The smaller telescope at the top of the picture acts as a "finder"; the
field of view of the large telescope is so restricted that it is
difficult to recognise, as it were, the part of the heavens being
surveyed. The smaller telescope takes in a larger area and enables the
precise object to be examined to be easily selected.]
[Illustration: MODERN DIRECT-READING SPECTROSCOPE
(_By A. Hilger, Ltd._)
The light is brought through one telescope, is split up by the prism,
and the resulting spectrum is observed through the other telescope.]
But there is a device whereby the power of these giant instruments,
great as it is, can be still further heightened. That device is the
simple one of allowing the photographic plate to take the place of the
human eye. Nowadays an astronomer seldom spends the night with his eye
glued to the great telescope. He puts a photographic plate there. The
photographic plate has this advantage over the eye, that it builds up
impressions. However long we stare at an object too faint to be seen, we
shall never see it. With the photographic plate, however, faint
impressions go on accumulating. As hour after hour passes, the star
which was too faint to make a perceptible impression on the plate goes
on affecting it until finally it makes an impression which can be made
visible. In this way the photographic plate reveals to us phenomena in
the heavens which cannot be seen even through the most powerful
telescopes.
Telescopes of the kind we have been discussing, telescopes for exploring
the heavens, are mounted _equatorially_; that is to say, they are
mounted on an inclined pillar parallel to the axis of the earth so that,
by rotating round this pillar, the telescope is enabled to follow the
apparent motion of a star due to the rotation of the earth. This motion
is effected by clock-work, so that, once adjusted on a star, and the
clock-work started, the telescope remains adjusted on that star for any
length of time that is desired. But a great official observatory, such
as Greenwich Observatory or the Observatory at Paris, also has _transit_
instruments, or telescopes smaller than the equatorials and without the
same facility of movement, but which, by a number of exquisite
refinements, are more adapted to accurate measurements. It is these
instruments which are chiefly used in the compilation of the _Nautical
Almanac_. They do not follow the apparent motions of the stars. Stars
are allowed to drift across the field of vision, and as each star
crosses a small group of parallel wires in the eye-piece its precise
time of passage is recorded. Owing to their relative fixity of position
these instruments can be constructed to record the _positions_ of stars
with much greater accuracy than is possible to the more general and
flexible mounting of equatorials. The recording of transit is
comparatively dry work; the spectacular element is entirely absent;
stars are treated merely as mathematical points. But these observations
furnish the very basis of modern mathematical astronomy, and without
them such publications as the _Nautical Almanac_ and the _Connaissance
du Temps_ would be robbed of the greater part of their importance.
§ 2