vision. Secchi knew little or nothing concerning stars whose
spectra contain bright lines, except as to the isolated
bright-line spectra of a few nebulae, and as to the bright
hydrogen lines in gamma Cassiopeia, and his system did not
include these.

One of the most comprehensive investigations ever undertaken by
a single institution was that of classifying the stars as to
their spectra, over the entire sky, substantially down to and
including the stars of eighth magnitude, by the Harvard College
Observatory, as a memorial to the lamented Henry Draper.
Professor Pickering and his associates have formulated a
classification system which is now in universal use. It starts
with the bright-line nebulae, passes to the bright-line stars,
and then to the stars in which the helium absorption lines are
prominent. The latter are called the helium stars, or
technically the Class B stars. The next main division includes
the stars in which hydrogen absorption is prominent, called
Class A. Classes B and A are blue stars. Then follows in
succession Class F, composed of bluish-yellow stars, which is
in a sense a transition class between the hydrogen stars and
those resembling our Sun, the latter called Class G. The Class
G stars are yellow. Class K stars are the yellowish-red; Class
M, the red; and Class N, the extremely red. Each of these
classes has several subdivisions which make the transition from
one main class to the next main class fairly gradual, and not
per saltum; though it should be said that the relationship of
Class N to Class M spectra is not clear. The illustration, Fig.
17, brings out the principal features of the spectra of Classes
B to M. The spectrum becomes more complicated as we pass from