eye, outside the tube, in which case it is called the Newtonian
telescope; or a mirror at R may be placed perpendicularly, and send
the rays through [Page 45] an opening in the mirror at M. This form
is called the Gregorian telescope. Or the mirror M may be slightly
inclined to the coming rays, so as to bring the point F entirely
outside the tube, in which case it is called the Herschelian
telescope. In either case the image may be magnified, as in the
refracting telescope.

[Illustration: Fig. 14.--Reflecting Telescope.]

Reflecting telescopes are made of all sizes, up to the Cyclopean
eye of the one constructed by Lord Rosse, which is six feet in
diameter. The form of instrument to be preferred depends on the
use to which it is to be put. The loss of light in passing through
glass lenses is about two-tenths. The loss by reflection is often
one-half. In view of this peculiarity and many others, it is held
that a twenty-six-inch refractor is fully equal to any six-foot
reflector.

The mounting of large telescopes demands the highest engineering
ability. The whole instrument, with its vast weight of a twenty-six-inch
glass lens, with its accompanying tube and appurtenances, must be
pointed as nicely as a rifle, and held as steadily as the axis
of the globe. To give it the required steadiness, the foundation
on which it is placed is sunk deep in the earth, far from rail or
other roads, and no part of the observatory is allowed to touch
this support. When a star is once found, the earth swiftly rotates
the telescope away from it, and it passes out of the field. To
avoid this, clock-work is so arranged that the great telescope