But, unfortunately, the pole is not a visible point. The observer has therefore to refer his position to the direction of gravity, which is done by a very ingenious use of a basin of quicksilver. The basin is set on a firm sup port on the ground under the telescope, and the latter is pointed directly downward. By mounting up to the eye-piece and looking down, the observer looks perpendicularly into the basin of mercury. A combination of re flectors is then arranged in the eye-piece of the telescope so that he can, at the same time, see the threads in his eye-piece and the images of these threads as reflected from the basin of mercury. When a telescope is so adjusted that the image and the thread coincide, he knows that the line of sight of his telescope is truly vertical. He then reads the microscope of his circle and so determines what the reading of his circle is for the vertical position. He knows that if the telescope is pointed at the zenith, the reading will be different by exactly He thus determines the exact distance at which the heavenly bodies crossed the me ridian north or south of his zenith. From this, the determination of the declination is, in principle, a simple matter.
The Equatorial.— One of the most import ant arrangements of nature with which the astronomer has to deal is the diurnal motion. This takes place so slowly that, in looking at the stars, we do not notice it unless we watch for some time. But, if we point a telescope at a heavenly body, it magnifies the diurnal motion as much as it does the object. The result is that such a body, seen in a fixed tele scope, is continually traveling across the field of view, and the instrument has to be moved continually to keep up with it.
In order to avoid this inconvenience, it is necessary that, if measures are to be made up on the body, or if it is to be continuously studied, the telescope must move to correspond. This is brought about by mounting it upon an axis parallel to that of the earth, and there fore oblique to the horizon, called the polar axis. The inclination to the horizon must be equal to the latitude of the place. All great telescopes are thus mounted. The way in which this is done will be seen by the ac companying picture of the great telescope mounted at Pulkova, Russia. In order to keep the telescope pointed at the object it must be turned upon the polar axis by clock work, moving it steadily at a rate equal to the diurnal motion of the object observed. In reality the telescope is then pointed in a fixed direction, the motion of the earth being simply neutralized by the clock-work of the telescope carrying the latter in the opposite direction. The equatorial telescope must also have a second axis, called the declination axis, in order that it may be pointed at stars in different declinations. The direction is deter mined by circles attached to the telescope, which show, at any time, to what declination on the celestial sphere the instrument is pointed. By a combination of contrivances the astronomer can point his telescope by day at any star bright enough to be visible in it; or, by night at any object visible to the naked eye, of which he knows the right ascension and declination. He first turns his until one divided circle corresponds to the declination of the star, and then clamps it in that position. Then, looking at his sidereal clock and taking the difference between the sidereal time and the right ascension of the star, he turns his telescope on the polar axis until the other circle shows the correct point ing. Then he starts the clock-work which sets the telescope in motion, and looking into the eye-piece, sees the required object. Every large telescope is also supplied with a finder. This consists of a smaller telescope fastened to the larger one in such a way that the cen tre of the field of view is the same in both. But the finder has a lower magnifying power, and therefore a much larger field. into it and recognizing the object he wishes to observe, the observer moves the telescope until the object is seen on the cross-threads of the finder. Then he knows that it is in the field of view of the large telescope.
Application of Photography to Astron omy.— From the time that photographic methods were introduced, the idea of taking pictures of the heavenly bodies by such methods must have occurred to astronomers.
About the year 1840, Prof. J. W. Draper of New York put this method into practice by taking a daguerreotype of the moon. Shortly after our present system of photography was devised, several American astronomers car ried the experiment yet farther. Notable among these were G. P. Bond, first assistant and afterward director of the Harvard Ob servatory; and L. M. Rutherford of New York, who was the possessor of an excellent telescope, and brought the method to a high state of perfection.
The. principle on which a photograph of a heavenly body is taken is extremely simple. A telescope is pointed at the body so that the image of the latter is formed in its focus. A sensitized plate is placed in the focus and ex posed for the necessary time. This may be only a fraction of a second, or it may be sev eral hours. Unless the exposure is very brief, it is necessary that the telescope shall be kept in motion, so as to follow the object in its apparent diurnal course. When the exposure is completed, the image is developed in the usual way. In photographing, the ordinary telescope, as used for eye observations, is not well suited to the purpose, for the reason that the chromatic aberration is not the same for the visual and for the photographic rays. It is necessary to have a somewhat stronger crown lens or a weaker flint lens if a tele scope is to be used in photographing than if it is to he used by the eye. But the necessity of having telescopes of the two kinds is now, to a certain extent, done away with by the use of sensitized plates which are especially sensi tive to the visual rays. By putting in an ab sorbing screen through which the rays must pass before they reach the focus and which allows only the visual rays to pass, very ac curate photographs can be taken by the plates. This defect is felt only in the refracting tele scope. A reflecting telescope brings all the rays, of whatever color, to one and the same focus, and therefore may be used either for photographing or for seeing. Improvements made in recent times in the sensitiveness of photographic plates have given an enormous extension to this method in astronomy. It is now found that celestial objects completely invisible to ordinary vision can be photo graphed. While only a few thousand nebula have been catalogued as visible to the naked eye, it is found that there are hundreds of thousands which admit of being determined by photography.
Photography is now employed for two dis tinct purposes. The first is simply that of forming a picture of the sky, or rather of the stars in the sky. For this purpose the best telescope is one as large as can conveniently be obtained, but of short focal length. A great enterprise in this direction was started to 1::7 by an association of astronomers who met at the Paris Observatory, and put into operation a plan of photographing the entire. heavens on from 10,000 to 20,000 plates, each two degrees square. The labor of measuring these plates and of afterward reducing the measures from them is so great that at least 30 years will probably elapse before this cat alogue is completed and published. A similar object is reached on a different system at the Harvard Observatory. There photographs are being constantly taken with telescopes much shorter than those used for the international chart. In this way new stars are from time to time discovered and variations in the light of different stars are brought out. The other purpose is that of exact measurement. When the astronomer had to determine the respec tive distances of stars in the same field of view he has hitherto generally depended on the filar or other micrometer. The use of this instrument is laborious. When the photo graphic method is used he simply takes a pic ture of the stars he wishes to measure and at any convenient time, places it under a measuring engine supplied with sliding micro scopes, and measures off the distance on his negative. The result of these two applications is that photography is now slowly supplanting eye observations in an important fraction of the astronomical work of the world.