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]
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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 Wilson but the Lick Observatory to which we have to give the honour. The eminent director of this observatory, W. W. Campbell, has in a high degree developed the accuracy in the determination of radial velocities and has moreover carried out such determinations in a large scale. The “Bulletin” No. 229 (1913) of the Lick Observatory contains the radial velocity of 915 stars. At the observatory of Lund, where as far as possible card catalogues of the attributes of the stars are collected, Gyllenberg has made a catalogue of this kind for the radial velocities. The total number of stars in this catalogue now amounts to 1640.[14]
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Finally I shall briefly mention some comprehensive works on more special questions regarding the stellar system.
On variable stars there is published every year by Hartwig in the “Vierteljahrschrift der astronomischen Gesellschaft” a catalogue of all known variable stars with needful information about their minima &c. This is the completest and most reliable of such catalogues, and is always up to date. A complete historical catalogue of the variables is given in “Geschichte und Literatur des Lichtwechsels der bis Ende 1915 als sicher veränderlich anerkannten Sterne nebst einem Katalog der Elemente ihres Lichtwechsels” von G. Müller und E. Hartwig. Leipzig 1918, 1920.
On nebulae we have the excellent catalogues of Dreyer, the “New General Catalogue” (N. G. C.) of 1890 in the “Memoirs of the Astronomical Society” vol. 49, the “Index catalogues” (I. C.) in the same memoirs, vols. 51 and 59 (1895 and 1908). These catalogues contain all together 13226 objects.
Regarding other special attributes I refer in the first place to the important Annals of the Harvard Observatory. Other references will be given in the following, as need arises.