[Gk. tele, far; and skopein, to see.] An instrument consisting of a tube and magnifying glasses for seeing things at a distance. In reflecting telescopes the image is formed by one or two concave mirrors, a large one at the lower end and a small one at the upper end. Sir William Herschel's telescope contained one mirror. In refracting telescopes the image is formed by refraction in an object-glass, and is magnified by an eye-glass. The largest refracting telescopes, made by Alvan S. Clark, are that at the Lick Observatory, Mount Hamilton, California, which has a 36-inch object-glass, and magnifies from 180 to 3,000 diameters ; and the Yerkes of the Chicago University, with a 40-inch object-glass and 64 feet of focal length. One of 48-inch object-glass was shown at the Paris Exposition of 1900. The Lick telescope separated the closest double stars known to us, and discovered the fifth satellite of Jupiter. Lord Rosse's telescope at Parsonstown is a reflector with 72-inch aperture. Common's reflector at Ealing is 5 feet in diameter. If the brightness of a star seen with the eye alone is one, with a 2-inch telescope it is 100 times as bright; with a 4-inch telescope it is 400 timesas bright; with an 8-inch telescope it is 1,6oo times as bright; with a 32-inch telescope it is 25,600 times as bright; with a 36-inch telescope it is 32,400 times as bright. That is, stars can be seen with the 36-inch telescope which are 30,000 times fainter than the faintest stars visible to the naked eye.
THE LICK TELESCOPE.