ch stronger in these
stars than in the sun, while many new lines also appear. These
dissimilarities are, however, of less importance than the peculiar
absorption bands in the red, yellow, and green parts of the spectrum,
overlying the metallic lines, and being sharply defined on the side
towards the violet and shading off gradually towards the red end of the
spectrum. Bands of this kind belong to chemical combinations, and this
appears to show that somewhere in the atmospheres of these distant suns
the temperature is low enough to allow stable chemical combinations to
be formed. The most important star of this kind is Betelgeuze or a
Orionis, the red star of the first magnitude in the shoulder of Orion;
but it is of special importance to note that many variable stars of long
period have spectra of Type III.a. Sir Norman Lockyer predicted in 1887
that bright lines, probably of hydrogen, would eventually be found to
appear at the maximum of brightness, when the smaller swarm is supposed
to pass through the larger one, and this was soon afterwards confirmed
by the announcement that Professor Pickering had found a number of
hydrogen lines bright on photographs, obtained at Harvard College
Observatory, of the spectrum of the remarkable variable, Mira Ceti, at
the time of maximum. Professor Pickering has since then reported that
bright lines have been found on the plates of forty-one previously known
variables of this class, and that more than twenty other stars have been
detected as variables by this peculiarity of their spectrum; that is,
bright lines being seen in them suggested that the stars were variable,
and further photometric investigations corroborated the fact.

The second subdivision (III.b) contains only comparatively faint stars,
of which none exceed the fifth magnitude, and is limited to a small
number of red stars. The strongly marked bands in their spectra are
sharply defined and dark on the red side, while they fade away gradually
towards the violet, exactly the reverse of what we see in the spectra of
III.a. These bands appear to arise from the absorption due to
hydrocarbon vapours present in the atmospheres of these stars; but there
are also some lines visible which indicate the presence of metallic
vapours, sodium being certainly among these. There can be little doubt
that these stars represent the last stage in the life of a sun, when it
has cooled down considerably and is not very far from actual extinct