by
-0m.16 if they are to be reduced to the Harvard scale. The difference
between the two catalogues however is due to some extent to the colour
of the stars, as has been shown by Messrs. MUeLLER and KEMPF.

25. _Photographic magnitudes._ Our knowledge of this subject is still
rather incomplete. The most comprehensive catalogue is the
"Actinometrie" by SCHWARZSCHILD (1912), containing the photographic
magnitudes of all stars in B. D. down to the magnitude 7m.5 between the
equator and a declination of +20 deg.. In all, 3522 stars. The photographic
magnitudes are however not reduced for the zero-point (compare Sec.6).

These is also a photometric photographic catalogue of the stars nearest
to the pole in PARKHURST's "Yerkes actinometry" (1912),[11] which
contains all stars in B. D. brighter than 7m.5 between the pole and 73 deg.
northern declination. The total number of stars is 672.

During the last few years the astronomers of Harvard and Mount Wilson
have produced a collection of "standard photographic magnitudes" for
faint stars. These stars, which are called the _polar sequence_,[12] all
lie in the immediate neighbourhood of the pole. The list is extended
down to the 20th magnitude. Moreover similar standard photographic
magnitudes are given in H. A. 71, 85 and 101.

A discussion of the _colour-index_ (_i.e._, the difference between the
photographic and the visual magnitudes) will be found in L. M. II, 19.
When the visual magnitude and the type of spectrum are known, the
photographic magnitude may be obtained, with a generally sufficient
accuracy, by adding the colour-index according to the table 1 in Sec.15
above.

26. _Stellar spectra._ Here too we find the Harvard Observatory to be
the leading one. The same volume of the Annals of the Harvard
Observatory (H. 50) that contains the most complete catalogue of visual
magnitudes, also gives the spectral types for all the stars there
included, _i.e._, for all stars to 6m.5. Miss CANNON, at the Harvard
Observatory, deserves the principal credit for this great work. Not
content with this result she is now publishing a still greater work
embracing more than 200000 stars. The first four volumes of this work
are now published and contain the first twelve hours of right
ascension, so that half the work is now printed.[13]

27. _Radial velocity._ In this matter, again, we find America to be the
leading nation, though, this time, it is not the Harvard or the Mount
W