improved until about 150,000 photographic plates, showing from
time to time the places of countless millions of stars among their
fellows are now stored at the Harvard Observatory. Not less
remarkable than this wealth of material has been the development
of skill in working it up. Some idea of the work will be obtained
by reflecting that, thirty years ago, careful study of the heavens
by astronomers devoting their lives to the task had resulted in
the discovery of some two or three hundred stars, varying in their
light. Now, at Harvard, through keen eyes studying and comparing
successive photographs not only of isolated stars, but of clusters
and agglomerations of stars in the Milky Way and elsewhere,
discoveries of such objects numbering hundreds have been made, and
the work is going on with ever-increasing speed. Indeed, the
number of variable stars now known is such that their study as
individual objects no longer suffices, and they must hereafter be
treated statistically with reference to their distribution in
space, and their relations to one another, as a census classifies
the entire population without taking any account of individuals.

The works just mentioned are concerned with the stars. But the
heavenly spaces contain nebulae as well as stars; and photography
can now be even more successful in picturing them than the stars.
A few years ago the late lamented Keeler, at the Lick Observatory,
undertook to see what could be done by pointing the Crossley
reflecting telescope at the sky and putting a sensitive
photographic plate in the focus. He was surprised to find that a
great number of nebulae, the existence of which had never before
been suspected, were impressed on the plate. Up to the present
time the positions of about 8000 of these objects have been
listed. Keeler found that there were probably 200,000 nebulae in