MOUNT WILSON OBSERVATORY. 231 



tribution of stars, he deriv.es the attraction of all the stars on any point 

 within the system, expressed in terms of the attraction of a star of 

 average mass. Systematic motion of the stars is necessary to produce 

 equilibrium. Such motion, parallel to the plane of the Milky Way, is 

 actually observed in the star-streams. A rotatory motion around the 

 axis toward the pole of the Milky Way is therefore assumed (p. 283). 



The formula derived from the kinetic theory of gases for the baro- 

 metric determination of altitudes is then applied to the stars lying 

 along the rotation axis, on the assumption that their velocities are 

 approximately distributed according to Maxwell's law. This leads 

 to values of the average effective mass of a star ranging from 2.2 at 

 198 parsecs to 1.4 at 1,660 parsecs. 



The centrifugal forces in the plane of the Milky Way are so deter- 

 mined as to give the same average effective mass for regions of equal 

 star density in this plane and on the rotation axis. The linear tan- 

 gential velocities corresponding to these forces vary but little with the 

 distance. For the greater part of the system they are practically 

 constant and equal to 19.5 km. The direction of rotation is indeter- 

 minate. If two groups are assumed to be moving in opposite direc- 

 tions, their relative velocity is 39 km., which agrees closely with the 

 observed relative velocity of these two star streams. 



Although admittedly speculative in many particulars, the value of a 

 theory of star-streaming, giving results in harmony with observation, 

 is obvious. Both the weak and strong points of the theory are fully 

 discussed in a Contribution soon to be published by this Observatory. 



Professor Russell has extended Saha's theory of ionization in solar 

 and stellar atmospheres to the case where atoms of several kinds are 

 present. He finds that the proportions of ionized atoms for different 

 elements depend only upon the temperature, though the elements of 

 lower ionization potential are always most highly ionized, those that 

 have lost one electron giving ordinary enhanced lines, and those that 

 have lost two electrons giving ''super-enhanced" lines, such as certain 

 silicon and oxygen lines studied by Fowler (p. 238). 



Professor Russell has also found Majorana's theory of the absorption 

 of gravitation by matter to be untenable when tested by the motions of 

 the moon and planets and the phenomena of the tides. Majorana's 

 experimental results, if confirmed, may be explained as a change of 

 mass caused by the mutual influence of two bodies (p. 285). 



Dr. Bertil Lindblad, of the University of Upteala, who has spent 

 nearly a year at the Observatory, has made an important extension of 

 the method of determining the absolute magnitude of a star from its 

 spectrum. The continuous spectrum of A and B stars between X3895 

 and X3907, when compared with the adjoining region toward the red, 

 is less intense in stars of low luminosity than in brighter stars. By 

 finding the exposure ratio necessary to give equal photographic im- 



