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PLEASURES OF THE
TELESCOPE
AN ILLUSTRATED GUIDE FOR AMATEUR ASTRONOMERS
AND A POPULAR DESCRIPTION OF THE CHIEF
WONDERS OF THE HEAVENS FOR
GENERAL READERS
BY
GARRETT P. SERVISS
AUTHOR OF ASTRONOMY WITH AN OPERA-GLASS
"This being made, He yearned for worlds to make
From other chaos out beyond our night--
For to create is still God's prime delight.
The large moon, all alone, sailed her dark lake,
And the first tides were moving to her might;
Then Darkness trembled, and began to quake
Big with the birth of stars, and when He spake
A million worlds leapt into radiant light."
LLOYD MIFFLIN.
_WITH MANY ILLUSTRATIONS_
NEW YORK D. APPLETON AND COMPANY 1901
COPYRIGHT, 1901,
BY D. APPLETON AND COMPANY.
PREFACE
By the introduction of a complete series of star maps, drawn specially
for the use of the amateur and distributed through the body of the work,
thus facilitating consultation, it is believed that this book makes a
step in advance of its predecessors. The maps show all of the stars
visible to the naked eye in the regions of sky represented, and, in
addition, some stars that can only be seen with optical aid. The latter
have been placed in the maps as guide posts in the telescopic field to
assist those who are searching for faint and inconspicuous objects
referred to in the text. As the book was not written for those who
possess the equipment of an observatory, with telescopes driven by
clockwork and provided with graduated circles, right ascensions and
declinations are not given. All of the telescopic phenomena described
are, however, represented in the maps. Star clusters are indicated by a
conventional symbol, and nebulae by a little white circle; while a small
cross serves to mark the places where notable new stars have appeared.
The relative magnitudes of the stars are approximately shown by the
dimensions of their symbols in the maps, the smaller stars being
represented by white dots and the larger by star-shaped figures.
In regard to binary stars, it should be remembered that, in many cases,
their distances and angles of position change so rapidly that any
statement concerning them remains valid only for a few years at the
most. There is also much confusion among the measurements announced by
different authorities. In general, the most recent measurements
obtainable in 1900 are given in the text, but the observer who wishes to
study close and rapid binaries will do well to revise his information
about them as frequently as possible. An excellent list of double stars
kept up to date, will be found in the annual Companion to the
Observatory, published in London.
In the lunar charts the plan of inserting the names of the principal
formations has been preferred to that usually followed, of indicating
them only by numbers, accompanied by a key list. Even in the most
detailed charts of the moon only a part of what is visible with
telescopes can be shown, and the representation, at best, must be merely
approximate. It is simply a question of what to include and what to
omit; and in the present case the probable needs of the amateur observer
have governed the selection--readiness and convenience of reference
being the chief aim.
It should, perhaps, be said here that the various chapters composing
this book--like those of "Astronomy with an Opera-glass"--were, in their
original form, with the single exception of Chapter IX, published in
Appletons' Popular Science Monthly. The author, it is needless to say,
was much gratified by the expressed wish of many readers that these
scattered papers should be revised and collected in a more permanent
form. As bearing upon the general subject of the book, a chapter has
been added, at the end, treating on the question of the existence of
planets among the stars. This also first appeared in the periodical
above mentioned.
In conclusion, the author wishes for his readers as great a pleasure in
the use of the telescope as he himself has enjoyed.
G. P. S.
BOROUGH OF BROOKLYN, NEW YORK, _January, 1901_.
CONTENTS
CHAPTER I PAGE
THE SELECTION AND TESTING OF A GLASS 1
How to get a good telescope--Difference between reflectors and
refractors--How a telescope is made achromatic--The way to test
a telescope on stars.
CHAPTER II
IN THE STARRY HEAVENS 19
Orion and its wonders, Lepus, Canis Major, Argo, Monoceros,
Canis Minor, and the Head of Hydra.
CHAPTER III
FROM GEMINI TO LEO AND ROUND ABOUT 38
The zodiacal constellations Gemini, Cancer, and Leo, and their
neighbors Auriga, the Lynx, Hydra, Sextans, and Coma Berenices.
CHAPTER IV
VIRGO AND HER NEIGHBORS 57
Crater and Corvus, Hydra, Virgo, the "Field of the Nebulae,"
Libra, Booetes, and the great Arcturus, Canes Venatici, and
Corona Borealis.
CHAPTER V
IN SUMMER STAR-LANDS 75
Scorpio and its red-green gem, Ophiuchus, Sagittarius, Scutum
Sobieskii, Capricornus, Serpens, Hercules, Draco, Aquila, and
Delphinus.
CHAPTER VI
FROM LYRA TO ERIDANUS 97
Lyra and its brilliant Vega, Cygnus, Vulpecula, Aquarius,
Equuleus, Pegasus, Cetus, and Eridanus.
CHAPTER VII
PISCES, ARIES, TAURUS, AND THE NORTHERN MARS 117
The first double star ever discovered, the Pleiades and their
photographic wonders, the Royal Family of the Sky, Andromeda,
Cassiopeia, Perseus and Cepheus, Ursa Major, Camelopardalus,
Ursa Minor, and the Pole Star.
CHAPTER VIII
SCENES ON THE PLANETS 139
Jupiter, its belts and its moons--Saturn, the ringed
planet--Saturn's moons and Roche's limit--Mars and its white
polar caps and so-called seas and continents--Venus and her
atmosphere--The peculiar rotations of Venus and Mercury.
CHAPTER IX
THE MOUNTAINS AND PLAINS OF THE MOON AND THE SPECTACLES OF THE
SUN 156
Peculiarities of the lunar landscapes--The so-called seas, the
craters, the ring mountains, the inclosed plains, the mountain
ranges, Tycho's mysterious streaks, and other lunar features
described--How to view the sun and its spots.
CHAPTER X
ARE THERE PLANETS AMONG THE STARS? 183
Significance of Dr. See's observations--Why our telescopes do
not show planets circling around distant suns--Reasons for
thinking that such planets may exist--The bearing of stellar
evolution on the question.
PLEASURES OF THE TELESCOPE
CHAPTER I
THE SELECTION AND TESTING OF A GLASS
"O telescope, instrument of much knowledge, more precious than any
scepter! Is not he who holds thee in his hand made king and lord of
the works of God?"--JOHN KEPLER.
If the pure and elevated pleasure to be derived from the possession and
use of a good telescope of three, four, five, or six inches aperture
were generally known, I am certain that no instrument of science would
be more commonly found in the homes of intelligent people. The writer,
when a boy, discovered unexpected powers in a pocket telescope not more
than fourteen inches long when extended, and magnifying ten or twelve
times. It became his dream, which was afterward realized, to possess a
more powerful telescope, a real astronomical glass, with which he could
see the beauties of the double stars, the craters of the moon, the spots
on the sun, the belts and satellites of Jupiter, the rings of Saturn,
the extraordinary shapes of the nebulae, the crowds of stars in the Milky
Way, and the great stellar clusters. And now he would do what he can to
persuade others, who perhaps are not aware how near at hand it lies, to
look for themselves into the wonder-world of the astronomers.
There is only one way in which you can be sure of getting a good
telescope. First, decide how large a glass you are to have, then go to a
maker of established reputation, fix upon the price you are willing to
pay--remembering that good work is never cheap--and finally see that the
instrument furnished to you answers the proper tests for a telescope of
its size. There are telescopes and telescopes. Occasionally a rare
combination of perfect homogeneity in the material, complete harmony
between the two kinds of glass of which the objective is composed, and
lens surfaces whose curves are absolutely right, produces a telescope
whose owner would part with his last dollar sooner than with it. Such
treasures of the lens-maker's art can not, perhaps, be commanded at
will, yet, they are turned out with increasing frequency, and the best
artists are generally able, at all times, to approximate so closely to
perfection that any shortcoming may be disregarded.
In what is said above I refer, of course, to the refracting telescope,
which is the form of instrument that I should recommend to all amateurs
in preference to the reflector. But, before proceeding further, it may
be well to recall briefly the principal points of difference between
these two kinds of telescopes. The purpose of a telescope of either
description is, first, to form an image of the object looked at by
concentrating at a focus the rays of light proceeding from that object.
The refractor achieves this by means of a carefully shaped lens, called
the object glass, or objective. The reflector, on the other hand, forms
the image at the focus of a concave mirror.
[Illustration: IMAGE AT THE FOCUS OF A LENS.]
A very pretty little experiment, which illustrates these two methods of
forming an optical image, and, by way of corollary, exemplifies the
essential difference between refracting and reflecting telescopes, may
be performed by any one who possesses a reading glass and a magnifying
hand mirror. In a room that is not too brightly illuminated pin a sheet
of white paper on the wall opposite to a window that, by preference,
should face the north, or away from the position of the sun. Taking
first the reading glass, hold it between the window and the wall
parallel to the sheet of paper, and a foot or more distant from the
latter. By moving it to and fro a little you will be able to find a
distance, corresponding to the focal length of the lens, at which a
picture of the window is formed on the paper. This picture, or image,
will be upside down, because the rays of light cross at the focus. By
moving the glass a little closer to the wall you will cause the picture
of the window to become indistinct, while a beautiful image of the
houses, trees, or other objects of the outdoor world beyond, will be
formed upon the paper. We thus learn that the distance of the image from
the lens varies with the distance of the object whose image is formed.
In precisely a similar manner an image is formed at the focus of the
object glass of a refracting telescope.
[Illustration: IMAGE AT THE FOCUS OF A CONCAVE MIRROR.]
Take next your magnifying or concave mirror, and detaching the sheet of
paper from the wall, hold it nearly in front of the mirror between the
latter and the window. When you have adjusted the distance to the focal
length of the mirror, you will see an image of the window projected upon
the paper, and by varying the distance, as before, you will be able to
produce, at will, pictures of nearer or more remote objects. It is in
this way that images are formed at the focus of the mirror of a
reflecting telescope.
Now, you will have observed that the chief apparent difference between
these two methods of forming an image of distant objects is that in the
first case the rays of light, passing through the transparent lens, are
brought to a focus on the side opposite to that where the real object
is, while in the second case the rays, being reflected from the
brilliant surface of the opaque mirror, come to a focus on the same side
as that on which the object itself is. From this follows the most
striking difference in the method of using refracting and reflecting
telescopes. In the refractor the observer looks toward the object; in
the reflector he looks away from it. Sir William Herschel made his great
discoveries with his back to the sky. He used reflecting telescopes.
This principle, again, can be readily illustrated by means of our simple
experiment with a reading glass and a magnifying mirror. Hold the
reading glass between the eye and a distant object with one hand, and
with the other hand place a smaller lens such as a pocket magnifier,
near the eye, and in line with the reading glass. Move the two carefully
until they are at a distance apart equal to the sum of the focal lengths
of the lenses, and you will see a magnified image of the distant object.
In other words, you have constructed a simple refracting telescope. Then
take the magnifying mirror, and, turning your back to the object to be
looked at, use the small lens as before--that is to say, hold it between
your eye and the mirror, so that its distance from the latter is equal
to the sum of the focal lengths of the mirror and the lens, and you will
see again a magnified image of the distant object. This time it is a
reflecting telescope that you hold in your hands.
The magnification of the image reminds us of the second purpose which is
subserved by a telescope. A telescope, whether refracting or reflecting,
consists of two essential parts, the first being a lens, or a mirror, to
form an image, and the second a microscope, called an eyepiece, to
magnify the image. The same eyepieces will serve for either the
reflector or the refractor. But in order that the magnification may be
effective, and serve to reveal what could not be seen without it, the
image itself must be as nearly perfect as possible; this requires that
every ray of light that forms the image shall be brought to a point in
the image precisely corresponding to that from which it emanates in the
real object. In reflectors this is effected by giving a parabolic form
to the concave surface of the mirror. In refractors there is a twofold
difficulty to be overcome. In the first place, a lens with spherical
surfaces does not bend all the rays that pass through it to a focus at
precisely the same distance. The rays that pass near the outer edge of
the lens have a shorter focus than that of the rays which pass near the
center of the lens; this is called spherical aberration. A similar
phenomenon occurs with a concave mirror whose surface is spherical. In
that case, as we have seen, the difficulty is overcome by giving the
mirror a parabolic instead of a spherical form. In an analogous way the
spherical aberration of a lens can be corrected by altering its curves,
but the second difficulty that arises with a lens is not so easily
disposed of: this is what is called chromatic aberration. It is due to
the fact that the rays belonging to different parts of the spectrum
have different degrees of refrangibility, or, in other words, that they
come to a focus at different distances from the lens; and this is
independent of the form of the lens. The blue rays come to a focus
first, then the yellow, and finally the red. It results from this
scattering of the spectral rays along the axis of the lens that there is
no single and exact focus where all meet, and that the image of a star,
for instance, formed by an ordinary lens, even if the spherical
aberration has been corrected, appears blurred and discolored. There is
no such difficulty with a mirror, because there is in that case no
refraction of the light, and consequently no splitting up of the
elements of the spectrum.
In order to get around the obstacle formed by chromatic aberration it is
necessary to make the object glass of a refractor consist of two lenses,
each composed of a different kind of glass. One of the most interesting
facts in the history of the telescope is that Sir Isaac Newton could see
no hope that chromatic aberration would be overcome, and accordingly
turned his attention to the improvement of the reflecting telescope and
devised a form of that instrument which still goes under his name. And
even after Chester More Hall in 1729, and John Dollond in 1757, had
shown that chromatic aberration could be nearly eliminated by the
combination of a flint-glass lens with one of crown glass, William
Herschel, who began his observations in 1774, devoted his skill entirely
to the making of reflectors, seeing no prospect of much advance in the
power of refractors.
A refracting telescope which has been freed from the effects of
chromatic aberration is called achromatic. The principle upon which its
construction depends is that by combining lenses of different dispersive
power the separation of the spectral colors in the image can be
corrected while the convergence of the rays of light toward a focus is
not destroyed. Flint glass effects a greater dispersion than crown glass
nearly in the ratio of three to two. The chromatic combination consists
of a convex lens of crown backed by a concave, or plano-concave, lens of
flint. When these two lenses are made of focal lengths which are
directly proportional to their dispersions, they give a practically
colorless image at their common focus. The skill of the telescope-maker
and the excellence of his work depend upon the selection of the glasses
to be combined and his manipulation of the curves of the lenses.
[Illustration: ACHROMATIC OBJECT GLASS.
_a_, crown glass; _b_, flint glass.]
Now, the reader may ask, "Since reflectors require no correction for
color dispersion, while that correction is only approximately effected
by the combination of two kinds of lenses and two kinds of glass in a
refractor, why is not the reflector preferable to the refractor?"
The answer is, that the refractor gives more light and better
definition. It is superior in the first respect because a lens transmits
more light than a mirror reflects. Professor Young has remarked that
about eighty-two per cent of the light reaches the eye in a good
refractor, while "in a Newtonian reflector, in average condition, the
percentage seldom exceeds fifty per cent, and more frequently is lower
than higher." The superiority of the refractor in regard to definition
arises from the fact that any distortion at the surface of a mirror
affects the direction of a ray of light three times as much as the same
distortion would do at the surface of a lens. And this applies equally
both to permanent errors of curvature and to temporary distortions
produced by strains and by inequality of temperature. The perfect
achromatism of a reflector is, of course, a great advantage, but the
chromatic aberration of refractors is now so well corrected that their
inferiority in that respect may be disregarded. It must be admitted that
reflectors are cheaper and easier to make, but, on the other hand, they
require more care, and their mirrors frequently need resilvering, while
an object glass with reasonable care never gets seriously out of order,
and will last for many a lifetime.
Enough has now, perhaps, been said about the respective properties of
object glasses and mirrors, but a word should be added concerning
eyepieces. Without a good eyepiece the best telescope will not perform
well. The simplest of all eyepieces is a single double-convex lens. With
such a lens the magnifying power of the telescope is measured by the
ratio of the focal length of the objective to that of the eye lens.
Suppose the first is sixty inches and the latter half an inch; then the
magnifying power will be a hundred and twenty diameters--i. e., the disk
of a planet, for instance, will be enlarged a hundred and twenty times
along each diameter, and its area will be enlarged the square of a
hundred and twenty, or fourteen thousand four hundred times. But in
reckoning magnifying power, diameter, not area, is always considered.
For practical use an eyepiece composed of an ordinary single lens is
seldom advantageous, because good definition can only be obtained in the
center of the field. Lenses made according to special formulae, however,
and called solid eyepieces, give excellent results, and for high powers
are often to be preferred to any other. The eyepieces usually furnished
with telescopes are, in their essential principles, compound
microscopes, and they are of two descriptions, "positive" and
"negative." The former generally goes under the name of its inventor,
Ramsden, and the latter is named after great Dutch astronomer, Huygens.
The Huygens eyepiece consists of two plano-convex lenses whose focal
lengths are in the ratio of three to one. The smaller lens is placed
next to the eye. Both lenses have their convex surfaces toward the
object glass, and their distance apart is equal to half the sum of their
focal lengths. In this kind of eyepiece the image is formed between the
two lenses, and if the work is properly done such an eyepiece is
achromatic. It is therefore generally preferred for mere seeing
purposes. In the Ramsden eyepiece two plano-convex lenses are also used,
but they are of equal focal length, are placed at a distance apart equal
to two thirds of the focal length of either, and have their convex sides
facing one another. With such an eyepiece the image viewed is beyond the
farther or field lens instead of between the two lenses, and as this
fact renders it easier to adjust wires or lines for measuring purposes
in the focus of the eyepiece, the Ramsden construction is used when a
micrometer is to be employed. In order to ascertain the magnifying power
which an eyepiece gives when applied to a telescope it is necessary to
know the equivalent, or combined, focal length of the two lenses. Two
simple rules, easily remembered, supply the means of ascertaining this.
The equivalent focal length of a negative or Huygens eyepiece is equal
to half the focal length of the larger or field lens. The equivalent
focal length of a positive or Ramsden eyepiece is equal to three fourths
of the focal length of either of the lenses. Having ascertained the
equivalent focal length of the eyepiece, it is only necessary to divide
it into the focal length of the object glass (or mirror) in order to
know the magnifying power of your telescope when that particular
eyepiece is in use.
[Illustration: NEGATIVE EYEPIECE.]
[Illustration: POSITIVE EYEPIECE.]
A first-class object glass (or mirror) will bear a magnifying power of
one hundred to the inch of aperture when the air is in good
condition--that is, if you are looking at stars. If you are viewing the
moon, or a planet, better results will always be obtained with lower
powers--say fifty to the inch at the most. And under ordinary
atmospheric conditions a power of from fifty to seventy-five to the inch
is far better for stars than a higher power. With a five-inch telescope
that would mean from two hundred and fifty to three hundred and
seventy-five diameters, and such powers should only be applied for the
sake of separating very close double stars. As a general rule, the
lowest power that will distinctly show what you desire to see gives the
best results. The experienced observer never uses as high powers as the
beginner does. The number of eyepieces purchased with a telescope should
never be less than three--a very low power--say ten to the inch; a very
high power, seventy-five or one hundred to the inch, for occasional use;
and a medium power--say forty to the inch--for general use. If you can
afford it, get a full battery of eyepieces--six or eight, or a
dozen--for experience shows that different objects require different
powers in order to be best seen, and, moreover, a slight change of power
is frequently a great relief to the eye.
There is one other thing of great importance to be considered in
purchasing a telescope--the mounting. If your glass is not well mounted
on a steady and easily managed stand, you might better have spent your
money for something more useful. I have endured hours of torment while
trying to see stars through a telescope that was shivering in the wind
and dancing to every motion of the bystanders, to say nothing of the
wriggling contortions caused by the application of my own fingers to the
focusing screw. The best of all stands is a solid iron pillar firmly
fastened into a brick or stone pier, sunk at least four feet in the
ground, and surmounted by a well-made equatorial bearing whose polar
axis has been carefully placed in the meridian. It can be readily
protected from the weather by means of a wooden hood or a rubber sheet,
while the tube of the telescope may be kept indoors, being carried out
and placed on its bearing only when observations are to be made. With
such a mounting you can laugh at the observatories with their cumbersome
domes, for the best of all observatories is the open air. But if you
dislike the labor of carrying and adjusting the tube every time it is
used, and are both fond of and able to procure luxuries, then, after
all, perhaps, you had better have the observatory, dome, draughts and
all.
The next best thing in the way of a mounting is a portable tripod stand.
This may be furnished either with an equatorial bearing for the
telescope, or an altazimuth arrangement which permits both up-and-down
and horizontal motions. The latter is cheaper than the equatorial and
proportionately inferior in usefulness and convenience. The essential
principle of the equatorial bearing is motion about two axes placed at
right angles to one another. When the polar axis is in the meridian, and
inclined at an angle equal to the latitude of the place, the telescope
can be moved about the two axes in such a way as to point to any quarter
of the sky, and the motion of a star, arising from the earthy rotation,
can be followed hour after hour without disturbing the instrument. When
thus mounted, the telescope may be driven by clockwork, or by hand with
the aid of a screw geared to a handle carrying a universal joint.
And now for testing the telescope. It has already been remarked that the
excellence of a telescope depends upon the perfection of the image
formed at the focus of the objective. In what follows I have only a
refractor in mind, although the same principles would apply to a
reflector. With a little practice anybody who has a correct eye can form
a fair judgment of the excellence of a telescopic image. Suppose we have
our telescope steadily mounted out of doors (if you value your peace of
mind you will not try to use a telescope pointed out of a window,
especially in winter), and suppose we begin our observations with the
pole star, employing a magnifying power of sixty or seventy to the inch.
Our first object is to see if the optician has given us a good glass. If
the air is not reasonably steady we had better postpone our experiment
to another night, because we shall find that the star as seen in the
telescope flickers and "boils," and behaves in so extraordinary a
fashion that there is no more definition in the image than there is
steadiness in a bluebottle buzzing on a window pane. But if the night is
a fine one the star image will be quiescent, and then we may note the
following particulars: The real image is a minute bright disk, about one
second of arc in diameter if we are using a four-and-a-half or five-inch
telescope, and surrounded by one very thin ring of light, and the
fragments, so to speak, of one or possibly two similar rings a little
farther from the disk, and visible, perhaps, only by glimpses. These
"diffraction rings" arise from the undulatory nature of light, and their
distance apart as well as the diameter of the central disk depend upon
the length of the waves of light. If the telescope is a really good one,
and both object glass and eyepiece are properly adjusted, the disk will
be perfectly round, slightly softer at the edge, but otherwise equally
bright throughout; and the ring or rings surrounding it will be exactly
concentric, and not brighter on one side than on another. Even if our
telescope were only two inches or two inches and a half in aperture we
should at once notice a little bluish star, the mere ghost of a star in
a small telescope, hovering near the polar star. It is the celebrated
"companion," but we shall see it again when we have more time to study
it. Now let us put the star out of focus by turning the focusing screw.
Suppose we turn it in such a way that the eyepiece moves slightly
outside the focus, or away from the object glass. Very beautiful
phenomena immediately begin to make their appearance. A slight motion
outward causes the little disk to expand perceptibly, and just as this
expansion commences, a bright-red point appears at the precise center of
the disk. But, the outward motion continuing, this red center
disappears, and is replaced by a blue center, which gradually expands
into a sort of flare over the middle of the disk. The disk itself has in
the mean time enlarged into a series of concentric bright rings,
graduated in luminosity with beautiful precision from center toward
circumference. The outermost ring is considerably brighter, however,
than it would be if the same gradation applied to it as applies to the
inner rings, and it is surrounded, moreover, on its outer edge by a
slight flare which tends to increase its apparent width. Next let us
return to the focus and then move the eyepiece gradually inside the
focal point or plane. Once more the star disk expands into a series of
circles, and, if we except the color phenomena noticed outside the
focus, these circles are precisely like those seen before in
arrangement, in size, and in brightness. If they were not the same, we
should pronounce the telescope to be imperfect. There is one other
difference, however, besides the absence of the blue central flare, and
that is a faint reddish edging around the outer ring when the expansion
inside the focus is not carried very far. Upon continuing to move the
eyepiece inside or outside the focus we observe that the system of rings
becomes larger, while the rings themselves rapidly increase in number,
becoming at the same time individually thinner and fainter.
[Illustration: THE STAR IMAGE.]
By studying the appearance of the star disk when in focus and of the
rings when out of focus on either side, an experienced eye can readily
detect any fault that a telescope may have. The amateur, of course, can
only learn to do this by considerable practice. Any glaring and serious
fault, however, will easily make itself manifest. Suppose, for example,
we observe that the image of a star instead of being perfectly round is
oblong, and that a similar defect appears in the form of the rings when
the eyepiece is put out of focus. We know at once that something is
wrong; but the trouble may lie either in the object glass, in the
eyepiece, in the eye of the observer himself, or in the adjustment of
the lenses in the tube. A careful examination of the image and the
out-of-focus circles will enable us to determine with which of these
sources of error we have to deal. If the star image when in focus has a
sort of wing on one side, and if the rings out of focus expand
eccentrically, appearing wider and larger on one side than on the other,
being at the same time brightest on the least expanded side, then the
object glass is probably not at right angles to the axis of the tube and
requires readjustment. That part of the object glass on the side where
the rings appear most expanded and faintest needs to be pushed slightly
inward. This can be effected by means of counterscrews placed for that
purpose in or around the cell. But it, after we have got the object
glass properly squared to the axis of the tube or the line of sight, the
image and the ring system in and out of focus still appear oblong, the
fault of astigmatism must exist either in the objective, the eyepiece,
or the eye. The chances are very great that it is the eye itself that is
at fault. We may be certain of this if we find, on turning the head so
as to look into the telescope with the eye in different positions, that
the oblong image turns with the head of the observer, keeping its major
axis continually in the same relative position with respect to the eye.
The remedy then is to consult an oculist and get a pair of cylindrical
eyeglasses. If the oblong image does not turn round with the eye, but
does turn when the eyepiece is twisted round, then the astigmatism is in
the latter. If, finally, it does not follow either the eye or the
eyepiece, it is the objective that is at fault.
But instead of being oblong, the image and the rings may be misshapen in
some other way. If they are three-cornered, it is probable that the
object glass is subjected to undue pressure in its cell. This, if the
telescope has been brought out on a cool night from a warm room, may
arise from the unequal contraction of the metal work and the glass as
they cool off. In fact, no good star image can be got while a telescope
is assuming the temperature of the surrounding atmosphere. Even the air
inclosed in the tube is capable of making much trouble until its
temperature has sunk to the level of that outside. Half an hour at least
is required for a telescope to adjust itself to out-of-door temperature,
except in the summer time, and it is better to allow an hour or two for
such adjustment in cold weather. Any irregularity in the shape of the
rings which persists after the lenses have been accurately adjusted and
the telescope has properly cooled may be ascribed to imperfections, such
as veins or spots of unequal density in the glass forming the objective.
[Illustration: THE OUT-OF-FOCUS RINGS.
1, Correct figure; 2 and 3, spherical aberration.]
The spherical aberration of an object glass may be undercorrected or
overcorrected. In the former case the central rings inside the focus
will appear faint and the outer ones unduly strong, while outside the
focus the central rings will be too bright and the outer ones too
feeble. But if the aberration is overcorrected the central rings will be
overbright inside the focus and abnormally faint outside the focus.
[Illustration: TWO VIEWS OF MARS IN 1892.
The smaller with a three-and-three-eighths-inch telescope; the larger
with a nine-inch.]
Assuming that we have a telescope in which no obvious fault is
discernible, the next thing is to test its powers in actual work. In
what is to follow I shall endeavor to describe some of the principal
objects in the heavens from which the amateur observer may expect to
derive pleasure and instruction, and which may at the same time serve
as tests of the excellence of his telescope. No one should be deterred
or discouraged in the study of celestial objects by the apparent
insignificance of his means of observation. The accompanying pictures of
the planet Mars may serve as an indication of the fact that a small
telescope is frequently capable of doing work that appears by no means
contemptible when placed side by side with that of the greater
instruments of the observatories.