150 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1912. 



or fusiform show the spkal form on plates taken with special care; 

 second, all exposures sufficiently long to photograph one of these 

 objects lead to the discovery of many other similar objects. The 

 number of spirals is much greater than had been supposed, and they 

 may include the majority of the nebulas. These results were obtained 

 on Mount Hamilton, Cal., where a rich American, James Lick, has 

 founded the observatory wliich bears his name. No astronomer can 

 visit this model observatory without envy and admiration. 



The order in which we take up the objects in the rich collection of 

 Keeler may evidently be open to criticism until an accord is established 

 upon a definite theory. No one, surely, would suppose that the nebulae 

 have always existed just as they are or that they have acquired a 

 final shape. We must look upon them as still in the process of change. 

 The question we will for the moment consider is whether they are in 

 the process of condensation or expansion; whether the spirals are 

 flowing out from or into the center. 



Before forming a too hasty decision, let us first examine the larger, 

 more massive nebulae, where the spirals are small and unimportant 

 in comparison with the central nucleus. Afterwards we will con- 

 sider the more dilated ones, where the greater part of the matter 

 seems dispersed into the spirals. Then we will consider in which 

 direction it is easier to suppose the transition. 



In each class we shall place in the first rank those which have the 

 most nearly circular appearance; that is, those whose plane is normal 

 to our line of sight and which will enable us to interpret better the 

 other nebulae seen at less favorable angles or even edgewise. 



Having completed that task, we needs must ask of what are the 

 spkal nebulae formed; in what way are they changing? Could we 

 answer these two questions, then we would ask two others still more 

 ambitious. How were the spiral nebulae formed, and what will be 

 their end ? But such questions may for a long while yet be prema- 

 ture, and I believe I thus voice the opinion of our master, Poincare, 

 if I rightly interpret the conclusioife stated in his recent book on 

 cosmic hypotheses. 



It seems to me that the elements of the spiral nebulae can be nought 

 else than collections of groups of stars, whence comes the abundance 

 of the luminous points scattered in the outer portions of the spirals 

 where they can be separately seen. A cosmic cloud formed of sub- 

 tler elements could never show such sharp outlines, nor reveal such 

 clear-cut di\dsions. The continuous spectra must lead us to sup- 

 pose that even in the central portions stars predominate, enveloped, 

 if you will, in a common atmosphere which diffuses their light. 



Some might argue that if the spirals are formed of stars they would 

 be brighter. I do not see that necessity. The distance of the 



