THE SEMI-CENTENNIAL ANNIVERSARY 59 



So that now, neglecting the stars of Sechi's fourth type, we 

 have still four types, which will be the helium stars, and the first, 

 second and third types of stars — helium, first, second and third. 

 Now, there is much evidence to show that this classification in 

 this order is a natural one. I mean that the order is really an 

 order of evolution, the helium stars being the stars of recent 

 birth; while we get to the older and older stars as we pass from 

 the helium stars to the first, from the first to the second, and from 

 the second to the third. I will adopt this classification in what 

 follows, although well aware of the fact that all astronomers 

 do not agree with me. I feel justified in this course, not only 

 because I think it is the opinion of the great majority of our 

 eminent spectroscopists, but also because the very facts which I 

 wish to put before you about star-streaming strongly confirm it. 



When we wish to penetrate into the history of the system, it 

 seems natural to investigate the problem of star-streaming 

 separately for these four classes of stars in the order of their 

 evolution. There are some difficulties, mainly the consequence 

 of the scantiness of materials. Still, however, even now it has 

 been possible to carry that investigation through in such a way 

 as to establish a few facts, and to give clear indications of other 

 facts. Of these I will consider only the two following, about the 

 reality of which I think there can be left hardly any doubt. 



First, the older the stars, the greater the internal velocity, and 

 second, the older the stars, the richer the second stream, at least 

 in comparison with the first stream. And I wish to consider 

 some of the inferences to which these facts lead us. In the first 

 place, the facts at once lead us back to the question just now 

 mentioned, about the order of evolution of the individual stars. 

 For this regularity in the increase in the internal velocity and 

 also in the richness of the second stream exists only if we arrange 

 the stars in their order, helium, first, second and third, or (of 

 course I need not say so) in the reverse order, third, second, first, 

 helium; but in no other arrangement. 



Therefore, with the same right that we expect that all the 

 properties of the stars will change with age, gradually, and not 



