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THE POPULAR SCIENCE MONTHLY 



Keeler's photography of the nebulae led him to open another chapter 

 in nebular investigation with the startling discovery that "most of the 

 nebulae have the spiral structure." This applied not only to the faint 

 nebulae which he discovered, but to the nebulas already known. Keeler's 

 successors have confirmed this discovery: it is certain that the great 

 majority of the nebulae have the spiral form. What the relative num- 

 ber of spirals and formless nebulae may be remains for the future to de- 



VI 



XVffi 



5 3 

 Fig. 6. Distribution of Faint Nebula discovered at Mount Wilson. 



cide. The spirals vary all the way from the great. Andromeda nebula 

 down to those so small that the photographic plate is just able to sep- 

 arate the details of structure; and there is no reason to doubt that more 

 powerful instruments would show still smaller objects to have the spiral 

 structure. 



There are irregular nebulae of all sizes. The brilliant Orion nebula 

 is diminutive in size compared with the faint nebulosity, discovered by 

 William H. Pickering in 1889, which forms the background for almost 

 the whole of the constellation of Orion. The well-known nebulous 

 structure connected with the brighter Pleiades stars is small in com- 

 parison with the area covered by a faint exterior nebulosity discovered 

 by Barnard in 1893. There are very great irregular nebulae such as 

 the Network Nebula in Cygnus and the nebulous background in the 

 Greater Magellanic Cloud. Barnard's wonderful photographs of the 

 Milky Way have recorded many extensive nebulous fields, especially in 

 regions where the background of the galaxy shows relatively few stars 

 (see Fig. 2). 



The so-called planetary nebulae are of special interest, as we shall 

 learn in the sequel. Small in size, all more or less dense, some quite 



