The Wonders of Optics

by Fulgence Marion

Fig. 43.—Section of an Astronomical Telescope.

The convex lens which serves as an object-glass, gives at a b a reversed image of the star A B. The small convex lens which acts the part of an eye-piece, enlarged this reversed image without changing its position, and causes it to be seen in the line A´ B´. This eye-piece is fixed at the extremity of a tube, which is smaller than that containing the object-glass, and slides easily backwards and forwards from the spot where the image a b is found. The latter is an indispensable condition, for it is rare to meet two persons whose eyes are of the same focus; besides, the image a b will fall at a different spot for objects at different distances: thus, if you are looking at the moon, and suddenly turn the instrument on to a distant nebula, you will find that the eye-piece requires adjusting. In showing ordinary observers an object in the telescope, it is well to insist on their moving the eye-piece backwards and forwards until distinct vision is obtained, for it often happens that people will say they see an object quite distinctly, when it is in reality misty, and will generally refuse to allow the focus to be altered. It is very singular how human vanity or complaisance will step in when some persons are looking through a telescope. They seem to think that there is some disgrace or rudeness involved in their not being able to see what their predecessors at the instrument have seen. Poor John Leech leaves us an amusing instance of this in a comic cut inserted in one of the early numbers of our old friend Punch. A gentleman is endeavouring to show a lady a distant steamboat through a telescope, but she has it accidentally pointed at two swans that are swimming on the margin of the lake below; consequently when he asks her if she sees the steamer, she replies that “she sees it most distinctly, and there are two of them,” a pretty good proof that the instrument was not only pointed at the wrong object, but was out of focus as well.

In constructing a telescope similar to the one described above, the object-glass ought to be of considerable diameter and of long focus; the eye-piece, on the contrary, should be comparatively small and of short focus. A little consideration will show the reason of this. An object-glass of long focus will form a large image at the point a b, and the eye-piece of short focus will magnify this image more than another lens of less convexity. It is, however, on the size, length of focus, and perfection of workmanship of the objective that the excellence of the telescope depends; large object-glasses are consequently rare, and are only to be found in observatories of the first class. The object-glass of the large telescope at the Observatory at Paris is nearly fifteen inches in diameter, and the highest magnifying power capable of being employed with it is 3,000. The Observatory of Pulkowa, near St. Petersburg, possesses a similar instrument, and the Observatory at Chicago, United States, a still larger one, measuring between eighteen and nineteen inches in diameter. But the largest of all is an objective in the possession of Mr. Buckingham, an amateur astronomer, who has an observatory near London, which is twenty inches in diameter, and twenty-eight feet in focal length.

The eye-pieces of astronomical telescopes are of different powers, and are changed according to the class of object to be observed. Thus, in taking a general view of the moon, a low power would be used. If you wished to examine any particular mountain, you would raise the magnifying power by inserting a stronger eye-piece. The power used also depends on the state of the atmosphere. For instance, on warm evenings, when the air is charged with moisture, the tremulousness of the atmosphere is so great, that it is often only possible to use the very lowest power. By combining four convex lenses together, we obtain what is called a terrestrial or erecting eye-piece, which has the property of re-reversing the image formed by the objective. The eye-pieces of all telescopes for use on land or at sea are made on this principle. The same effect may be obtained, as we have already shown in [fig. 41], by using a concave lens, but in this the field of view is much diminished.

Hitherto we have only spoken of refracting telescopes or those instruments provided with a convex object-glass, to collect and refract the rays of light given off by the object we are desirous of examining; but there is another and very important class of instruments, in which the object-glass is replaced by a reflecting mirror. The first reflecting telescope was invented by Dr. Gregory, an English philosopher, about 1650. It consisted of a brass tube, at the lower extremity of which was fixed a concave mirror made of metal, and provided with a hole in its centre for the insertion of the small tube containing the eye-glass. Towards the other end of the telescope was placed a second and smaller mirror, which reflected the image formed by the large mirror, through the eye-piece to the eye. The following figure will show the path of the rays in the Gregorian telescope.

Fig. 44.—Section of the Gregorian Telescope.

The rays A B, proceeding from the object at which the instrument is pointed, are first reflected from the surface of the principal mirror M M on to the small mirror m, whence they proceed to form a magnified image at a b, which is then again enlarged by the eye-piece appearing to the eye as if placed at A´ B´. The focus in the Gregorian is altered, not by sliding the eye-piece backwards and forwards but by moving the mirror m, which is provided with a long screw, to which is attached a handle. At first sight a reflecting telescope has the appearance of a very stumpy-looking refracting instrument, but one instant’s examination will show the observer that the usual object-glass is absent at the end of the tube. In [fig. 45] we have a Gregorian telescope, mounted on a tripod stand.

Fig. 45.—Gregorian Telescope.