242 CARNEGIE INSTITUTION OF WASHINGTON. 



although the result requires confirmation. The A's, however, at least 

 from A3 on, behave in an opposite manner, the stars of high luminosity 

 being very definitely bluer than fainter objects of the same spectral 

 class. For F stars the relation is a transition from the characteristics 

 of the A's to those of the G's. With increasing absolute magnitude the 

 color first changes toward red and then, as the lower luminosities are 

 approached, shifts back toward the blue. 



Since most of the brighter stars are giants, the variation of color- 

 index with spectral type hitherto found refers to stars of high lumi- 

 nosity. For stars of zero absolute magnitude, the present observations 

 agree with previous results in showing a change in color-index, which, 

 as far as K5, is nearly proportional to the change in spectrum. Beyond 

 this point, through spectral classes Ma, Mb, and Mc, the color of the 

 giants remains sensibly constant. For the dwarfs, the variation of 

 color with spectrum is much less regular, and can not be inferred from 

 the numerical results hitherto available. 



Investigation of Stae-Clusters. 



In continuation of his systematic photometric study of stellar clus- 

 ters, Mr. Shapley has completed a special investigation of the typical 

 globular cluster Messier 68 (N. G. C. 4590). All the characteristic 

 conditions and laws previously found in the study of other globular 

 systems are without exception verified by the work on this cluster. 

 The results include the verification of (1) the color law for giant stars 

 in clusters; (2) the constancy of the median magnitude of cluster-type 

 variables; (3) the numerical relation of the median magnitude to the 

 mean magnitude of the brighter stars; (4) the anomalous form of the 

 general luminosity curve in stellar clusters; and (5) the applicability 

 of luminosity methods to the determination of distances. 



A preliminary determination of the distances of 70 open clusters 

 has been made on the basis of (1) 30 spectrograms of some 200 faint 

 stars in various northern groups, obtained with a slitless spectrograph of 

 small dispersion at the 80-foot focus of the 60-inch reflector; (2) more 

 than 100 direct photographs at the primary focus of the 60-inch; 

 (3) magnitudes and colors of about 2,000 stars; and (4) the measure- 

 ment of the diameters and form of all known open clusters on Franklin- 

 Adams charts. Harvard photographs, or Mount Wilson plates. The 

 study of the distribution of these open systems in space throws some 

 light on the structure of the nearer parts of the galactic system, and 

 supports the hypothesis that hght from the most distant objects along 

 the galactic plane may be obstructed by dark nebulous matter. 



In last year's report mention is made of the possibiHty of interpreting 

 star-streaming, as well as the decrease of stellar density with distance 

 from the sun, as phenomena connected with the translational and 

 rotational motions of a large local cluster situated in the star-fields of the 



