MOUNT WILSON OBSERVATORY. 185 



the star, as measured with the aid of other lines, appears to be only 

 about — 15 km., the enhanced lines on June 10 showed displacements, 

 proportional to the wave-length, which should apparently be ascribed 

 to a velocity of the corresponding gases amounting to —1,460 km. 

 These displacements increased daily, until on June 15 they corre- 

 sponded to a velocity of — 1,630 km. The explanation of this apparent 

 velocity, uniformly accelerating for several days, of the vapors of iron, 

 titanium, chromium, strontium, and other elements in the atmosphere 

 of the star can not yet be given, but it must have an important bearing 

 upon the theory of temporary stars. It recalls the expanding nebula 

 about Nova Persei, but in the present case no such nebula has been 

 detected. 



The completion of the determination of the photographic magni- 

 tudes of stars in 115 of the Selected Areas of Kapteyn, and the advanced 

 stage of the task of completing these magnitudes for 24 additional 

 areas, brings this extensive research to a point where pubUcation of 

 the details will soon be feasible. An intercomparison of the areas 

 of each zone shows a very satisfactory accordance of the results, 

 leaving no doubt as to the high precision of the measures. Nearly 

 half the photographs required for the determination of photovisual 

 magnitudes in the Selected Areas have also been secured. 



The photographic method of determining the colors of stars, de- 

 scribed in the armual report for 1916, has been utilized in a study of 

 the dependence of color upon absolute magnitude in stars of the same 

 spectral type. In the case of red stars the effect is very consider- 

 able, giants and dwarfs indicating a difference amounting to about 

 half a magnitude in the color index. Further use of this method, 

 especially in conjunction with spectroscopic observations, promises 

 to yield results of interest. 



The continuation of a comprehensive investigation of globular star- 

 clusters has resulted in an important addition, comprising novel and 

 radical features, to existing theories of cosmogony. The absolute 

 distances of all known globular clusters have been derived, directly 

 or indirectly, from a remarkable relation which gives the absolute 

 magnitude of a typical Cepheid variable star when its period of Hght 

 variation is known. The clusters are thus found to range in distance 

 from 6,500 to nearly 70,000 parsecs, and to be secondary organizations 

 in comparison with the Galaxy, which appears vastly greater in 

 extent than hitherto supposed. Less-complete evidence also supports 

 the view that the spiral nebulae, instead of being "island universes," 

 are part and parcel of a single, all-embracing sidereal system. The 

 average brightness of the faint temporary stars discovered in spiral 

 nebulae, of which 14 have now been found, may serve as a valuable 

 means of measuring the distances and dimensions of these objects. 

 The general investigation also indicates that a large, open, flattened 



