TELESCOPE (from Gk. .7-7/Xeas6a-or, tele slcopos, far-seeing, from riThe, tile, afar + crKoweiv, skopein, to look). Essentially a lens or mirror to form an image of a distant object and a miscroseope to enable the observer to examine this image in detail. The invention of the tele scope was doubtless accomplished in Holland, but there is no little confusion and controversy to be encountered in attempting to determine the origi nal inventor. Tradition has it that about the beginning of the seventeenth century one Jansen, a spectacle-maker of Middleburg, Holland, con structed a telescope about 16 inches in length which he exhibited to Prince Maurice and the Archduke Albert, who, appreciating the impor tance of the discovery, paid him a sum of money to keep it concealed. Another spectacle-maker, Lippershey, made application (1608) to the States General for a patent for a telescope, as also did Metius, a professor of mathematics, but in the former instance, at least, it was refused, as the apparatus was already known. It seems certain that the instrument was known more or less about Europe, but the honor of its invention is usually given to Galileo, who was the first to describe the instrument and exhibit it in a com plete form (May, 1609). Galileo having heard that a device to magnify distant objects had been constructed in France or Holland, immediately distant object which can be received on a screen, as in the camera obscura (q.v.). If instead of the screen we use a simple microscope or lens to observe this image which is formed in the air, we have an astronomical telescope. This will ap pear from the accompanying diagram.
C represents a convex lens, the object glass of a telescope; A is a distant object and B is a real and inverted image formed by the lens. The size of this image depends upon the ratio of the two conjugate foci or distances from the lens to the image and the object. As the distance of the lens from the object is necessarily fixed, depend ing of course on the positions of the object and the observer, in order to obtain as large an im age as possible, it is necessary to increase the focal length of the lens. From the discussion of lenses in the article on LIGHT (q.v.) it will appear that in order to obtain a large focal length for a telescope an object glass with large radii of curvature is essential. This, however,
is not feasible in actual practice, as was found by the early astronomers after experiences with telescopes having focal lengths as great as 600 feet; and lenses of shorter focal length but of improved construction were adopted. The next question involved is the size of the object glass, or, as it is technically expressed, its aperture. The larger the lens the greater will be the amount of light transmitted. The pupil of the eye through which the light producing an im age passes normally is about 1-5 of an inch in diameter, consequently as much more light will pass through the telescope as the square of the diameter of the object glass is times greater than the square of the diameter of the pupil. But if the image formed is too large, then the light will be distributed and the image will lack brilliancy and not be plainly visible. Taking these facts into consideration, it would appear that the greater the focal length of the lens and the greater its surface, the more satisfactory would be the image produced. This would be the case were it not for the fact that both spherical and chromatic aberration (q.v.) increase with the aperture of a lens, and that the larger the piece of glass, the greater the difficulty of secur ing homogeneity and freedom from imperfections. Increasing the focal length increases the diffi culties involved in mounting the telescope as well as in the manufacture of the objective itself. To observe the image formed by the objective an eyepiece is used, whose action is simply to mag nify the image.
The image furnished by the object glass is in verted and is merely magnified by the eyepiece. In what is known as the terrestrial telescope there is an additional lease or lenses added to erect or invert the image so that it will appear to the eye in its natural position. This lens merely forms a new image which is then viewed by the eyepiece as in the case of the astronomical tele scope. the action being shown in the accompany ing diagram, the erecting lens and the eyepiece being in reality a compound microscope to view the image furnished by the objective.