confined to its geometrical properties. A serious study of the
evolution of the stars must seek to determine, first of all,
what the stars really are, what their chemical constitutions
and physical conditions are; and how they are related to each
other as to their physical properties. The application of the
spectroscope has advanced our knowledge of the subject by leaps
and bounds. This wonderful instrument, assisted by the
photographic plate, enables every visible celestial body to
write its own record of the conditions existing in itself,
within limits set principally by the brightness of the body.
Such records physicists have succeeded to some extent in
duplicating in their laboratories; and the known conditions
under which the laboratory experiments have been conducted are
the Rosetta Stones which are enabling us to interpret, with
more or less success, the records written by the stars.

It is well known that the ordinary image of a star, whether
formed by the eye alone, or by the achromatic telescope and the
eye combined, contains light of an infinite variety of colors
corresponding, speaking according to the mechanical theory of
light, to waves of energy of an infinite variety of lengths
which have traveled to us from the star. In the point image of
a star, these radiations fall in a confused heap. and the
observer is unable to say that radiations corresponding to any
given wave-lengths are present or absent. When the star's light
has been passed through the prism, or diffracted from the
grating of a spectroscope, these rays are separated one from
another and arranged side by side in perfect order, ready for
the observer to survey them and to determine which ones are
present in superabundance and which other ones are lacking