THE SEMI-CENTENNIAL ANNIVERSARY 63 



I shall offer no solution. I simply wish to point out that here 

 is a problem which may be interesting to the physicist no less 

 than to the astronomer. 



Passing to other inferences, I wish to draw your attention to 

 a question already alluded to. Does the observed fact of the 

 preference of the star motions for two definite directions lead us 

 with necessity to the assumption that our system has been formed 

 by the meeting of two independent star clouds? Or is it still 

 possible, and in that case more plausible, to explain it without the 

 sacrifice of the unity of the system? In other words, is it a dual 

 system, or is it one unit? 



Suppose a very elongated system of stars which are originally 

 at rest; now let these be left to their mutual attraction; then it 

 is evident that those stars, under the influence of that gravitation 

 will fall together. The stars at the extreme ends of this elongated 

 system will fall toward each other, in a motion that will follow, 

 in the main, the axis of that system of stars. 



In the main, there w r ill be deviations because, of course, the 

 system is pretty long in comparison to its breadth; but still, it is 

 not linear. In other words, there will be set up two streams of 

 stars, in which all the stars will be moving preferentially and 

 there will be deviations. Those star-streams will be exactly 

 opposite each other. There is no difficulty in that. True it is 

 that the streams of stars we see make an angle of about one hun- 

 dred degrees, but it is evident that if w r e have two streams mov- 

 ing in exactly opposite directions, and we view these streams 

 from the earth which itself has a motion in space, the conse- 

 quence will then be that those opposite streams will be seen to 

 make an angle, and it is easy to determine the earth's motion 

 in such a way as to bring it in perfect harmony with the obser- 

 vation. Thus far there is no objection. It might seem that this 

 evolution of the system, which saves the unity of the system, 

 might explain the matter; but there are further consequences. 



In an elongated universe, as here supposed, both the longi- 

 tudinal motion (what we in this lecture have called the stream 

 motion) and the deviations therefrom (what in this lecture have 



