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 glass 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 glass, 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 glass 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 glass 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.
[Illustration 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 glass 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 glass 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-glass in that the coating of silver is put on the front surface, so that the light does not pass through the glass. 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 glass 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 class.
Next to the moon, Saturn is among the most beautiful of celestial objects. Its aspect, however, varies with its position in its orbit. 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-glass 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 "crape 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 latitude 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.
[Illustration with caption: THE GREAT REFRACTOR OF THE NATIONAL OBSERVATORY AT WASHINGTON]
Of the satellites of Saturn the brightest is Titan, 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 orbit, can thereby render a valuable service to astronomy.