CHAPTER I.

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[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.]