameter; and at the autumn equinox the sun is
+descending and the same description applies, only that the diameter
+(inclined 23-1/2 deg. to the horizon) which has its _right_ end uppermost,
+now represents the ecliptic. For intermediate dates, use the following
+little table:--
+
+--------------------------------------------------------------------------
+Date.              |Dec. 22|Jan. 5|Jan. 20|Feb. 4|Feb. 19|Mar. 5 |Mar. 21
+(Circiter.)        |       |June 6|May 21 |May 5 |Apr. 20|Apr. 5 |
+-------------------+-------+------+-------+------+-------+-------+--------
+Inclination of     |Left   |Left  |Left   |Left  |Left   |Left   |Left
+Ecliptical Diameter|       |      |       |      |       |       |
+of Sun to the      |0 deg. 0'  |6 deg.24' |12 deg.14' |17 deg.3' |20 deg.36' |22 deg.44' |23 deg.27'
+Horizon.[17]       |Right  |Right |Right  |Right |Right  |Right  |Right
+-------------------+-------+------+-------+------+-------+-------+--------
+Date.              |       |Dec. 7|Nov. 22|Nov. 7|Oct. 23|Oct. 8 |
+(Circiter.)        |Jan. 21|July 7|July 23|Aug. 6|Aug. 23|Sept. 7|Sept. 23
+--------------------------------------------------------------------------
+
+Now if our observer describe a circle, and draw a diameter inclined
+according to above table, this diameter would represent the sun's
+equator if the axis of the sun were square to the ecliptic-plane. But
+this axis is slightly inclined, the effect of which is, that on or about
+June 10 the sun is situated as shown in fig. 14 with respect to the
+ecliptic _ab_; on or about September 11 he is situated as shown in fig.
+13; on or about December 11 as shown in fig. 12; and on or about March
+10 as shown in fig. 15. The inclination of his equator to the ecliptic
+being so small, the student can find little difficulty in determining
+with sufficient approximation the relation of the sun's polar axis to
+the ecliptic on intermediate days, since the equator is never more
+_inclined_ than in figs. 12 and 14, never more _opened out_ than in
+figs. 13 and 15. Having then drawn a line to represent the sun's
+ecliptical diameter inclined to the horizontal diameter as above
+described, and having (with this line to correspond to _ab_ in figs.
+12-15) drawn in the sun's equator suitably inclined and opened out, he
+has the sun's actual presentation (at noon) as seen with an erecting
+eye-piece. Holding his picture upside down, he has the sun's
+presentation as seen with an astronomical eye-piece--and, finally,
+looking at his picture from behind (without inverting it), he has the
+presentation seen when the sun is projected on the screen. Hence, if he
+make a copy of this last view of his diagram upon the centre of his
+screen, and using a low power, bring the whole of the sun's image to
+coincide with the circle thus drawn (to a suitable scale) on the screen,
+he will at once see what is the true position of the different
+sun-spots. After a little practice the construction of a suitably sized
+and marked circle on the screen will not occupy more than a minute or
+two.
+
+[Illustration: _Fig. 12._]
+
+[Illustration: _Fig. 13._]
+
+[Illustration: _Fig. 14._]
+
+[Illustration: _Fig. 15._]
+
+It must be noticed that the sun's apparent diameter is not always the
+same. He is nearer to us in winter than in summer, and, of course, his
+apparent diameter is greater at the former than at the latter season.
+The variation of the apparent diameter corresponds (inversely) to the
+variation of distance. As the sun's greatest distance from the earth is
+93,000,000 miles (pretty nearly) and his least 90,000,000, his greatest,
+mean, and least apparent diameters are as 93, 91-1/2, and 90
+respectively; that is, as 62, 61, and 60 respectively.
+
+Mr. Howlett considers that with a good 3-inch telescope, applied in the
+manner we have described, all the solar features may be seen, except the
+separate granules disclosed by first-class instruments in the hands of
+such observers as Dawes, Huggins, or Secchi. Faculae may, of course, be
+well seen. They are to be looked for near spots which lie close to the
+sun's limb.
+
+When the sun's general surface is carefully scrutinised, it is found to
+present a mottled appearance. This is a somewhat delicate feature. It
+results, undoubtedly, from the combined effect of the granules
+separately seen in powerful instruments. Sir John Herschel has stated
+that he cannot recognise the marbled appearance of the sun with an
+achromatic. Mr. Webb, however, has seen this appearance with such a
+telescope, of moderate power, used with direct vision; and certainly I
+can corroborate Mr. Howlett in the statement that this appearance may be
+most distinctly seen when the image of the sun is received within a
+well-darkened room.
+
+My space will not permit me to enter here upon the discussion of any of
+those interesting speculations which have been broached concerning solar
+phenomena. We may hope that the great eclipse of August, 1868, which
+promises to be the most favourable (for effective observation) that has
+ever taken place, will afford astronomers the opportunity of resolving
+some important questions. It seems as if we were on the verge of great
+discoveries,--and certainly, if persevering and well-directed labour
+would seem in any case to render such discoveries due as man's just
+reward, we may well say that he deserves shortly to reap a harvest of
+exact knowledge respecting solar phenomena.
+
+
+
+
+THE END.
+
+
+
+FOOTNOTES:
+
+[Footnote 1: Such a telescope is most powerful with the shortest sight.
+It may be remarked that the use of a telescope often reveals a
+difference in the sight of the two eyes. In my own case, for instance, I
+have found that the left eye is very short-sighted, the sight of the
+right eye being of about the average range. Accordingly with my left eye
+a 5-1/2-foot object-glass, alone, forms an effective telescope, with
+which I can see Jupiter's moons quite distinctly, and under favourable
+circumstances even Saturn's rings. I find that the full moon is too
+bright to be observed in this way without pain, except at low
+altitudes.]
+
+[Footnote 2: Betelgeuse--commonly interpreted the Giant's
+Shoulder--_ibt-al-jauza_. The words, however, really signify, "the
+armpit of the central one," Orion being so named because he is divided
+centrally by the equator.]
+
+[Footnote 3: I have never been able to see more than four with a
+3-3/4-inch aperture. I give a view of the trapezium as seen with an
+8-inch equatorial.]
+
+[Footnote 4: Sir W. Herschel several times saw [epsilon] Lyrae as a
+double. Bessel also relates that when he was a lad of thirteen he could
+see this star double. I think persons having average eye-sight could see
+it double if they selected a suitable hour for observation. My own
+eye-sight is not good enough for this, but I can distinctly see this
+star wedged whenever the line joining the components is inclined about
+45 deg. to the horizon, and also when Lyra is near the zenith.]
+
+[Footnote 5: They were so described by Admiral Smyth in 1839. Mr. Main,
+in 1862, describes them as straw-coloured and reddish, while Mr. Webb,
+in 1865, saw them pale-yellow and _lilac_!]
+
+[Footnote 6: Or the observer may sweep from [omicron] towards [nu],
+looking for R about two-fifths of the way from [omicron] to [nu].]
+
+[Footnote 7: Here a single period only is taken, to get back to a
+convenient hour of the evening.]
+
+[Footnote 8: Here a single period only is taken, to get back to a
+convenient hour of the evening.]
+
+[Footnote 9: I have constructed a zodiac-chart, which will enable the
+student to mark in the path of a planet, at any season of the year, from
+the recorded places in the almanacs.]
+
+[Footnote 10: It is convenient to remember that through precession a
+star near the ecliptic shifts as respects the R.A. and Dec. lines,
+through an arc of one degree--or nearly twice the moon's diameter--in
+about 72 years, all other stars through a less arc.]
+
+[Footnote 11: Mercury is best seen when in quadrature to the sun, but
+_not_ (as I have seen stated) at those quadratures in which he attains
+his maximum elongation from the sun. This will appear singular, because
+the maximum elongation is about 27 deg., the minimum only about 18 deg.. But it
+happens that in our northern latitudes Mercury is always _south_ of the
+sun when he attains his maximum elongation, and this fact exercises a
+more important effect than the mere amount of elongation.]
+
+[Footnote 12: It does not seem to me that the difficulty of detecting
+Mercury is due to the difficulty "of identifying it amongst the
+surrounding stars, during the short time that it can be seen" (Hind's
+'Introduction to Astronomy'). There are few stars which are comparable
+with Mercury in brilliancy, when seen under the same light.]
+
+[Footnote 13: I may notice another error sometimes made. It is said that
+the shadow of a satellite _appears_ elliptical when near the edge of the
+disc. The shadow is _in reality_ elliptical when thus situated, but
+_appears_ circular. A moment's consideration will show that this should
+be so. The part of the disc concealed by a _satellite_ near the limb is
+also elliptical, but of course appears round.]
+
+[Footnote 14: From a paper by Mr. Breen, in the 'Popular Science
+Review,' October, 1864.]
+
+[Footnote 15: 'Intellectual Observer' for July, 1867, to which magazine
+the reader is referred for full details of Mr. Howlett's method of
+observation, and for illustrations of the appliances he made use of, and
+of some of his results.]
+
+[Footnote 16: As the sun does not attain such an altitude as 18 deg. during
+two months in the year, it is well to notice that the true length of the
+sun's apparent solar diameter is determinable even immediately after
+sun-rise, if the line of graduation is made to coincide with the
+_horizontal_ diameter of the picture on the screen--for refraction does
+not affect the length of this diameter.]
+
+[Footnote 17: The words "Left" and "Right" indicate which end of the
+sun's ecliptical diameter is uppermost at the dates in upper or lower
+row respectively.]
+
+
+
+
+LONDON:
+
+PRINTED BY W. CLOWES AND SONS, DUKE STREET, STAMFORD STREET, AND CHARING
+CROSS.
+
+
+
+
+
+End of the Project Gutenberg EBook of Half-hours with the Telescope
+by Richard A. Proctor
+
+*** END OF THIS PROJECT GUTENBERG EBOOK HALF-HOURS WITH THE TELESCOPE ***
+
+***** This file should be named 16767.txt or 16767.zip *****
+This and all associated files of various formats will be found in:
+        https://www.gutenberg.org/1/6/7/6/16767/
+
+Produced by Jason Isbell and the Online Distributed
+Proofreading Team at https://www.pgdp.net
+
+
+Updated editions will replace the previous one--the old editions
+will be renamed.
+
+Creating the works from public domain print editions 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.  Special rules,
+set forth in the General Terms of Use part of this license, apply to
+copying and distributing Project Gutenberg-tm electronic works to
+protect the PROJECT GUTENBERG-tm concept and trademark.  Project
+Gutenberg is a registered trademark, and may not be used if you
+charge for the eBooks, unless you receive specific permission.  If you
+do not charge anything for copies of this eBook, complying with the
+rules is very easy.  You may use this eBook for nearly any purpose
+such as creation of derivative works, reports, performances and
+research.  They may be modified and printed and given away--you may do
+practically ANYTHING with public domain eBooks.  Redistribution is
+subject to the trademark license, especially commercial
+redistribution.
+
+
+
+*** START: FULL LICENSE ***
+
+THE FULL PROJECT GUTENBERG LICENSE
+PLEASE READ THIS BEFORE YOU DISTRIBUTE OR USE THIS WORK
+
+To protect the Project Gutenberg-tm mission of promoting the free
+distribution of electronic works, by using or distributing this work
+(or any other work associated in any way with the phrase "Project
+Gutenberg"), you agree to comply with all the terms of the Full Project
+Gutenberg-tm License (available with this file or online at
+https://gutenberg.org/license).
+
+
+Section 1.  General Terms of Use and Redistributing Project Gutenberg-tm
+electronic works
+
+1.A.  By reading or using any part of this Project Gutenberg-tm
+electronic work, you indicate that you have read, understand, agree to
+and accept all the terms of this license and intellectual property
+(trademark/copyright) agreement.  If you do not agree to abide by all
+the terms of this agreement, you must cease using and return or destroy
+all copies of Project Gutenberg-tm electronic works in your possession.
+If you paid a fee for obtaining a copy of or access to a Project
+Gutenberg-tm electronic work and you do not agree to be bound by the
+terms of this agreement, you may obtain a refund from the person or
+entity to whom you paid the fee as set forth in paragraph 1.E.8.
+
+1.B.  "Project Gutenberg" is a registered trademark.  It may only be
+used on or associated in any way with an electronic work by people who
+agree to be bound by the terms of this agreement.  There are a few
+things that you can do with most Project Gutenberg-tm electronic works
+even without complying with the full terms of this agreement.  See
+paragraph 1.C below.  There are a lot of things you can do with Project
+Gutenberg-tm electronic works if you follow the terms of this agreement
+and help preserve free future access to Project Gutenberg-tm electronic
+works.  See paragraph 1.E below.
+
+1.C.  The Project Gutenberg Literary Archive Foundation ("the Foundation"
+or PGLAF), owns a compilation copyright in the collection of Project
+Gutenberg-tm electronic works.  Nearly all the individual works in the
+collection are in the public domain in the United States.  If an
+individual work is in the public domain in the United States and you are
+located in the United States, we do not claim a right to prevent you from
+copying, distributing, performing, displaying or creating derivative
+works based on the work as long as all references to Project Gutenberg
+are removed.  Of course, we hope that you will support the Project
+Gutenberg-tm mission of promoting free access to electronic works by
+freely sharing Project Gutenberg-tm works in compliance with the terms of
+this agreement for keeping the Project Gutenberg-tm name associated with
+the work.  You can easily comply with the terms of this agreement by
+keeping this work in the same format with its attached full Project
+Gutenberg-tm License when you share it without charge with others.
+
+1.D.  The copyright laws of the place where you are located also govern
+what you can do with this work.  Copyright laws in most countries are in
+a constant state of change.  If you are outside the United States, check
+the laws of your country in addition to the terms of this agreement
+before downloading, copying, displaying, performing, distributing or
+creating derivative works based on this work or any other Project
+Gutenberg-tm work.  The Foundation makes no representations concerning
+the copyright status of any work in any country outside the United
+States.
+
+1.E.  Unless you have removed all references to Project Gutenberg:
+
+1.E.1.  The following sentence, with active links to, or other immediate
+access to, the full Project Gutenberg-tm License must appear prominently
+whenever any copy of a Project Gutenberg-tm work (any work on which the
+phrase "Project Gutenberg" appears, or with which the phrase "Project
+Gutenberg" is associated) is accessed, displayed, performed, viewed,
+copied or distributed:
+
+This eBook is for the use of anyone anywhere 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
+
+1.E.2.  If an individual Project Gutenberg-tm electronic work is derived
+from the public domain (does not contain a notice indicating that it is
+posted with permission of the copyright holder), the work can be copied
+and distributed to anyone in the United States without paying any fees
+or charges.  If you are redistributing or providing access to a work
+with the phrase "Project Gutenberg" associated with or appearing on the
+work, you must comply either with the requirements of paragraphs 1.E.1
+through 1.E.7 or obtain permission for the use of the work and the
+Project Gutenberg-tm trademark as set forth in paragraphs 1.E.8 or
+1.E.9.
+
+1.E.3.  If an individual Project Gutenberg-tm electronic work is posted
+with the permission of the copyright holder, your use and distribution
+must comply with both paragraphs 1.E.1 through 1.E.7 and any additional
+terms imposed by the copyright holder.  Additional terms will be linked
+to the Project Gutenberg-tm License for all works posted with the
+permission of the copyright holder found at the beginning of this work.
+
+1.E.4.  Do not unlink or detach or remove the full Project Gutenberg-tm
+License terms from this work, or any files containing a part of this
+work or any other work associated with Project Gutenberg-tm.
+
+1.E.5.  Do not copy, display, perform, distribute or redistribute this
+electronic work, or any part of this electronic work, without
+prominently displaying the sentence set forth in paragraph 1.E.1 with
+active links or immediate access to the full terms of the Project
+Gutenberg-tm License.
+
+1.E.6.  You may convert to and distribute this work in any binary,
+compressed, marked up, nonproprietary or proprietary form, including any
+word processing or hypertext form.  However, if you provide access to or
+distribute copies of a Project Gutenberg-tm work in a format other than
+"Plain Vanilla ASCII" or other format used in the official version
+posted on the official Project Gutenberg-tm web site (www.gutenberg.org),
+you must, at no additional cost, fee or expense to the user, provide a
+copy, a means of exporting a copy, or a means of obtaining a copy upon
+request, of the work in its original "Plain Vanilla ASCII" or other
+form.  Any alternate format must include the full Project Gutenberg-tm
+License as specified in paragraph 1.E.1.
+
+1.E.7.  Do not charge a fee for access to, viewing, displaying,
+performing, copying or distributing any Project Gutenberg-tm works
+unless you comply with paragraph 1.E.8 or 1.E.9.
+
+1.E.8.  You may charge a reasonable fee for copies of or providing
+access to or distributing Project Gutenberg-tm electronic works provided
+that
+
+- You pay a royalty fee of 20% of the gross profits you derive from
+     the use of Project Gutenberg-tm works calculated using the method
+     you already use to calculate your applicable taxes.  The fee is
+     owed to the owner of the Project Gutenberg-tm trademark, but he
+     has agreed to donate royalties under this paragraph to the
+     Project Gutenberg Literary Archive Foundation.  Royalty payments
+     must be paid within 60 days following each date on which you
+     prepare (or are legally required to prepare) your periodic tax
+     returns.  Royalty payments should be clearly marked as such and
+     sent to the Project Gutenberg Literary Archive Foundation at the
+     address specified in Section 4, "Information about donations to
+     the Project Gutenberg Literary Archive Foundation."
+
+- You provide a full refund of any money paid by a user who notifies
+     you in writing (or by e-mail) within 30 days of receipt that s/he
+     does not agree to the terms of the full Project Gutenberg-tm
+     License.  You must require such a user to return or
+     destroy all copies of the works possessed in a physical medium
+     and discontinue all use of and all access to other copies of
+     Project Gutenberg-tm works.
+
+- You provide, in accordance with paragraph 1.F.3, a full refund of any
+     money paid for a work or a replacement copy, if a defect in the
+     electronic work is discovered and reported to you within 90 days
+     of receipt of the work.
+
+- You comply with all other terms of this agreement for free
+     distribution of Project Gutenberg-tm works.
+
+1.E.9.  If you wish to charge a fee or distribute a Project Gutenberg-tm
+electronic work or group of works on different terms than are set
+forth in this agreement, you must obtain permission in writing from
+both the Project Gutenberg Literary Archive Foundation and Michael
+Hart, the owner of the Project Gutenberg-tm trademark.  Contact the
+Foundation as set forth in Section 3 below.
+
+1.F.
+
+1.F.1.  Project Gutenberg volunteers and employees expend considerable
+effort to identify, do copyright research on, transcribe and proofread
+public domain works in creating the Project Gutenberg-tm
+collection.  Despite these efforts, Project Gutenberg-tm electronic
+works, and the medium on which they may be stored, may contain
+"Defects," such as, but not limited to, incomplete, inaccurate or
+corrupt data, transcription errors, a copyright or other intellectual
+property infringement, a defective or damaged disk or other medium, a
+computer virus, or computer codes that damage or cannot be read by
+your equipment.
+
+1.F.2.  LIMITED WARRANTY, DISCLAIMER OF DAMAGES - Except for the "Right
+of Replacement or Refund" described in paragraph 1.F.3, the Project
+Gutenberg Literary Archive Foundation, the owner of the Project
+Gutenberg-tm trademark, and any other party distributing a Project
+Gutenberg-tm electronic work under this agreement, disclaim all
+liability to you for damages, costs and expenses, including legal
+fees.  YOU AGREE THAT YOU HAVE NO REMEDIES FOR NEGLIGENCE, STRICT
+LIABILITY, BREACH OF WARRANTY OR BREACH OF CONTRACT EXCEPT THOSE
+PROVIDED IN PARAGRAPH F3.  YOU AGREE THAT THE FOUNDATION, THE
+TRADEMARK OWNER, AND ANY DISTRIBUTOR UNDER THIS AGREEMENT WILL NOT BE
+LIABLE TO YOU FOR ACTUAL, DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE OR
+INCIDENTAL DAMAGES EVEN IF YOU GIVE NOTICE OF THE POSSIBILITY OF SUCH
+DAMAGE.
+
+1.F.3.  LIMITED RIGHT OF REPLACEMENT OR REFUND - If you discover a
+defect in this electronic work within 90 days of receiving it, you can
+receive a refund of the money (if any) you paid for it by sending a
+written explanation to the person you received the work from.  If you
+received the work on a physical medium, you must return the medium with
+your written explanation.  The person or entity that provided you with
+the defective work may elect to provide a replacement copy in lieu of a
+refund.  If you received the work electronically, the person or entity
+providing it to you may choose to give you a second opportunity to
+receive the work electronically in lieu of a refund.  If the second copy
+is also defective, you may demand a refund in writing without further
+opportunities to fix the problem.
+
+1.F.4.  Except for the limited right of replacement or refund set forth
+in paragraph 1.F.3, this work is provided to you 'AS-IS' WITH NO OTHER
+WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO
+WARRANTIES OF MERCHANTIBILITY OR FITNESS FOR ANY PURPOSE.
+
+1.F.5.  Some states do not allow disclaimers of certain implied
+warranties or the exclusion or limitation of certain types of damages.
+If any disclaimer or limitation set forth in this agreement violates the
+law of the state applicable to this agreement, the agreement shall be
+interpreted to make the maximum disclaimer or limitation permitted by
+the applicable state law.  The invalidity or unenforceability of any
+provision of this agreement shall not void the remaining provisions.
+
+1.F.6.  INDEMNITY - You agree to indemnify and hold the Foundation, the
+trademark owner, any agent or employee of the Foundation, anyone
+providing copies of Project Gutenberg-tm electronic works in accordance
+with this agreement, and any volunteers associated with the production,
+promotion and distribution of Project Gutenberg-tm electronic works,
+harmless from all liability, costs and expenses, including legal fees,
+that arise directly or indirectly from any of the following which you do
+or cause to occur: (a) distribution of this or any Project Gutenberg-tm
+work, (b) alteration, modification, or additions or deletions to any
+Project Gutenberg-tm work, and (c) any Defect you cause.
+
+
+Section  2.  Information about the Mission of Project Gutenberg-tm
+
+Project Gutenberg-tm is synonymous with the free distribution of
+electronic works in formats readable by the widest variety of computers
+including obsolete, old, middle-aged and new computers.  It exists
+because of the efforts of hundreds of volunteers and donations from
+people in all walks of life.
+
+Volunteers and financial support to provide volunteers with the
+assistance they need, is critical to reaching Project Gutenberg-tm's
+goals and ensuring that the Project Gutenberg-tm collection will
+remain freely available for generations to come.  In 2001, the Project
+Gutenberg Literary Archive Foundation was created to provide a secure
+and permanent future for Project Gutenberg-tm and future generations.
+To learn more about the Project Gutenberg Literary Archive Foundation
+and how your efforts and donations can help, see Sections 3 and 4
+and the Foundation web page at https://www.pglaf.org.
+
+
+Section 3.  Information about the Project Gutenberg Literary Archive
+Foundation
+
+The Project Gutenberg Literary Archive Foundation is a non profit
+501(c)(3) educational corporation organized under the laws of the
+state of Mississippi and granted tax exempt status by the Internal
+Revenue Service.  The Foundation's EIN or federal tax identification
+number is 64-6221541.  Its 501(c)(3) letter is posted at
+https://pglaf.org/fundraising.  Contributions to the Project Gutenberg
+Literary Archive Foundation are tax deductible to the full extent
+permitted by U.S. federal laws and your state's laws.
+
+The Foundation's principal office is located at 4557 Melan Dr. S.
+Fairbanks, AK, 99712., but its volunteers and employees are scattered
+throughout numerous locations.  Its business office is located at
+809 North 1500 West, Salt Lake City, UT 84116, (801) 596-1887, email
+business@pglaf.org.  Email contact links and up to date contact
+information can be found at the Foundation's web site and official
+page at https://pglaf.org
+
+For additional contact information:
+     Dr. Gregory B. Newby
+     Chief Executive and Director
+     gbnewby@pglaf.org
+
+
+Section 4.  Information about Donations to the Project Gutenberg
+Literary Archive Foundation
+
+Project Gutenberg-tm depends upon and cannot survive without wide
+spread public support and donations to carry out its mission of
+increasing the number of public domain and licensed works that can be
+freely distributed in machine readable form accessible by the widest
+array of equipment including outdated equipment.  Many small donations
+($1 to $5,000) are particularly important to maintaining tax exempt
+status with the IRS.
+
+The Foundation is committed to complying with the laws regulating
+charities and charitable donations in all 50 states of the United
+States.  Compliance requirements are not uniform and it takes a
+considerable effort, much paperwork and many fees to meet and keep up
+with these requirements.  We do not solicit donations in locations
+where we have not received written confirmation of compliance.  To
+SEND DONATIONS or determine the status of compliance for any
+particular state visit https://pglaf.org
+
+While we cannot and do not solicit contributions from states where we
+have not met the solicitation requirements, we know of no prohibition
+against accepting unsolicited donations from donors in such states who
+approach us with offers to donate.
+
+International donations are gratefully accepted, but we cannot make
+any statements concerning tax treatment of donations received from
+outside the United States.  U.S. laws alone swamp our small staff.
+
+Please check the Project Gutenberg Web pages for current donation
+methods and addresses.  Donations are accepted in a number of other
+ways including including checks, online payments and credit card
+donations.  To donate, please visit: https://pglaf.org/donate
+
+
+Section 5.  General Information About Project Gutenberg-tm electronic
+works.
+
+Professor Michael S. Hart was the originator of the Project Gutenberg-tm
+concept of a library of electronic works that could be freely shared
+with anyone.  For thirty years, he produced and distributed Project
+Gutenberg-tm eBooks with only a loose network of volunteer support.
+
+
+Project Gutenberg-tm eBooks are often created from several printed
+editions, all of which are confirmed as Public Domain in the U.S.
+unless a copyright notice is included.  Thus, we do not necessarily
+keep eBooks in compliance with any particular paper edition.
+
+
+Most people start at our Web site which has the main PG search facility:
+
+     https://www.gutenberg.org
+
+This Web site includes information about Project Gutenberg-tm,
+including how to make donations to the Project Gutenberg Literary
+Archive Foundation, how to help produce our new eBooks, and how to
+subscribe to our email newsletter to hear about new eBooks.
diff --git a/16767.zip b/16767.zip
new file mode 100644
index 0000000..cad0ddc
--- /dev/null
+++ b/16767.zip
diff --git a/LICENSE.txt b/LICENSE.txt
new file mode 100644
index 0000000..6312041
--- /dev/null
+++ b/LICENSE.txt
@@ -0,0 +1,11 @@
+This eBook, including all associated images, markup, improvements,
+metadata, and any other content or labor, has been confirmed to be
+in the PUBLIC DOMAIN IN THE UNITED STATES.
+
+Procedures for determining public domain status are described in
+the "Copyright How-To" at https://www.gutenberg.org.
+
+No investigation has been made concerning possible copyrights in
+jurisdictions other than the United States. Anyone seeking to utilize
+this eBook outside of the United States should confirm copyright
+status under the laws that apply to them.
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..81869ba
--- /dev/null
+++ b/README.md
@@ -0,0 +1,2 @@
+Project Gutenberg (https://www.gutenberg.org) public repository for
+eBook #16767 (https://www.gutenberg.org/ebooks/16767)