Two principal directions might be pointed out in which photography was
of great service to the astronomer. It enabled him within the
comparatively short time of a single exposure to secure permanently
with great exactness the relative positions of hundreds or even of
thousands of stars, or the minute features of nebulæ or other objects,
or the phenomena of a passing eclipse, a task which by means of the
eye and hand could only be accomplished, if done at all, after a very
great expenditure of time and labor. Photography put it in the power
of the astronomer to accomplish in the short span of his own life, and
so enter into their fruition, great works which otherwise must have
been passed on by him as a heritage of labor to succeeding
generations. The second great service which photography rendered was
not simply an aid to the powers the astronomer already possessed. On
the contrary, the plate, by recording light waves which were both too
small and too large to excite vision in the eye, brought him into a
new region of knowledge, such as the infra-red and the ultra-violet
parts of the spectrum, which must have remained forever unknown but
for artificial help.