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 Müller 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 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. Müller 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°. In all, 3522 stars. The photographic magnitudes are however not reduced for the zero-point (compare [§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° 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 [§15] above.