Contributions Volume 6, Nos. 106-123 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1916 Excerpt: ...in some detail in Sections VII and XII. Palmer divides the stars visible on one of Keeler's Lick Observatory plates into two groups with respect to magnitude, counting 1016 bright and 4466 faint stars, with very few of intermediate brightness. It is suggested that the law of distribution is different for the two groups. Perrine goes farther and finds this separation into two classes of magnitudes a marked characteristic in 30 of the 35 globular clusters examined by him (including Messier 13), and states further: "Perhaps a third of the whole number of stars in each cluster lies between the eleventh and thirteenth photographic magnitudes, while almost all of the remainder are very faint, being about the sixteenth magnitude." There can be little doubt that this last statement is quantitatively very much in error. The idea of the division of cluster magnitudes into two distinct groups has always been prominent, especially in visual work, but there is room for a strong suspicion that in most cases it is only a subjective phenomenon, and that in the others it is not so conspicuous as commonly supposed. Some of the Mount Wilson observations are being made on clusters in which this condition is said to be the most marked. 1 Loc. cit. a Loc. cit. and Lick Observatory Bulletins, 5, 102, 1909. The integrated spectrum of Messier 13 has been observed by Fath at the Lick Observatory1 and at Mount Wilson.2 The work by Pease on the spectra of individual stars in the cluster will be discussed in a later section of this paper. H. THE OBSERVATIONAL DATA The magnitudes contained in the catalogue (Section V) are derived from the complete measurement of two pairs of plates, made on different nights and each consisting of one isochromatic and one ordinary photographic ...