April 1, 1886.] 



♦ KNO^A^LEDGE ♦ 



18i 



arrangement of the stars there wholly negativing our initial 

 supposition, we must expect to be misled. With all respect 

 for the elder Struve's labours, I must admit that this seems 

 to me to be what he has done in his famous distribution 

 of the stars according to zones of given galactic polar 

 limits. 



Consider, however, fig. 1, which represents the galaxy as 

 actually seen in the heavens,* and it becomes wholly impos- 

 sible to believe that we have to deal with the projection upon 

 the celestial sphere of a widely-exteuded cloven disc of stars. 

 The view does not account for one of the peculiarities of the 

 calaxy proper, however justly it may seem applicable to the 

 sidereal system. The great break in Ai-go (opposite line 1 

 in our fisure) is of itself sufficient to negative the disc 



Milky Way on any leasonable hypothesis. It was easy to see 

 that, whatever hypothesis we adopt, we must be prepared to 

 admit of the existence of great irregularities. In fact, as 

 .such a stream as I conceive the Milky Way to be would be 

 subject to a number of attractions, swaying its length here 

 in one direction, there in another, those irregularities were 

 lo be looked for independently of any considerations founded 

 upon the observed appearance of the Milky Way. But 

 there were certain features which I felt that any hypothesis 

 for which support could reasonably be claimed ought to 

 explain. 



The difficulty I found was in conceiving how, fiist, the 

 interruptions, secondly, the variations of hrilliancij, and 

 thirdly, hiciince in the Milky Way, could be accounted for by 



theory ; so is the coal-siick near Crux, and .so are the some- 

 whatsimilar vacancies in Cygnus and Argo. The fact, too, 

 that the second stream (which has led to the a.ssumption 

 that the sidereal disc is cloven) is not continuous, is one 

 which cannot possibly be explained on the disc theory. 



But, although one may feel convinced that the galaxy is 

 really a stream of relatively small stars surrounding our 

 heavens, it has always seemed to me a very difficult matter 

 to account for the various phenomena presented by the 



* The mode of projection must be conceived to be as follows: — 

 Suppose that on a celestial globe a band is taken, including the 

 whole of the Milky Way, and that this band is spread as a long, 

 straight slip on a plane surface. If, then, we conceive the band 

 turned into a circular strip, from the uniform contraction of one edge, 

 we shall have such a map as fig. 1. 



any single stream however shaped. An exphmation, which 

 accounted for the interruption opposite Hue 1, left the 

 interruption opposite line i unaccounted for. Again, I did 

 not find it easy to account for the sudden access of brilliancy 

 at 8, the extreme faintness at 7, or the fact that of the two 

 branches starting from 5 the fainter becomes presently the 

 brighter, and vice versd. The three coal-sacks also were 

 a great mvsterv to me. 



I have again and again attacked the problem (which now 

 seems perfectly simple and easy) without being able to 

 imagine a stream of reasonable figure which would account 

 for these peculiarities. 



At length (more than two ye;trs after I had come to the 

 conclusion that the galaxy is really a spirally formed ring of 

 generally circular section, by looking on the break opposite 



