ly illustrated by a set of original spectrographic plates, while
precision was added to the adopted classification by the separation of
helium from hydrogen stars. The spectrum of the exotic substance
terrestrially captured in 1895 is conspicuous by absorption, as Vogel,
Lockyer, and Deslandres promptly recognised in a considerable number of
white stars, among them the Pleiades and most of the brilliants in
Orion. Mr. McClean, whose valuable spectrographic survey of the heavens
was completed at the Cape in 1897, found reason to conclude that they
are in the first stage of development from gaseous nebulae;[1384] and in
this the Tulse Hill investigators unhesitatingly concur.

The strongest evidence for the primitive state of white stars is found
in their nebular relations. The components of groups, still involved and
entangled with "silver braids" of cosmic mist, show, perhaps invariably,
spectra of the helium type, occasionally crossed by bright rays.
Possibly all such stars have passed through a bright-line stage; but
further evidence on the point is needed. Relative density furnishes
another important test of comparative age, and Sirian stars are, on the
whole, undoubtedly more bulky proportionately to their mass than solar
stars. The rule, however, seems to admit of exceptions; hence the change
from one kind of spectrum to the other is not inevitably connected with
the attainment of a particular degree of condensation. There is reason
to believe that it is anticipated in the more massive globes, despite
their comparatively slow cooling, as a consequence of the greater power
of gravity over their investing vaporous envelopes. This conclusion is
enforced by the relations of double-star spectra. The fact that, in
unequal pairs, the chief star most frequently shows a solar, its
companion a Sirian, spectrum can scarcely be otherwise explained than by
admitting that, while the sequence of types is pursued in an invariable
order, it is pursued much more rapidly in larger than in small orbs. It
need not, indeed, be supposed that all stars are identical in
constitution, and present identical life-histories.[1385]
Individualities in the one, and divergencies in the other, must be
allowed for. Yet the main track is plainly continuous, and leads by
insensible gradations from nebulae through helium stars to the Sirian,
and onward to the solar type, whence, by an inevitable transition,
fluted, or "Antarian,"[1386] spectra develop.