58 NATIONAL ACADEMY OF SCIENCES 



deviations will be rare, and very great deviations will be deci- 

 dedly exceptional. The motions of the two individual bubbles 

 will still be clearly discernible. 



So it is that with the stars in the sky we have recognized two 

 clearly defined preferential motions. These directions make an 

 angle of about one hundred degrees. The stars are not moving 

 in these directions only. Small deviations are frequent; greater 

 deviations are somewhat rare; very great deviations are deci- 

 dedly exceptional. This describes what is actually observed in 

 the sky. 



We may say that all investigations made since the first an- 

 nouncement of star-streaming in 1904 — investigations based on 

 very different materials — all agree in the establishment of these 

 two preferential directions of motion among the stars. We find 

 them in the brightest stars; we find them in the fainter stars; 

 they show in the swift moving stars; they show in the slow mov- 

 ing stars. They betray their existence not only in the radial 

 motions but as well in the motion at right angles to the visible 

 ray. 



Our conception of two independent star clouds is one of these 

 investigations. Whether this interpretation is the correct one, is 

 a question of evolution of the system and will have to be con- 

 sidered presently. Our conclusion will eventually be in favor of 

 the two-cloud theory; and so, for the sake of greater clearness, I 

 will continue to use this conception. 



In the study of the history of the system, we start from what 

 we know, or think we know, about the evolution of the separate 

 stars. 



The stars have been classified by Sechi into four spectral 

 classes. We have a far more elaborate classification, but for 

 the present purpose Sechi's classification will do. The stars 

 of the fourth type are so few in number that we may, for the 

 present, neglect them. 



On the other hand, there is a class of stars showing the helium 

 lines, that formerly was classified with all the other classified 

 stars, but it is now considered as a separate class — the helium 

 stars. 



