MOUNT WILSON OBSERVATORY. 243 



greater galactic system. Stars of spectral type B are especially suitable 

 for determining the extent and position of this local cluster. Assisted 

 by Miss Richmond, Mr. Shapley has studied diagrammatically the 

 distribution of B stars on the surface of the sky, showing that those 

 fainter than apparent magnitude 7 are practically all concentrated 

 narrowly to the galactic circle, while all but a few of those brighter 

 than apparent magnitude 6 show independence of the galactic circle 

 and a distinct concentration to the equator of the flattened local 

 cluster. In the part of the sky where the circles of the Galaxy proper 

 and the local cluster appear most widely separated, the projection on 

 the sky of stars of the local cluster gives rise to a faintly visible second- 

 ary Milky Way. These new results support both the hypothesis of 

 the existence of a local cluster or cloud and the related theory of star- 

 streaming. 



In a series of nine short papers printed together as Mount Wilson 

 Contribution No. 161, Mr. and Mrs. Shapley have continued their 

 study of the structure of the galactic system. An interesting part 

 of this work relates to the distribution of spirals and to certain prop- 

 erties of their systematic recessional motion, suggesting that the 

 whole galactic system may be rapidly moving through space. Ap- 

 parently the spirals are not distant stellar organizations or "island 

 universes," but truly nebular structures of great volume which in 

 general are actively repelled from stellar systems. A tentative cos- 

 mogonic hj^othesis has been formulated to account for the motions, 

 distribution, and observed structure of clusters and spiral nebulae. 

 For the purpose of further testing the premises and conclusions of the 

 proposed interpretation, various investigations have been taken up, 

 among which the following are the most significant: (1) a search for 

 additional globular clusters, which so far has yielded 17 extremely dis- 

 tant systems; (2) the verification of the method of diameters in the 

 estimation of distances of globular clusters; (3) the successful use of 

 the integrated magnitudes of clusters as criteria of distance; (4) a 

 discussion of the dimensions of the local cluster, as shown by the 

 spectral data in the Henry Draper Catalogue; (5) the frequency of 

 various absolute magnitudes in clusters, including an attempt to 

 ascertain the relative number of dwarf stars; (6) an inquiry into the 

 radiation of stellar energy, with a consideration of its bearing on the 

 speed of spectral evolution (as illustrated by phenomena of globular clus- 

 ters and Cepheid variables) and on the age of the sun and the earth.^ 



Variable Stars. 



A theoretical investigation by Mr. Shapley and Mr. Nicholson of the 

 form of the spectral lines of a spherically pulsating star bears on the 

 hypothesis that pulsations of single stars underlie the numerous phe- 



^ Publications of the Astronomical Society of the Pacific, October 1918, and June 1919; 

 Nature, March 13, 1919, and June 12, 1919. 



