This marvellous instrument, we need hardly remind our readers, is of two distinct kinds--that in which light is gathered together into a focus by _refraction_, and that in which the same end is attained by _reflection_. The image formed is in each case viewed through a magnifying lens, or combination of lenses, called the eye-piece. Not for above a century after the "optic glasses" invented or stumbled upon by the spectacle-maker of Middelburg (1608) had become diffused over Europe, did the reflecting telescope come, even in England, the place of its birth, into general use. Its principle (a sufficiently obvious one) had indeed been suggested by Mersenne as early as 1639;[305] James Gregory in 1663[306] described in detail a mode of embodying that principle in a practical shape; and Newton, adopting an original system of construction, actually produced in 1668 a tiny speculum, one inch across, by means of which the apparent distance of objects was reduced thirty-nine times. Nevertheless, the exorbitantly long tubeless refractors, introduced by Huygens, maintained their reputation until Hadley exhibited to the Royal Society, January 12, 1721,[307] a reflector of six inches aperture, and sixty-two in focal length, which rivalled in performance, and of course indefinitely surpassed in manageability, one of the "aerial" kind of 123 feet.
The concave-mirror system now gained a decided ascendant, and was brought to unexampled perfection by James Short of Edinburgh during the years 1732-68. Its resources were, however, first fully developed by William Herschel. The energy and inventiveness of this extraordinary man marked an epoch wherever they were applied. His ardent desire to measure and gauge the stupendous array of worlds which his specula revealed to him, made him continually intent upon adding to their "space-penetrating power" by increasing their light-gathering surface. These, as he was the first to explain,[308] are in a constant proportion one to the other. For a telescope with twice the linear aperture of another will collect four times as much light, and will consequently disclose an object four times as faint as could be seen with the first, or, what comes to the same, an object equally bright at twice the distance. In other words, it will possess double the space-penetrating power of the smaller instrument. Herschel's great mirrors--the first examples of the giant telescopes of modern times--were then primarily engines for extending the bounds of the visible universe; and from the sublimity of this "final cause" was derived the vivid enthusiasm which animated his efforts to success.