The telescope may be likened to a wondrous cyclopean eye, endued with
superhuman power, by which the astronomer extends the reach of his
vision to the further heavens, and surveys galaxies and universes
compared with which the solar system is but an atom floating in the air.
The transit may be compared to the measuring rod which he lays from
planet to planet, and from star to star, to ascertain and mark off the
heavenly spaces, and transfer them to his note-book; the clock is that
marvelous apparatus by which he equalizes and divides into nicely
measured parts a portion of that unconceived infinity of duration,
without beginning and without end, in which all existence floats as on a
shoreless and bottomless sea.
In the contrivance and the execution of these instruments, the utmost
stretch of inventive skill and mechanical ingenuity has been put forth.
To such perfection have they been carried, that a single second of
magnitude or space is rendered a distinctly visible and appreciable
quantity. "The arc of a circle," says Sir J. Herschell, "subtended by
one second, is less than the 200,000th part of the radius, so that on a
circle of six feet in diameter, it would occupy no greater linear extent
than 1-5700 part of an inch, a quantity requiring a powerful microscope
to be discerned at all."[A] The largest body in our system, the sun,
whose real diameter is 882,000 miles, subtends, at a distance of
95,000,000 miles, but an angle of little more than 32; while so
admirably are the best instruments constructed, that both in Europe
and America a satellite of Neptune, an object of comparatively
inconsiderable diameter, has been discovered at a distance of 2,850
millions of miles.
[Footnote A: _Outlines_, § 131.]