MOUNT WILSON OBSERVATORY. 205 



it has now been possible to derive estimates of the absolute parallax 

 of all known globular clusters. Their distances greatly exceed those 

 previously appearing in stellar studies, and the analysis of their 

 spatial distribution brings to light numerous interesting points that 

 have some bearing on the problem of the structure of the universe. 

 In particular, it appears that the globular clusters are secondary 

 organizations as compared with the whole galactic system, which in 

 volume is at least a hundred thousand times that commonly accepted 

 heretofore. Various synopses of the observed results and their inter- 

 pretation are cited in the bibliography accompanying this report. 

 Some of the more interesting features may be summarized as follows: 



1. The mid-galactic region, an equatorial segment some 100,000 

 parsecs in diameter and 4,000 parsecs thick, contains practically all 

 the isolated stars, open clusters, planetary and diffuse nebulae of which 

 we know, but is essentially devoid of globular clusters and spiral 

 nebulae. 



2. The globular clusters are congregated to both sides of this 

 galactic zone and there is some evidence that they are disrupted and 

 scattered when they enter the dense stellar regions. It is suggested 

 that the open galactic clusters are the remnants of globular systems 

 that have entered the equatorial segment. 



3. The center of the galactic system, which is in the direction of 

 the star clouds of Sagittarius, is many thousand parsecs from the 

 solar system. 



4. The globular clusters vary in distance from 6,500 to nearly 70,000 

 parsecs. 



5. The total number of known globular clusters is now 75, a sys- 

 tematic examination of faint suspected objects having yielded during 

 the year only 6 new ones, all of which are very remote. 



6. Through investigations of colors in clusters more distant than 

 50,000 parsecs, the upper limit of the coefficient of the scattering of 

 light in space has been still further reduced. 



7. An observational test of the speed of spectral evolution empha- 

 sizes the fact that unknown sources of energy are involved in the 

 radiation of stars. 



Galactic Planes in Globular Clusters. 



That the typical globular cluster is not spherical, but oblately 

 spheroidal, has become evident from studies of star-distribution on 

 Mount Wilson photographs. The somewhat flattened form is revealed 

 by an elongation of the diagram, in polar coordinates, of the density 

 of the stars; and occasionally the elongation can be detected on our 

 plates merely from an inspection of the photographic image. It now 

 appears that on good photographs of small scale (the Franklin- Adams 

 Charts, for instance) the direction of elongation may be directly 



