TELESCOPE (Gr. tde-skopos, far seeing) consists essentially of a lens or mirror, to form, within our reach, an image of a distant object; and a microscope (q.v.), to examine this image in detail. Its invention is ascribed to various individuals living about the end of the 16th c.; but there is no doubt that Galilei (q.v.) was the first to apply it td any purpose other than the gratification of mere curiosity.
The space at our disposal will not allow of our entering into any minute details, so we propose to give: first, a general idea of the mode in which a telescope acts, in the course of which we shall incidentally show how the magnifying power and the bright ness of the image depend on the dimensions of the various parts of the instrument; wond, to point out the various causes of imperfection, which in all telescopes are unavoidable, and how these are reduced to as small an amount as possible; third, to mention the most important of the many forms which have been devised, and the processes by which these delicate instruments are practically constructed.
When a lens is employed, as in a camera obscura, to form an image of an object, as AB in fig. 1, the distance of the image from the lens depends on the focal length of the lens, and also on the distance of the object. Practically, with telescopes, the dis tance of the image from the lens is, on account of the remoteness of the object, the focal length of the lens, Also the image of any point, A, of the object lies in the prolonga tion of the line joining A with the center, C, of the lens. Join AC, and produce it to a, Ca being made equal to the focal length of the lens, a is the point at which the image of A is formed. Similarly at b the image of B is formed. Thus the image is inverted, and, seen from C, the image and the object subtend equal angles, or look equally large. When a concave mirror forms an image, the effect is as in fig. 2, where C is now the center of the sphere of which the mirror is a portion. When the object, AB, is at a great distance, the image. ab, is inverted, and is formed half way between C and the mirror. As before, object and image subtend equal angles at C. In order to see these
images, the eye must be placed at some such point as E in each of the figures.
So much for the formation within our reach of an image of a distant body. We must next show the action of a lens when employed to magnify this image. When an object, as ab in fig. 3, is placed rather nearer to a lens than its focal length, rays which pass from the object through the lens appear to have come, not from the object, but from an enlarged image as a/3, at a greater distance from the lens—but subtending, as before, the same angle at the center, c, of the lens. In practice, the lens is so adjusted as to form the image, afi, at a distance of about 10 in. from c, in which case the eye sees it most distinctly, and the distance of ab from the lens is then (practically) the focal length of the lens.
We now combine the first and third diagrams, and we have the common astronomical telescope. The magnifying power is obviously to be measured by the increase in the angle which is subtended by the image al3 (fig. 4), over that which is subtended by the object, AB. The angle at C Ls the measure of the apparent size of the object; that at c, of the apparent size of the image. And it is easy to see from the quadrilateral Ca,eb in the figure that these angles are inversely as the sides Ca and ac. (For instance, if Ca have six times the length of ac, the angle at C will be only of that at c.) Hence the magnifying power is to be found by dividing the focal length of the object-lens by that of the eye-lens. In practice, the lenses are so mounted in tubes that their distance may be increased or diminished at pleasure. When the object, AB, comes nearer the observer, its image, ab, is formed further from the lens C. Hence, for near objects, the telescope requires to be pulled out. Again, the distance of most distinct vision differs for different people, so that even when AB is at a fixed distance, short-sighted and long-sighted eyes require the eye-lens to be removed from, or advanced to, ab, so that for each eye aft may be formed at the distance at which it can be most distinctly seen.