532 SCIENCE PROGRESS 



as determined by Shapley, and could not be regarded as an 

 island universe. 



Curtis further argues that if the Andromeda nebula is at a 

 distance of 500,000 light-years, its diameter would be 17,000 

 light-years, and therefore, in his opinion, of the same order of 

 size as our Galaxy. Adopting, however, Shapley's dimensions 

 for the size to be comparable, the distance assigned for the 

 Andromeda nebula would be much greater than is admissible. 



As an argument against the spirals being members of our 

 Galactic System, Curtis supposes that all the spirals are of 

 approximately equal size, the differences in angular diameter 

 being due to differences in distance. He then concludes that 

 even if the distance of the Andromeda nebulae is only 20,000 

 light-years, the distances of the fainter nebulae must extend to 

 10,000,000 light-years, or far outside our Galactic System. 

 There is at present no evidence to support this assumption, and 

 it seems strange that Curtis should deny that Cepheid variables 

 in different parts of the Galactic System obey the same lumino- 

 sity-period relationship, but should accept the hypothesis that 

 the spiral nebulae are of the same size wherever they occur. 



The balance of evidence seems to be decidedly against the 

 hypothesis that the spirals are comparable in size with our 

 Galactic System. It may further be mentioned that Scares 's 

 measurements of the surface brightness of several spirals show 

 that they are much brighter than that of the Milky Way, and 

 Reynolds, from a study of the distribution of light and colour 

 in typical spirals, has concluded that they cannot be stellar 

 systems. Shapley considers the spirals to be nebulous bodies 

 of great dimensions and distances, but not island universes. 

 It will be possible to form some more definite conclusions as to 

 their distances in the course of years, when, by the accumulation 

 of material, it will become possible to determine with some 

 accuracy the proper motion, or at least to assign an upper 

 limit for the motions. 



In conclusion, Shapley's summary of the results of accepting 

 the restricted scale of the Galactic System may be quoted : 



" If the distance of globular clusters must be decreased to 

 one-tenth, the light-emitting power of their stars can be only 

 a hundredth that of local cluster stars of the same spectral and 

 photometric types. As a consequence, I believe Russell's 

 illuminative theory of spectral evolution would have to be 

 largely abandoned, and Eddington's brilliant theory of gaseous 

 giant stars would need to be greatly modified or given up 

 entirely. Now both of these modern theories have their 

 justification : first, in the fundamental nature of their concepts 

 and postulates ; and second, in their great success in fitting 

 observational facts. 



