ng type. The two calcium lines H and K
(396.9, 393.4 [mu][mu]) strongly pronounced.

Among the stars of this type are found a great many bright stars
(compare the third chapter), such as Polaris, Canopus, Procyon.

_Type G_ (Sun type). Numerous metallic lines together with relatively
faint hydrogen lines.

To this class belong the sun, Capella, [alpha] Centauri and other bright
stars.

_Type K._ The hydrogen lines still fainter. The K-line attains its
maximum intensity (is not especially pronounced in the figure of plate
III).

This is, next to the A-type, the most numerous type (1142 stars) among
the bright stars.

We find here [gamma] Andromedae, [beta] Aquilae, Arcturus, [alpha]
Cassiopeiae, Pollux and Aldebaran, which last forms a transition to the
next type.

_Type M._ The spectrum is banded and belongs to SECCHI's third type. The
flutings are due to titanium oxide.

Only 190 of the stars visible to the naked eye belong to this type.
Generally they are rather faint, but we here find Betelgeuze, [alpha]
Herculis, [beta] Pegasi, [alpha] Scorpii (Antares) and most variables of
long period, which form a special sub-type _Md_, characterized by bright
hydrogen lines together with the flutings.

Type M has two other sub-types Ma and Mb.

_Type N_ (SECCHI's fourth type). Banded spectra. The flutings are due to
compounds of carbon.

Here are found only faint stars. The total number is 241. All are red.
27 stars having this spectrum are variables of long period of the same
type as Md.

The spectral types may be summed up in the following way:--

White stars:--SECCHI's type I:--Harvard B and A,
Yellow " :-- " " II:-- " F, G and K,
Red " :-- " " III:-- " M,
" " :-- " " IV:-- " N.

The Harvard astronomers do not confine themselves to the types mentioned
above, but fill up the intervals between the types with sub-types which
are designated by the name of the type followed by a numeral 0, 1, 2,
..., 9. Thus the sub-types between A and F have the designations A0, A1,
A2, ..., A9, F0, &c. Exceptions are made as already indicated, for the
extreme types O and M.

11. _Spectral index._ It may be gathered from the above description that
the definition of the types implies many vague moments. Especially in
regard to the G-type are very different definitions indeed accepted,
even at Harvard.[6] It is also a defect that the definitions do not
direc