Very — Stellar Revolutions within the Galaxy. 129 



Kovv if the outer and more thinly distributed stars are revolv- 

 ing around great galactic aggregations of suflScient mass to 

 control the general motion, the orbital velocities must increase 

 in the vicinity of the controlling mass. JS^evertheless, the 

 dense aggregation of stars itself, having been accumulated 

 during an immense time by gravity, ought to be relatively at 

 rest, because of frequent collisions in the crowded centers. 

 We have seen that stars of the first type are more abundant in 

 the Milky Way. Kapteyn has shown that, between magni- 

 tudes and 6*5, there are 19*3 times as many stars of type II 

 as of type I having annual proper motions greater than 0''*50, 

 but only 0*6 as many with proper motions less than 0'''03. 

 This, taken in conjunction with the galactic condensation of 

 the first-type stars, doubtless means, primarily, that there are 

 comparatively few stars of the first type outside the Galaxy ; 

 but the small proper motions of stars of the first type may, in 

 part, be due to the smallness of their linear motions. Professor 

 Frost announced at the meeting of the Astronomical and Astro- 

 physical Society of America, held in Washington, December, 

 1902, that the members of a group of Orion stars (all pre- 

 sumably galactic) have exceptionally small velocities in the 

 line of sight (about 6*5 kilometers per second)."^ As far as it 

 goes, this observation favors a value for the galactic parallax 

 not far from 0^'*03, with the consequences which have been 

 noted. It is desirable that a large number of galactic first-type 

 stars shall have their velocities in the line of sight and their 

 proper motions measured, in order that this important point 

 may be definitely settled. 



Starting from these general principles, let us take up the 

 mechanism of the Galaxy more in detail. 



(1) Considering the form and structure of the Galaxy : (a) 

 From analogy with nebular forms, we may say that the Milky 

 Way is either spiral or annular in its major stream, but in any 

 case it is substantially uniplanar in its leading features, (h) 

 The Galaxy, however, taken as a complete system, is divisible 

 into these directed or segregated stellar streams, and subordinate 

 or satellite hosts whose interlacing movements weave a common 

 envelope to the more condensed portions, which is presumably 

 of spheroidal form, and which, judging from analogy with the 

 outer shell of a spheroidal cluster, may extend to possibly ten 

 times the radius of the condensed nucleus. 



(2) I assume that, in the stellar universe, there are both 



* " Eadial Velocities of Twenty Stars having Spectra of the Orion Type " : 

 Edwin B. Frost and Walter S. Adams, Science, xvii, 324, 1903. I have 

 applied corrections for the solar motion. " The angular proper motions of 

 these stars are also small." In the Publications of the Washburn Observa- 

 tory, vol. xi, p. 34, Mr. Albert S. Flint shows that the proper motions of 

 the stars diminish more rapidly than their parallaxes. 



