nation 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.

It seems probable that the seven-foot telescope constructed by him in
1775--that is within little more than a year af