elves and as a waste of the chief
object of astrophysicists' greed--light. Reflectors, then, are in this
respect specially adapted to photographic and spectrographic use. But
they have a countervailing drawback. The penalties imposed by bigness
are for them peculiarly heavy. Perfect definition becomes with
increasing size, more and more difficult to attain; once attained, it
becomes more and more difficult to keep. For the huge masses of material
employed to form great object-glasses or mirrors tend with every
movement to become deformed by their own weight. Now, the slightest
bending of a mirror is fatal to its performance, the effect being
doubled by reflection; while in a lens alteration of figure is
compensated by the equal and contrary flexures of the opposing surfaces,
so that the emergent beams pursue much the same paths as if the curves
of the refracting medium had remained theoretically perfect. For this
reason work of precision must remain the province of refracting
telescopes, although great reflectors retain the primacy in the
portraiture of the heavenly bodies, as well as in certain branches of
spectroscopy. Professor Hale, accordingly, summarised a valuable
discussion on the subject by asserting[1637] "that the astrophysicist
may properly consider the reflector to be an even more important part of
his instrumental equipment than the refractor." A new era in its
employment west of the Atlantic opened with the transfer from Halifax to
Mount Hamilton of the Crossley reflector. Its prerogatives in nebular
photography were splendidly indicated in 1899 by Professor Keeler's
exquisite and searching portrayals of the cloud-worlds of space, and
those obtained two years later, with a similar, though smaller,
instrument, by Professor Ritchey of the Yerkes Observatory, were fully
comparable with them. The performances of the Yerkes 5-foot reflector
still belong to the future.

Ambition as regards telescopic power is by no means yet satisfied. Nor
ought it to be. The advance of astrophysical researches of all kinds
depends largely upon light-grasp. For the spectroscopic examination of
stars, for the measurement of their motions in the line of sight, for
the discovery and study of nebulae, for stellar and nebular photography,
the cry continually is "more light." There is no enterprising head of an
observatory but must feel cramped in his designs if he can command no
more than 14 or 15 inches of aperture, and he aspires