ope--so far as the expression has any
definite meaning--is to be measured by the diameter of its object-glass.
There has, indeed, been some honourable rivalry between the various
civilised nations as to which should possess the greatest refracting
telescope. Among the notable instruments that have been successfully
completed is that erected in 1881 by Sir Howard Grubb, of Dublin, at the
splendid observatory at Vienna. Its dimensions may be estimated from the
fact that the object-glass is two feet and three inches in diameter.
Many ingenious contrivances help to lessen the inconvenience incident to
the use of an instrument possessing such vast proportions. Among them we
may here notice the method by which the graduated circles attached to
the telescope are brought within view of the observer. These circles are
necessarily situated at parts of the instrument which lie remote from
the eye-piece where the observer is stationed. The delicate marks and
figures are, however, easily read from a distance by a small auxiliary
telescope, which, by suitable reflectors, conducts the rays of light
from the circles to the eye of the observer.

[Illustration: Fig. 5.--Principle of Herschel's Refracting Telescope.]

Numerous refracting telescopes of exquisite perfection have been
produced by Messrs. Alvan Clark, of Cambridgeport, Boston, Mass. One of
their most famous telescopes is the great Lick Refractor now in use on
Mount Hamilton in California. The diameter of this object-glass is
thirty-six inches, and its focal length is fifty-six feet two inches. A
still greater effort has recently been made by the same firm in the
refractor of forty inches aperture for the Yerkes Observatory of the
University of Chicago. The telescope, which is seventy-five feet in
length, is mounted under a revolving dome ninety feet in diameter, and
in order to enable the observer to reach the eye-piece without using
very large step-ladders, the floor of the room can be raised and lowered
through a range of twenty-two feet by electric motors. This is shown in
Fig. 4, while the south front of the Yerkes Observatory is represented
in Fig. 6.

[Illustration: Fig. 6.--South Front of the Yerkes Observatory, Chicago.

(_From the Astrophysical Journal, Vol. vi., No. 1._)]

[Illustration: Fig. 7.--Lord Rosse's Telescope.]

Within the last few years two fine telescopes have been added to the
instrumental equipment of the Royal Observatory, Greenwich, both by Sir