as sources of light. The spectrum should, therefore, consist of
some of the bright lines which were observed in the nebular
spectrum, these proceeding from the outer strata of the star;
and of a continuous spectrum made up of radiations proceeding
from the deeper strata or core of the star, in which a few dark
lines may be introduced by the absorption from those parts of
the outer gaseous strata which lie between us and the core.

A few hundred stellar spectra resembling this description are
well known, discovered mostly at the Harvard Observatory. Their
details differ greatly, but they have certain features in
common. The bright lines of helium are extremely rare in stars,
but they have been observed in a few stellar spectra. The
bright lines of nebulium have never been observed in a true
star: they and the radiations in the ultra-violet known as at
3726A, seem to be confined to the nebular state; and the
absorption lines of nebulium have never been observed in any
spectrum. As soon as the stellar state is reached nebulium is
no longer in evidence. Stellar spectra containing bright lines
seem always to include hydrogen bright lines. This is as we
should expect; hydrogen is the lightest known gas, and it is
probably the substance which can best exist in the outer strata
of stars in general. The extensive outer strata of very young
stars seem to be composed largely of hydrogen, though other
elements are in some cases present, as indicated by the weaker
bright lines in a few cases. This preference of hydrogen for
the outermost strata is illustrated by several very interesting
observations of the nebulae. The nebulium lines are relatively
strong in the central denser parts of the Orion and Trifid
nebulae, but the hydrogen bright-lines are relatively very