ut the intervention of the atmosphere.

In addition to instruments like those already indicated, astronomers
have other means of following the motions of the heavenly bodies. Within
the last fifteen years photography has commenced to play an important
part in practical astronomy. This beautiful art can be utilised for
representing many objects in the heavens by more faithful pictures than
the pencil of even the most skilful draughtsman can produce. Photography
is also applicable for making charts of any region in the sky which it
is desired to examine. When repeated pictures of the same region are
made from time to time, their comparison gives the means of ascertaining
whether any star has moved during the interval. The amount and direction
of this motion may be ascertained by a delicate measuring apparatus
under which the photographic plate is placed.

If a refracting telescope is to be used for taking celestial
photographs, the lenses of the object-glass must be specially designed
for this purpose. The rays of light which imprint an image on the
prepared plate are not exactly the same as those which are chiefly
concerned in the production of the image on the retina of the human eye.
A reflecting mirror, however, brings all the rays, both those which are
chemically active and those which are solely visual, to one and the
same focus. The same reflecting instrument may therefore be used either
for looking at the heavens or for taking pictures on a photographic
plate which has been substituted for the observer's eye.

A simple portrait camera has been advantageously employed for obtaining
striking photographs of larger areas of the sky than can be grasped in a
long telescope; but for purposes of accurate measurement those taken
with the latter are incomparably better.

It is needless to say that the photographic apparatus, whatever it may
be, must be driven by delicately-adjusted clockwork to counteract the
apparent daily motion of the stars caused by the rotation of the earth.
The picture would otherwise be spoiled, just as a portrait is ruined if
the sitter does not remain quiet during the exposure.

Among the observatories in the United Kingdom the Royal Observatory at
Greenwich is of course the most famous. It is specially remarkable among
all the similar institutions in the world for the continuity of its
labours for several generations. Greenwich Observatory was founded in
1675 for the promotion of astronomy and