64 NATIONAL ACADEMY OF SCIENCES 



been called the internal motions) must gradually increase from 

 zero to the value we find them having at the present time. 



Now as to the internal velocities, this is exactly what we find 

 by observation. Do we find the same for the stream motion? 



Recent Mt. Wilson observations have enabled us to derive at 

 least a pretty reliable value of the relative stream velocities for 

 the first type stars. For the helium stars we can as yet only 

 assign a limit which the relative velocities of the two streams 

 must exceed; but for the older stars we have had reliable infor- 

 mation for some time. 



All these determinations show, contrary to what takes place 

 with the internal motion, that the relative velocity of the stream 

 motion does not change, or does not change very much, at least, 

 with age; certainly the stream motion is not nearly vanishing 

 for the helium stars. It seems to me that this consideration is 

 fatal to the present explanation. 



Professor Schwarzschild has developed a different theory, 

 which also leaves the universe a unit; but this theory also can not, 

 I think, be maintained. Among other things, we have, as a main 

 objection, the fact — which was not known at the time Professor 

 Schwarzschild proposed his theory- — that the richness of the two 

 streams is not the same for stars of different age. 



The tacit assumption is made, and must be made, under 

 Schwarzschild's theory that the two streams have the same num- 

 ber of stars. Now, this may be more or less approximately true 

 of the stars of the second and third types. It certainly is not 

 true of stars of the first type. 



While it is true that the first stream is three times richer than 

 the second stream, the richness of the helium stars is certainly not 

 a tenth of that of the first stream. 



The conclusion to be drawn from all of this seems pretty 

 obvious. It would seem that we are driven to the theory assumed 

 here, from the first, the theory of the two star-clouds, which, 

 owing to their initial velocity, have come to meet and intermingle 

 in space. 



