The New Student's Reference Work/Telescope
Tel′escope, an instrument for viewing distant objects, was invented, apparently, by Franz Lippershey, a Dutch spectacle-maker at Middelburg, in 1608. Shortly after this date Galileo heard of Lippershey’s instrument and made one for himself. With this instrument he discovered four satellites of Jupiter and the variable phases of Venus, thus completely establishing the heliocentric views of Copernicus. The telescope employed by Galileo essentially was an opera-glass, consisting of a small double-concave (or negative) lens immediately in front of the eye, combined with a double-convex (or positive) lens at some distance in front of the eye. (See Opera-Glass). But there are two reasons why the Galilean telescope is no longer used: It has a very limited field, i. e., no large extent of the object can be viewed with it at one time. (2) One cannot use a micrometer eye-piece with it. (See Micrometer.)
The telescope almost universally employed in modern times is the astronomical telescope made of two converging lenses, as shown in Fig. 1. Let it be borne in mind that the purpose of the telescope is twofold: To gather a large amount of light into a small bundle of rays, so that these can all enter the pupil of the eye and thus make any luminous point appear much brighter than it would to the unaided eye; (2) to magnify the angle subtended by two luminous points and thus make them appear farther apart than when seen by the naked eye. How the first purpose is accomplished will be evident from Fig. 1, where wf1 indicates the plane wave-front of the light coming from a distant star. The effect of the object-lens, L, is to convert this parallel bundle of rays into a conical bundle, having its apex or focus at F. The light leaving F is collected by the eye-lens, E, and converted again into a parallel beam, wf4, which is small enough, say of an inch, to enter the pupil of the eye. In this way nearly all the light which falls on the objective is made to enter the eye, making the star appear much brighter through the telescope than without it. How the second purpose of the telescope is accomplished will be clear from Fig. 2, which is the same as Fig. 1, except that it shows light coming from two distant stars, say a double star. Each star will send a beam of parallel rays to the objective, L. But the two beams of parallel rays which emerge from the eye-lens make a much larger angle with each other than do the two beams of parallel rays which enter the objective. The telescope thus magnifies the angular distance between two stars. Since the human eye cannot distinguish two points as separate unless they subtend an angle of two minutes, the telescope enables us to recognize many stars as double when to the naked eye they appear as single.
The magnifying power of a telescope is numerically equal to the ratio of the diameters of the incident and emergent beams.
The best specimens of the astronomical telescope in existence are the 36-inch glass of Lick Observatory and the 40-inch glass of Yerkes Observatory, both made by the late Alvan G. Clark. On good nights these instruments permit the use of a magnifying power of from 2,000 to 3,000.
The reflecting telescope is little used and can therefore not be discussed within the limits of this article. Since, however, the reflector is interesting from a historical point of view, the student will do well to consult chapter vi. of Miss Clerke’s History of Astronomy.