234 THE POPULAR SCIENCE MONTHLY 



opposite points, but simply that their components of motion parallel to 

 this line are considerably greater, on the average, than the components 

 in any other direction. We may visualize his ideas in the following 

 manner : 



Assume the existence, ages ago, of a great cluster or cloud of stars 

 distributed more or less uniformly through a certain vast volume of 

 space, whose individual motions were at random in both magnitude and 

 direction. Assume the existence of an entirely similar group of stars, 

 occupying another vast volume of space, whose internal motions were 

 also at random. Assume, further, that these two groups of stars were 

 traveling through space in such a way that they more or less completely 

 interpenetrated, with the result that the two groups of stars have now 

 become a single group. There are stars still moving in all directions, 

 with speeds of all dimensions within certain limits, and yet there exists 

 a preference for motion along and parallel to the line which originally 

 joined the centers of the two groups. Assume now that our Sun is 

 carrying the terrestrial observer through the combined group in a 

 direction making a considerable angle with the line of preferential mo- 

 tion : the apparent motions of the individual stars, as observed from the 

 solar system, would then have preferences for two directions very 

 different from the line joining the two original positions of the groups; 

 we should find a great number deviating by small angles from the two 

 preferential directions, a small number deviating to a greater extent, 

 and relatively few whose motions make large angles with the preferential 

 directions; and this is as the apparent motions of the stars have been 

 determined by many observers. 



Kapteyn's results depend upon proper motion data ; that is, upon 

 their apparent motions on the surface of the sky. Spectroscopic ob- 

 servations of stellar motions of approach and recession confirm that the 

 stars have preferential motions, but to a smaller degree than proper 

 motion data had indicated. 



No one doubts that preferential motions exist, but the explanation is 

 another matter. Kapteyn does not insist that our stellar system has 

 actually resulted from the intermingling of two star streams, yet he in- 

 clines more and more to this point of view, and the hypothesis does seem 

 to accord better with the observed facts than any other hitherto proposed. 

 A strong objection to it is its apparent improbability. It does not seem 

 reasonable that two great clouds of stars, containing all the stars now 

 in our sidereal universe, should have come together and interpenetrated 

 so completely as to have produced in an age when we happen to be 

 the observers a stellar system apparently spheroidal in form. When I 

 look at the Milky Way, completely encircling the sky, my mind is filled 

 with doubt. And if two o T eat galaxies of stars have traveled far and 

 come together, they will travel further and through each other and we 

 shall have two galaxies again, moving away from each other. It does 



