226 CURTIS: MODERN THEORIES OF SPIRAL NEBULAE 



great deal of evidence for such occulting matter in smaller areas 

 in our galaxy. Many such dark areas are observed around 

 certain of the diffuse nebulosities, or seen in projection on the 

 background furnished by such nebulosities or the denser por- 

 tions of the Milky Way; these appearances seem to be actual 

 "dark nebulae."^ The curious "rifts" in the Milky Way may 

 well be ascribed, at least in part, to such occulting matter. 



Though we thereby run the risk of arguing in a circle, the fact 

 that no spirals can be detected in our galactic plane, a natural 

 result of such a ring of occulting matter, would in itself appear 

 to lend some probability to the hypothesis. The pecuHar dis- 

 tribution of the spiral nebulae would then be explained as due, 

 not to an actual asymmetrical and improbable distribution in 

 space, but to a cause within our own galaxy, assumed to be a 

 spiral with a peripheral ring of occulting matter similar to that 

 observed in a large proportion of the spirals. The argument 

 that the spirals must be an integral feature of our own galaxy, 

 based on a relationship of avoidance, would then lose its force. 

 The explanation appears to be a possibility, even a strong prob- 

 ability, on the island universe theory, and I known of no other 

 explanation, on any theory, for the observed phenomenon of 

 nebular distribution about our galactic poles. 



SUMMARY 



The Spiral Nebulae as Island Universes. 



1 . On this theory, it is unnecessary to attempt to coordinate the 

 tremendous space-velocities of the spirals with the thirty-fold 

 smaller values found for the stars. Very high velocities have 

 been found for the Magellanic Clouds, which may possibly 

 be very irregular spirals, relatively close to our galaxy. 



2. There is some evidence for a spiral structure in our own 

 galaxy. 



3. The spectrum of the majority of the spirals is practically 

 identical with that given by a star cluster; a spectrum of this 

 general type is such as would be expected from a vast congeries 

 of stars. 



* Barnard, E. E. On the dark markings of the sky, with a catalogue of 182 such 

 objects. Astrophys. Journ. 49: i. 1919; Curtis, H. D. Dark nebulae. Publ. 

 Astron. Soc. Pacific 30: 65. 1918. 



