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. 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 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] 28. Finally I shall briefly mention some comprehensive works on more special questions regarding the stellar system.