s
without a telescope. A slit and an eye-piece are alone needed to view
it, and absorption of light by glass lenses is obviated--an advantage
especially sensible in dealing with the ultra- or infra-visible rays.

The high qualities of Rowland's great photographic map of the solar
spectrum were thus based upon his previous improvement of the
instrumental means used in its execution. The amount of detail shown in
it is illustrated by the appearance on the negatives of 150 lines
between H and K; and many lines depict themselves as double which, until
examined with a concave grating, had passed for one and indivisible. A
corresponding hand-drawing, for which M. Thollon received in 1886 the
Lalande Prize, exhibits, not the diffractive, but the prismatic spectrum
as obtained with bisulphide of carbon prisms of large dispersive power.
About one-third of the visible gamut of the solar radiations (A to _b_)
is covered by it; it includes 3,200 lines, and is over ten metres
long.[1655] The grating is an expensive tool in the way of light. Where
there is none to spare, its advantages must be foregone. They could not,
accordingly, be turned to account in stellar spectroscopy until the Lick
telescope was at hand to supply more abundant material for research. By
the use thus made possible of Rowland's gratings, Professor Keeler was
able to apply enormous dispersion to the rays of stars and nebulae, and
so to attain a previously unheard-of degree of accuracy in their
measurement. His memorable detection of nebular movement in line of
sight ensued as a consequence. Professor Campbell, his successor, has
since obtained, by the same means, the first satisfactory photographs of
stellar diffraction-spectra.

The means at the disposal of astronomers have not multiplied faster than
the tasks imposed upon them. Looking back to the year 1800, we cannot
fail to be astonished at the change. The comparatively simple and serene
science of the heavenly bodies known to our predecessors, almost perfect
so far as it went, incurious of what lay beyond its grasp, has developed
into a body of manifold powers and parts, each with its separate mode
and means of growth, full of strong vitality, but animated by a restless
and unsatisfied spirit, haunted by the sense of problems unsolved, and
tormented by conscious impotence to sound the immensities it perpetually
confronts.

Knowledge might be said, when the _Mecanique Celeste_ issued from the
press, to be