294 THE POPULAR SCIENCE MONTHLY 



the same author explains the dark ring around the nebula near Rho 

 Ophiuchi on the supposition that " the smaller and more slowly moving 

 immigrants . . . are stopped by the particles of the nebulae," and are 

 detained by the gathering crowd. But even if it could be demonstrated 

 that the stars are arranged in thin sheets, and that celestial bodies exist 

 of sufficient size and momentum to plow their way through great aggre- 

 gations of stars, demolishing everything in their track, it would still be 

 exceedingly improbable that only one layer of stars should exist in a 

 given direction, or that several rifts should coincide. On the other 

 hand, the presence of widely extended masses of dark absorbent matter 

 in the shape of branching streams, sheets or rings, situated between us 

 and the depths of star-strewn space, is not unlikely. 



The gaseous nebulas which form a part of the galactic structure are 

 often very extensive, and are of a great variety of shapes, being fre- 

 quently strangely irregular; but the more numerous white nebulas are 

 formed more nearly after a common pattern, although still with infinite 

 variation as to details. 10 In general, what is common to nearly all of 

 the white nebulas is a tendency to form a two-branched spiral, the 

 branches issuing from opposite sides of a central condensation, and 

 coiling either within the boundaries of a plane circular disk, or forming 

 a helix around a cylindrical directrix. The former figure is the more 

 characteristic, and is well exhibited in the Great Nebula in Andromeda. 



Another very remarkable and at present unique type is the transient 

 nebulosity which appeared around Nova Persei, issuing from the star 

 as a center, and expanding into the commencement of a vortical ring. 

 It was an electric phenomenon, an exhibition of canal rays, or positive 

 ions, on a grand scale. Facts from the history of these two bodies will 

 be found useful in preparing one of the necessary means for our quest. 



It is obvious that we require for this investigation of external 

 galaxies some scale of distances, and equally obvious that at present 

 such a scale can be only approximate. Indeed, it is probably this 

 uncertainty as to the scale on which the universe is constructed which 

 deters astronomers from attempting to discriminate between different 

 galactic orders. I propose to see if this uncertainty can be, in part, 

 removed. 



I propose to take the distance of the Andromeda nebula as our 

 celestial " yardstick," which may be called one andromede, and assuming 

 that when we consider a large number of nebulas, the average size does 

 not vary with the distance, and that consequently the average distances 

 may be taken inversely proportional to the angular diameters of the 

 objects, I shall classify the nebulas according to apparent size and 

 brightness. It is essential that the subdivision shall not be too minute. 



10 The class of white nebulae exhibits various stages of development, and 

 includes objects of mixed type. See E. A. Fath, "The Spectra of Spiral Nebulae 

 and Globular Star Clusters," Astrophysical Journal, Vol. 33, p. 58, January, 1911. 



