276 CARNEGIE INSTITUTION OF WASHINGTON. 



determination of the distances of groups of stars involved in nebulosity. 

 The data used were the spectroscopic parallaxes of Mr. Adams and his 

 associates (1,646 stars), the helium stars of Professor Kapteyn (434), 

 the visual binaries investigated by Jackson and Furner (556), the 

 Cepheid variables with Mr. Shapley's determinations of absolute 

 magnitude, and his results for more than a thousand giant stars in 

 globular clusters. The chart illustrating the results for the first two 

 of these groups is reproduced on page 270. Such groups as the 

 Hj^ades, the Pleiades, and the Ursa Major stream were also considered; 

 and finally correlations of color wdth apparent magnitude were ex- 

 amined for several clusters of unknowai or uncertam distance and for a 

 number of rich galactic fields. So far as the original purpose is con- 

 cerned, the results are not wholly conclusive, but a number of inter- 

 esting details were brought to light by plotting absolute magnitudes 

 against spectral types, or their equivalent color indices, in the manner 

 of the well-known diagram of Professor Russell. The curve of modal 

 values for the helium stars runs smoothly into that of the dwarfs. 

 The corresponding curve for the visual binaries has a different slope 

 from that defined by the helium stars and the stars of the spectroscopic 

 list, because of the use of a constant mass for the calculation of the 

 hypothetical parallaxes of the binaries. The Cepheids and pseudo- 

 Cepheids of our own system, in spite of high dispersion, present a 

 correlation of mean color with luminosity similar to that shown by the 

 Cepheids when absolute magnitudes are assigned in accordance with 

 the period luminosity' relation, and to that of the giant stars in globular 

 clusters. Further, in certain rich galactic fields there is a parallel 

 correlation of mean color with apparent magnitude (p. 265). The 

 inference is that the stars in the galactic clouds are similar in luminosity 

 to the Cepheids and pseudo-Cepheids of our own system and the most 

 luminous stars in the globular clusters. Other details will be discussed 

 in a forthcoming Contribution. 



The complicated character of the diagram, en page 273 raises a ques- 

 tion as to how the frequency curve for all tj^^es together would agree 

 with the luminosity and density functions determined by Professors 

 Kapteyn and van Rhijn. Mr. Scares has derived from these functions 

 an expression for the frequency curve of the absolute magnitudes of all 

 the stars brighter than any limit of apparent magnitude, which can be 

 compared directly with the results of observation. All things con- 

 sidered, the agreement is excellent. The only irregularity of impor- 

 tance arises from an exceptional congestion of K stars between M = 

 andM=+2. 



MASSES OF VISUAL BINARY STARS. 



The examination of the data bearing on absolute magnitude has led 

 Mr. Seares to a determination of the mean masses of visual binaries of 



