immense, and the highest skill was required for devising instruments
and methods to read off the star positions from the plates.

Then we have the Harvard College collection of photographic plates,
always being automatically added to; and their annex at Arequipa in
Peru.

Such catalogues vary in their degree of accuracy; and fundamental
catalogues of standard stars have been compiled. These require
extension, because the differential methods of the heliometer and the
camera cannot otherwise be made absolute.

The number of stars down to the fourteenth magnitude may be taken at
about 30,000,000; and that of all the stars visible in the greatest
modern telescopes is probably about 100,000,000.

_Nebulae and Star-clusters._--Our knowledge of nebulae really dates from
the time of W. Herschel. In his great sweeps of the heavens with his
giant telescopes he opened in this direction a new branch of
astronomy. At one time he held that all nebulae might be clusters of
innumerable minute stars at a great distance. Then he recognised the
different classes of nebulae, and became convinced that there is a
widely-diffused "shining fluid" in space, though many so-called nebulae
could be resolved by large telescopes into stars. He considered that
the Milky Way is a great star cluster, whose form may be conjectured
from numerous star-gaugings. He supposed that the compact "planetary
nebulae" might show a stage of evolution from the diffuse nebulae, and
that his classifications actually indicate various stages of
development. Such speculations, like those of the ancients about the
solar system, are apt to be harmful to true progress of knowledge