30-inch for Pulkowa, the 36-inch for Lick and the
40-inch for Yerkes. Each in turn was the largest yet made, and each time
the Clarks were called upon to surpass the world's record, which they
themselves had already established. Have we at length reached the limit
in size? If we include reflectors, no, since we have mirrors of 60
inches aperture at Mt. Wilson and Cambridge, and a still larger one of
100 inches has been undertaken. It is more than doubtful, however,
whether a further increase in size is a great advantage. Much more
depends on other conditions, especially those of climate, the kind of
work to be done and, more than all, the man behind the gun. The case is
not unlike that of a battleship. Would a ship a thousand feet long
always sink one of five hundred feet? It seems as if we had nearly
reached the limit of size of telescopes, and as if we must hope for the
next improvement in some other direction.

The second great advance in astronomy originated in America, and was in
an entirely different direction, the application of photography to the
study of the stars. The first photographic image of a star was obtained
in 1850, by George P. Bond, with the assistance of Mr. J.A. Whipple, at
the Harvard College Observatory. A daguerreotype plate was placed at the
focus of the 15-inch equatorial, at that time one of the two largest
refracting telescopes in the world. An image of [Greek: alpha] Lyrae was
thus obtained, and for this Mr. Bond received a gold medal at the first
international exhibition, that at the Crystal Palace, in London, in
1851. In 1857, Mr. Bond, then Professor Bond, director of the Harvard
Observatory, again took up the matter with collodion wet plates, and in
three masterly papers showed the advantages of photography in many ways.
The lack of sensitiveness of the wet plate was perhaps the only reason
why its use progressed but slowly. Quarter of a century later, with the
introduction of the dry plate and the gelatine film, a new start was
made. These photographic plates were very sensitive, were easily
handled, and indefinitely long exposures could be made with them. As a
result, photography has superseded visual observations, in many
departments of astronomy, and is now carrying them far beyond the limits
that would have been deemed possible a few years ago.

The third great advance in astronomy is in photographing the spectra of
the stars. The first photograph showing the lines in a stellar sp