f star-distances should be but few in number. The most
complete catalogues are those of BIGOURDAN in the Bulletin astronomique
XXVI (1909), of KAPTEYN and WEERSMA in the publications of Groningen Nr.
24 (1910), embracing 365 stars, and of WALKEY in the "Journal of the
British Astronomical Association XXVII" (1917), embracing 625 stars.
Through the spectroscopic method of ADAMS it will be possible to enlarge
this number considerably, so that the distance of all stars, for which
the spectrum is well known, may be determined with fair accuracy. ADAMS
has up to now published 1646 parallax stars.

23. _Proper motions._ An excellent catalogue of the proper motions of
the stars is LEWIS BOSS's "Preliminary General Catalogue of 6188 stars"
(1910) (B. P. C.). It contains the proper motions of all stars down to
the sixth magnitude (with few exceptions) and moreover some fainter
stars. The catalogue is considered by the editor only as a preliminary
to a greater catalogue, which is to embrace some 25000 stars and is now
nearly completed.

24. _Visual magnitudes._ The Harvard observatory has, under the
direction of PICKERING, made its principal aim to study the
magnitudes of the stars, and the history of this observatory is at the
same time the history of the treatment of this problem. PICKERING, in
the genuine American manner, is not satisfied with the three thirds of
the sky visible from the Harvard observatory, but has also founded a
daughter observatory in South America, at Arequipa in Peru. It is
therefore possible for him to publish catalogues embracing the whole
heaven from pole to pole. The last complete catalogue (1908) of the
magnitudes of the stars is found in the "Annals of the Harvard
Observatory T. 50" (H. 50). It contains 9110 stars and can be considered
as complete to the magnitude 6m.5. To this catalogue are generally
referred the magnitudes which have been adopted at the Observatory of
Lund, and which are treated in these lectures.

A very important, and in one respect even still more comprehensive,
catalogue of visual magnitudes is the "Potsdam General Catalogue" (P. G.
C.) by MUeLLER and KEMPF, which was published simultaneously with H. 50.
It contains the magnitude of 14199 stars and embraces all stars on the
northern hemisphere brighter than 7m.5 (according to B. D.). We have
already seen that the zero-point of H. 50 and P. G. C. is somewhat
different and that the magnitudes in P. G. C. must be increased