ens. The dome is capable of revolving so that the opening may be
turned towards that part of the sky where the object happens to be
situated. The next view (Fig. 3) exhibits a section through the dome,
showing the machinery by which the attendant causes it to revolve, as
well as the telescope itself. The eye of the observer is placed at the
eye-piece, and he is represented in the act of turning a handle, which
has the power of slowly moving the telescope, in order to adjust the
instrument accurately on the celestial body which it is desired to
observe. The two lenses which together form the object-glass of this
instrument are twelve inches in diameter, and the quality of the
telescope mainly depends on the accuracy with which these lenses have
been wrought. The eye-piece is a comparatively simple matter. It
consists merely of one or two small lenses; and various eye-pieces can
be employed, according to the magnifying power which may be desired. It
is to be observed that for many purposes of astronomy high magnifying
powers are not desirable. There is a limit, too, beyond which the
magnification cannot be carried with advantage. The object-glass can
only collect a certain quantity of light from the star; and if the
magnifying power be too great, this limited amount of light will be
thinly dispersed over too large a surface, and the result will be found
unsatisfactory. The unsteadiness of the atmosphere still further limits
the extent to which the image may be advantageously magnified, for every
increase of power increases in the same degree the atmospheric
disturbance.

A telescope mounted in the manner here shown is called an _equatorial_.
The convenience of this peculiar style of supporting the instrument
consists in the ease with which the telescope can be moved so as to
follow a star in its apparent journey across the sky. The necessary
movements of the tube are given by clockwork driven by a weight, so
that, once the instrument has been correctly pointed, the star will
remain in the observer's field of view, and the effect of the apparent
diurnal movement will be neutralised. The last refinement in this
direction is the application of an electrical arrangement by which the
driving of the instrument is controlled from the standard clock of the
observatory.

[Illustration: Fig. 4.--The Telescope at Yerkes Observatory, Chicago.

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

The power of a refracting telesc