EVOLUTION OF THE STARS 235 



not follow that the more distant and fainter stars will show the same 

 preferential motions as the brighter and nearer ones which led Kapteyn 

 to his hypothesis, though it should be said that a fairly extensive study 

 of stars fainter than the Kapteyn stars made by Comstock led to results 

 in good agreement with Kapteyn's. May it not be possible that the 

 preferential motions observed are in some way connected with rotational 

 phenomena within our stellar system, especially as the line of prefer- 

 ential motions lies approximately in the plane of the Milky Way, or 

 are local to what we may call our region of the system, and not be 

 true of the system as a whole ? 5 



An alternative hypothesis of prevailing stellar motions, proposed by 

 Schwarzschild, seems to have advantages from the point of view of prob- 

 ability, but it appears not to accord so well with the facts of observation. 

 Schwarzschild suggests that if from a given point we draw vectors 

 whose directions and lengths represent the directions and speeds of 

 existing stellar motions, then the outer extremities of these vectors will 

 define the surface of an ellipsoid (of preferential motions) having three 



unequal axes. 



(To be continued) 



s Turner has proposed the following explanation of the two star streams : 

 The whole mass of the stellar system exerts a gravitational influence on the 

 motion of each star in the system, and the individual stars revolve around the 

 center of mass of the system in their elongated orbits. One star stream com- 

 prises all those stars moving away from the center, and the other stream all 

 those stars moving toward the center. We can not doubt that the motions of the 

 individual stars are influenced by the gravitational attractions of the stellar 

 system and of the group of stars nearest to them ; but observational data on 

 stellar motions must be vastly more extensive than at present in order to test 

 Turner 's hypothesis. 



Halm, of the Cape of Good Hope, has given evidence of the existence of a 

 third star stream, much less extensive than Kapteyn's two streams. 



