instrument, the spectroscope, has been developed, the extent of
whose revelations we are just beginning to learn, although it has
been more than thirty years in use. The application of photography
has been so extended that, in some important branches of
astronomical work, the observer simply photographs the phenomenon
which he is to study, and then makes his observation on the
developed negative.

The world of astronomy is one of the busiest that can be found to-
day, and the writer proposes, with the reader's courteous consent,
to take him on a stroll through it and see what is going on. We
may begin our inspection with a body which is, for us, next to the
earth, the most important in the universe. I mean the sun. At the
Greenwich Observatory the sun has for more than twenty years been
regularly photographed on every clear day, with the view of
determining the changes going on in its spots. In recent years
these observations have been supplemented by others, made at
stations in India and Mauritius, so that by the combination of all
it is quite exceptional to have an entire day pass without at
least one photograph being taken. On these observations must
mainly rest our knowledge of the curious cycle of change in the
solar spots, which goes through a period of about eleven years,
but of which no one has as yet been able to establish the cause.

This Greenwich system has been extended and improved by an
American. Professor George E. Hale, formerly Director of the
Yerkes Observatory, has devised an instrument for taking
photographs of the sun by a single ray of the spectrum. The light
emitted by calcium, the base of lime, and one of the substances
most abundant in the sun, is often selected to impress the plate.