[Footnote: The following is a rough rule for getting an idea of the price of an achromatic objective, made to order, of the finest quality.

Take the cube of the diameter in inches, or, which is the same thing, calculate the contents of a cubical box which would hold a sphere of the same diameter as the clear aperture of the gla.s.s. The price of the gla.s.s will then range from $1 to $1.75 for each cubic inch in this box.

For example, the price of a four-inch objective will probably range from $64 to $112. Very small object-gla.s.ses of one or two inches may be a little higher than would be given by this rule. Instruments which are not first-cla.s.s, but will answer most of the purposes of the amateur, are much cheaper.]

[Ill.u.s.tration with caption: A VERY PRIMITIVE MOUNTING FOR A TELESCOPE.]

The tube for the telescope may be made of paper, by pasting a great number of thicknesses around a long wooden cylinder. A yet better tube is made of a simple wooden box. The best material, however, is metal, because wood and pasteboard are liable both to get out of shape, and to swell under the influence of moisture. Tin, if it be of sufficient thickness, would be a very good material. The brighter it is kept, the better. The work of fitting the objective into one end of a tin tube of double thickness, and properly adjusting it, will probably be quite within the powers of the ordinary amateur. The fitting of the eye-piece into the other end of the tube will require some skill and care both on his own part and that of his tinsmith.

Although the construction of the eye-piece is much easier than that of the objective, since the same accuracy in adjusting the curves is not necessary, yet the price is lower in a yet greater degree, so that the amateur will find it better to buy than to make his eye-piece, unless he is anxious to test his mechanical powers. For a telescope which has no micrometer, the Huyghenian or negative eye-piece, as it is commonly called, is the best. As made by Huyghens, it consists of two plano-convex lenses, with their plane sides next the eye, as shown in the figure.

[Ill.u.s.tration with caption: THE HUYGHENIAN EYE-PIECE.]

So far as we have yet described our telescope it is optically complete.

If it could be used as a spy-gla.s.s by simply holding it in the hand, and pointing at the object we wish to observe, there would be little need of any very elaborate support. But if a telescope, even of the smallest size, is to be used with regularity, a proper "mounting" is as essential as a good instrument. Persons unpractised in the use of such instruments are very apt to underrate the importance of those accessories which merely enable us to point the telescope. An idea of what is wanted in the mounting may readily be formed if the reader will try to look at a star with an ordinary good-sized spy-gla.s.s held in the hand, and then imagine the difficulties he meets with multiplied by fifty.

The smaller and cheaper telescopes, as commonly sold, are mounted on a simple little stand, on which the instrument admits of a horizontal and vertical motion. If one only wants to get a few glimpses of a celestial object, this mounting will answer his purpose. But to make anything like a study of a celestial body, the mounting must be an equatorial one; that is, one of the axes around which the telescope moves must be inclined so as to point towards the pole of the heavens, which is near the polar star. This axis will then make an angle with the horizon equal to the lat.i.tude of the place. The telescope cannot, however, be mounted directly on this axis, but must be attached to a second one, itself fastened to this one.

[Ill.u.s.tration with caption: SECTION OF THE PRIMITIVE MOUNTING. P P.

Polar axis, bearing a fork at the upper end A. Declination axis pa.s.sing through the fork E. Section of telescope tube C. Weight to balance the tube.]

When mounted in this way, an object can be followed in its diurnal motion from east to west by turning on the polar axis alone. But if the greatest facility in use is required, this motion must be performed by clock-work. A telescope with this appendage will commonly cost one thousand dollars and upward, so that it is not usually applied to very small ones.

We will now suppose that the reader wishes to purchase a telescope or an object-gla.s.s for himself, and to be able to judge of its performance. He must have the object-gla.s.s properly adjusted in its tube, and must use the highest power; that is, the smallest eye-piece, which he intends to use in the instrument. Of course he understands that in looking directly at a star or a celestial object it must appear sharp in outline and well defined. But without long practice with good instruments, this will not give him a very definite idea. If the person who selects the telescope is quite unpractised, it is possible that he can make the best test by ascertaining at what distance he can read ordinary print. To do this he should have an eye-piece magnifying about fifty times for each inch of aperture of the telescope. For instance, if his telescope is three inches clear aperture, then his eye-piece should magnify one hundred and fifty times; if the aperture is four inches, one magnifying two hundred times may be used. This magnifying power is, as a general rule, about the highest that can be advantageously used with any telescope. Supposing this magnifying power to be used, this page should be legible at a distance of four feet for every unit of magnifying power of the telescope. For example, with a power of 100, it should be legible at a distance of 400 feet; with a power of 200, at 800 feet, and so on. To put the condition into another shape: if the telescope will read the print at a distance of 150 feet for each inch of aperture with the best magnifying power, its performance is at least not very bad. If the magnifying power is less than would be given by this rule, the telescope should perform a little better; for instance, a three-inch telescope with a power of 60 should make this page legible at a distance of 300 feet, or four feet for each unit of power.

The test applied by the optician is much more exact, and also more easy. He points the instrument at a star, or at the reflection of the sun's rays from a small round piece of gla.s.s or a globule of quicksilver several hundred yards away, and ascertains whether the rays are all brought to a focus. This is not done by simply looking at the star, but by alternately pushing the eye-piece in beyond the point of distinct vision and drawing it out past the point. In this way the image of the star will appear, not as a point, but as a round disk of light. If the telescope is perfect, this disk will appear round and of uniform brightness in either position of the eye-piece. But if there is any spherical aberration or differences of density in different parts of the gla.s.s, the image will appear distorted in various ways. If the spherical aberration is not correct, the outer rim of the disk will be brighter than the centre when the eye-piece is pushed in, and the centre will be the brighter when it is drawn out. If the curves of the gla.s.s are not even all around, the image will appear oval in one or the other position. If there are large veins of unequal density, wings or notches will be seen on the image. If the atmosphere is steady, the image, when the eye-piece is pushed in, will be formed of a great number of minute rings of light. If the gla.s.s is good, these rings will be round, unbroken, and equally bright. We present several figures showing how these spectral images, as they are sometimes called, will appear; first, when the eye-piece is pushed in, and secondly, when it is drawn out, with telescopes of different qualities.

We have thus far spoken only of the refracting telescope, because it is the kind with which an observer would naturally seek to supply himself.

At the same time there is little doubt that the construction of a reflector of moderate size is easier than that of a corresponding refractor. The essential part of the reflector is a slightly concave mirror of any metal which will bear a high polish. This mirror may be ground and polished in the same way as a lens, only the tool must be convex.

[Ill.u.s.tration with caption: SPECTRAL IMAGES OF STARS; THE UPPER LINE SHOWING HOW THEY APPEAR WITH THE EYE-PIECE PUSHED IN, THE LOWER WITH THE EYE-PIECE DRAWN OUT.

A The telescope is all right B Spherical aberration shown by the light and dark centre C The objective is not spherical but elliptical D The gla.s.s not uniform--a very bad and incurable case E One side of the objective nearer than the other. Adjust it]

Of late years it has become very common to make the mirror of gla.s.s and to cover the reflecting face with an exceedingly thin film of silver, which can be polished by hand in a few minutes. Such a mirror differs from our ordinary looking-gla.s.s in that the coating of silver is put on the front surface, so that the light does not pa.s.s through the gla.s.s.

Moreover, the coating of silver is so thin as to be almost transparent: in fact, the sun may be seen through it by direct vision as a faint blue object. Silvered gla.s.s reflectors made in this way are extensively manufactured in London, and are far cheaper than refracting telescopes of corresponding size. Their great drawback is the want of permanence in the silver film. In the city the film will ordinarily tarnish in a few months from the sulphurous vapors arising from gaslights and other sources, and even in the country it is very difficult to preserve the mirror from the contact of everything that will injure it. In consequence, the possessor of such a telescope, if he wishes to keep it in order, must always be prepared to resilver and repolish it. To do this requires such careful manipulation and management of the chemicals that it is hardly to be expected that an amateur will take the trouble to keep his telescope in order, unless he has a taste for chemistry as well as for astronomy.

The curiosity to see the heavenly bodies through great telescopes is so wide-spread that we are apt to forget how much can be seen and done with small ones. The fact is that a large proportion of the astronomical observations of past times have been made with what we should now regard as very small instruments, and a good deal of the solid astronomical work of the present time is done with meridian circles the apertures of which ordinarily range from four to eight inches. One of the most conspicuous examples in recent times of how a moderate-sized instrument may be utilized is afforded by the discoveries of double stars made by Mr. S. W. Burnham, of Chicago.

Provided with a little six-inch telescope, procured at his own expense from the Messrs. Clark, he has discovered many hundred double stars so difficult that they had escaped the scrutiny of Maedler and the Struves, and gained for himself one of the highest positions among the astronomers of the day engaged in the observation of these objects. It was with this little instrument that on Mount Hamilton, California--afterward the site of the great Lick Observatory--he discovered forty-eight new double stars, which had remained unnoticed by all previous observers. First among the objects which show beautifully through moderate instruments stands the moon. People who want to see the moon at an observatory generally make the mistake of looking when the moon is full, and asking to see it through the largest telescope. Nothing can then be made out but a brilliant blaze of light, mottled with dark spots, and crossed by irregular bright lines. The best time to view the moon is near or before the first quarter, or when she is from three to eight days old. The last quarter is of course equally favorable, so far as seeing is concerned, only one must be up after midnight to see her in that position. Seen through a three or four inch telescope, a day or two before the first quarter, about half an hour after sunset, and with a magnifying power between fifty and one hundred, the moon is one of the most beautiful objects in the heavens.

Twilight softens her radiance so that the eye is not dazzled as it will be when the sky is entirely dark. The general aspect she then presents is that of a hemisphere of beautiful chased silver carved out in curious round patterns with a more than human skill. If, however, one wishes to see the minute details of the lunar surface, in which many of our astronomers are now so deeply interested, he must use a higher magnifying power. The general beautiful effect is then lessened, but more details are seen. Still, it is hardly necessary to seek for a very large telescope for any investigation of the lunar surface. I very much doubt whether any one has ever seen anything on the moon which could not be made out in a clear, steady atmosphere with a six-inch telescope of the first cla.s.s.

Next to the moon, Saturn is among the most beautiful of celestial objects. Its aspect, however, varies with its position in its...o...b..t.

Twice in the course of a revolution, which occupies nearly thirty years, the rings are seen edgewise, and for a few days are invisible even in a powerful telescope. For an entire year their form may be difficult to make out with a small telescope. These unfavorable conditions occur in 1907 and 1921. Between these dates, especially for some years after 1910, the position of the planet in the sky will be the most favorable, being in northern declination, near its perihelion, and having its rings widely open. We all know that Saturn is plainly visible to the naked eye, shining almost like a star of the first magnitude, so that there is no difficulty in finding it if one knows when and where to look. In 1906-1908 its oppositions occur in the month of September. In subsequent years, it will occur a month later every two and a half years. The ring can be seen with a common, good spy-gla.s.s fastened to a post so as to be steady. A four or five-inch telescope will show most of the satellites, the division in the ring, and, when the ring is well opened, the curious dusky ring discovered by Bond. This "c.r.a.pe ring," as it is commonly called, is one of the most singular phenomena presented by that planet.

It might be interesting to the amateur astronomer with a keen eye and a telescope of four inches aperture or upward to frequently scrutinize Saturn, with a view of detecting any extraordinary eruptions upon his surface, like that seen by Professor Hall in 1876. On December 7th of that year a bright spot was seen upon Saturn's equator. It elongated itself from day to day, and remained visible for several weeks. Such a thing had never before been known upon this planet, and had it not been that Professor Hall was engaged in observations upon the satellites, it would not have been seen then. A similar spot on the planet was recorded in 1902, and much more extensively noticed. On this occasion the spot appeared in a higher lat.i.tude from the planet's equator than did Professor Hall's. At this appearance the time of the planet's revolution on its axis was found to be somewhat greater than in 1876, in accordance with the general law exhibited in the rotations of the sun and of Jupiter. Notwithstanding their transient character, these two spots have afforded the only determination of the time of revolution of Saturn which has been made since Herschel the elder.

[Ill.u.s.tration with caption: THE GREAT REFRACTOR OF THE NATIONAL OBSERVATORY AT WASHINGTON]

Of the satellites of Saturn the brightest is t.i.tan, which can be seen with the smallest telescope, and revolves around the planet in fifteen days. Iapetus, the outer satellite, is remarkable for varying greatly in brilliancy during its revolution around the planet. Any one having the means and ability to make accurate photometrical estimates of the light of this satellite in all points of its...o...b..t, can thereby render a valuable service to astronomy.

The observations of Venus, by which the astronomers of the last century supposed themselves to have discovered its time of rotation on its axis, were made with telescopes much inferior to ours. Although their observations have not been confirmed, some astronomers are still inclined to think that their results have not been refuted by the failure of recent observers to detect those changes which the older ones describe on the surface of the planet. With a six-inch telescope of the best quality, and with time to choose the most favorable moment, one will be as well equipped to settle the question of the rotation of Venus as the best observer. The few days near each inferior conjunction are especially to be taken advantage of.

The questions to be settled are two: first, are there any dark spots or other markings on the disk? second, are there any irregularities in the form of the sharp cusps? The central portions of the disk are much darker than the outline, and it is probably this fact which has given rise to the impression of dark spots. Unless this apparent darkness changes from time to time, or shows some irregularity in its outline, it cannot indicate any rotation of the planet. The best time to scrutinize the sharp cusps will be when the planet is nearly on the line from the earth to the sun. The best hour of the day is near sunset, the half-hour following sunset being the best of all. But if Venus is near the sun, she will after sunset be too low down to be well seen, and must be looked at late in the afternoon.

The planet Mars must always be an object of great interest, because of all the heavenly bodies it is that which appears to bear the greatest resemblance to the earth. It comes into opposition at intervals of a little more than two years, and can be well seen only for a month or two before and after each opposition. It is hopeless to look for the satellites of Mars with any but the greatest telescopes of the world.

But the markings on the surface, from which the time of rotation has been determined, and which indicate a resemblance to the surface of our own planet, can be well seen with telescopes of six inches aperture and upward. One or both of the bright polar spots, which are supposed to be due to deposits of snow, can be seen with smaller telescopes when the situation of the planet is favorable.

The case is different with the so-called ca.n.a.ls discovered by Schiaparelli in 1877, which have ever since excited so much interest, and given rise to so much discussion as to their nature. The astronomer who has had the best opportunities for studying them is Mr. Percival Lowell, whose observatory at Flaggstaff, Arizona, is finely situated for the purpose, while he also has one of the best if not the largest of telescopes. There the ca.n.a.ls are seen as fine dark lines; but, even then, they must be fifty miles in breadth, so that the word "ca.n.a.l" may be regarded as a misnomer.

Although the planet Jupiter does not present such striking features as Saturn, it is of even more interest to the amateur astronomer, because he can study it with less optical power, and see more of the changes upon its surface. Every work on astronomy tells in a general way of the belts of Jupiter, and many speculate upon their causes. The reader of recent works knows that Jupiter is supposed to be not a solid ma.s.s like the earth, but a great globe of molten and vaporous matter, intermediate in const.i.tution between the earth and the sun. The outer surface which we see is probably a hot ma.s.s of vapor hundreds of miles deep, thrown up from the heated interior. The belts are probably cloudlike forms in this vaporous ma.s.s. Certain it is that they are continually changing, so that the planet seldom looks exactly the same on two successive evenings. The rotation of the planet can be very well seen by an hour's watching. In two hours an object at the centre of the disk will move off to near the margin.

The satellites of this planet, in their ever-varying phases, are objects of perennial interest. Their eclipses may be observed with a very small telescope, if one knows when to look for them. To do this successfully, and without waste of time, it is necessary to have an astronomical ephemeris for the year. All the observable phenomena are there predicted for the convenience of observers. Perhaps the most curious observation to be made is that of the shadow of the satellite crossing the disk of Jupiter. The writer has seen this perfectly with a six-inch telescope, and a much smaller one would probably show it well.

With a telescope of this size, or a little larger, the satellites can be seen between us and Jupiter. Sometimes they appear a little brighter than the planet, and sometimes a little fainter.

Of the remaining large planets, Mercury, the inner one, and Ura.n.u.s and Neptune, the two outer ones, are of less interest than the others to an amateur with a small telescope, because they are more difficult to see.

Mercury can, indeed, be observed with the smallest instrument, but no physical configurations or changes have ever been made out upon his surface. The question whether any such can be observed is still an open one, which can be settled only by long and careful scrutiny. A small telescope is almost as good for this purpose as a large one, because the atmospheric difficulties in the way of getting a good view of the planet cannot be lessened by an increase of telescopic power.

Ura.n.u.s and Neptune are so distant that telescopes of considerable size and high magnifying power are necessary to show their disks. In small telescopes they have the appearance of stars, and the observer has no way of distinguishing them from the surrounding stars unless he can command the best astronomical appliances, such as star maps, circles on his instrument, etc. It is, however, to be remarked, as a fact not generally known, that Ura.n.u.s can be well seen with the naked eye if one knows where to look for it. To recognize it, it is necessary to have an astronomical ephemeris showing its right ascension and declination, and star maps showing where the parallels of right ascension and declination lie among the stars. When once found by the naked eye, there will, of course, be no difficulty in pointing the telescope upon it.

Of celestial objects which it is well to keep a watch upon, and which can be seen to good advantage with inexpensive instruments, the sun may be considered as holding the first place. Astronomers who make a specialty of solar physics have, especially in this country, so many other duties, and their view is so often interrupted by clouds, that a continuous record of the spots on the sun and the changes they undergo is hardly possible. Perhaps one of the most interesting and useful pieces of astronomical work which an amateur can perform will consist of a record of the origin and changes of form of the solar spots and faculae. What does a spot look like when it first comes into sight?

Does it immediately burst forth with considerable magnitude, or does it begin as the smallest visible speck, and gradually grow? When several spots coalesce into one, how do they do it? When a spot breaks up into several pieces, what is the seeming nature of the process? How do the groups of brilliant points called faculae come, change, and grow? All these questions must no doubt be answered in various ways, according to the behavior of the particular spot, but the record is rather meagre, and the conscientious and industrious amateur will be able to amuse himself by adding to it, and possibly may make valuable contributions to science in the same way.

Still another branch of astronomical observation, in which industry and skill count for more than expensive instruments, is the search for new comets. This requires a very practised eye, in order that the comet may be caught among the crowd of stars which flit across the field of view as the telescope is moved. It is also necessary to be well acquainted with a number of nebulae which look very much like comets. The search can be made with almost any small telescope, if one is careful to use a very low power. With a four-inch telescope a power not exceeding twenty should be employed. To search with ease, and in the best manner, the observer should have what among astronomers is familiarly known as a "broken-backed telescope." This instrument has the eye-piece on the end of the axis, where one would never think of looking for it. By turning the instrument on this axis, it sweeps from one horizon through the zenith and over to the other horizon without the observer having to move his head. This is effected by having a reflector in the central part of the instrument, which throws the rays of light at right angles through the axis.

[Ill.u.s.tration: THE "BROKEN-BACKED COMET-SEEKER"]

How well this search can be conducted by observers with limited means at their disposal is shown by the success of several American observers, among whom Messrs. W. R. Brooks, E. E. Barnard, and Lewis Swift are well known. The cometary discoveries of these men afford an excellent ill.u.s.tration of how much can be done with the smallest means when one sets to work in the right spirit.

The larger number of wonderful telescopic objects are to be sought for far beyond the confines of the solar system, in regions from which light requires years to reach us. On account of their great distance, these objects generally require the most powerful telescopes to be seen in the best manner; but there are quite a number within the range of the amateur. Looking at the Milky Way, especially its southern part, on a clear winter or summer evening, tufts of light will be seen here and there. On examining these tufts with a telescope, they will be found to consist of congeries of stars. Many of these groups are of the greatest beauty, with only a moderate optical power. Of all the groups in the Milky Way the best known is that in the sword-handle of Perseus, which may be seen during the greater part of the year, and is distinctly visible to the naked eye as a patch of diffused light. With the telescope there are seen in this patch two closely connected cl.u.s.ters of stars, or perhaps we ought rather to say two centres of condensation.

Another object of the same cla.s.s is Proesepe in the constellation Cancer. This can be very distinctly seen by the naked eye on a clear moonless night in winter or spring as a faint nebulous object, surrounded by three small stars. The smallest telescope shows it as a group of stars.

Of all stellar objects, the great nebula of Orion is that which has most fascinated the astronomers of two centuries. It is distinctly visible to the naked eye, and may be found without difficulty on any winter night. The three bright stars forming the sword-belt of Orion are known to every one who has noticed that constellation. Below this belt is seen another triplet of stars, not so bright, and lying in a north and south direction. The middle star of this triplet is the great nebula. At first the naked eye sees nothing to distinguish it from other stars, but if closely scanned it will be seen to have a hazy aspect. A four-inch telescope will show its curious form. Not the least interesting of its features are the four stars known as the "Trapezium," which are located in a dark region near its centre. In fact, the whole nebula is dotted with stars, which add greatly to the effect produced by its mysterious aspect.

The great nebula of Andromeda is second only to that of Orion in interest. Like the former, it is distinctly visible to the naked eye, having the aspect of a faint comet. The most curious feature of this object is that although the most powerful telescopes do not resolve it into stars, it appears in the spectroscope as if it were solid matter shining by its own light.

The above are merely selections from the countless number of objects which the heavens offer to telescopic study. Many such are described in astronomical works, but the amateur can gratify his curiosity to almost any extent by searching them out for himself.

[Ill.u.s.tration with caption: NEBULA IN ORION]

Ever since 1878 a red spot, unlike any before noticed, has generally been visible on Jupiter. At first it was for several years a very conspicuous object, but gradually faded away, so that since 1890 it has been made out only with difficulty. But it is now regarded as a permanent feature of the planet. There is some reason to believe it was occasionally seen long before attention was first attracted to it.

Doubtless, when it can be seen at all, practice in observing such objects is more important than size of telescope.

VI

WHAT THE ASTRONOMERS ARE DOING