the heavens capable of being photographed with the Crossley
reflector. But the work of taking these photographs is so great,
and the number of reflecting telescopes which can be applied to it
so small, that no one has ventured to seriously commence it. It is
worthy of remark that only a very small fraction of these objects
which can be photographed are visible to the eye, even with the
most powerful telescope.

This demonstration of what the reflecting telescope can do may be
regarded as one of the most important discoveries of our time as
to the capabilities of astronomical instruments. It has long been
known that the image formed in the focus of the best refracting
telescope is affected by an imperfection arising from the
different action of the glasses on rays of light of different
colors. Hence, the image of a star can never be seen or
photographed with such an instrument, as an actual point, but only
as a small, diffused mass. This difficulty is avoided in the
reflecting telescope; but a new difficulty is found in the bending
of the mirror under the influence of its own weight. Devices for
overcoming this had been so far from successful that, when Mr.
Crossley presented his instrument to the Lick Observatory, it was
feared that little of importance could be done with it. But as
often happens in human affairs outside the field of astronomy,
when ingenious and able men devote their attention to the careful
study of a problem, it was found that new results could be
reached. Thus it was that, before a great while, what was supposed
to be an inferior instrument proved not only to have qualities not
before suspected, but to be the means of making an important
addition to the methods of astronomical investigation.