EVOLUTION OF THE STABS 229 



regular in outline, and a large proportion containing condensed or stellar 

 nuclei near their centers, they were called planetaries by Herschel, be- 

 cause, though faint, they present discs somewhat as the planets do, when 

 viewed under low power. 



There are several scores of so-called stellar nebula 1 . In moderate- 

 sized telescopes most of them look like ordinary stars. In large tele- 

 scopes many of them are hazy, but some are as well defined as stars. 

 The spectroscope shows that all are true nebula?. If they were much 

 closer to us we should doubtless see them as planetary nebulae. 



A few other interesting objects are known as ring nebulae, the most 

 noted case being the ring nebula in Lyra. 



Among the remarkable facts of the stellar universe are these: the 

 large irregular nebula?, the ring nebula?, the planetary nebula?, and the 

 stellar 4 nebula 3 , with relatively rare exceptions, are in or very close to the 

 Milky "Way : and, on the contrary, the spirals in or near the Milky Way 

 are of negligible number. The first group are without question an 

 integral part of our stellar system. The spirals, seem not to be closely 

 connected with our stellar system, yet their very avoidance of the Milky 

 Way shows that they bear some intimate relationship to it. There is 

 no occasion for surprise that a small group of special objects should be 

 in the Milky Way structure; but that the scores of thousands, and 

 perhaps hundreds of thousands, of spirals, should abhor the Milky Way 

 is a fact which immediately arrests our attention and calls for explana- 

 tion. Moore has suggested that their absence from the Milky Way may 

 be apparent and not real : that any absorbing or obstructing medium in 

 the Milky Way structure might prevent the light of the spirals from 

 reaching us, especially if the spirals are extremely distant. If the light 

 from very distant nebula? is absorbed or obstructed, as a function of the 

 angular distance from the galaxy, the nebula 1 near the poles of the 

 galaxy, other things being equal, should on the average be intrinsically 

 brighter than the nebula? in or near the Milky Way. Secondly, if such 

 an effect exists, long-exposure photographs on regions near the galaxy 

 should record nebula? in numbers more nearly equal to those recorded 

 by short exposures near the poles of the galaxy. An examination of 

 existing Crossley reflector photographs has led to negative results on this 

 question, and we must assume that the spiral nebula? really avoid the 

 Milky Way. 



The question of the distances of the spiral nebula? has long been held 

 in mind. The evidence, to which we shall refer later, is to the effect 

 that they are very far away, and accordingly that they are of enormously 



4 The terms irregular, ring, planetary and stellar are intended merely to 

 differentiate these objects as to their appearance in the telescope or on the 

 photographic plate. They do not in themselves indicate differences in constitu- 

 tion or physical condition. The ring, planetary and stellar nebulae have a great 

 many characteristics in common. 



