106 TWO NEW WORLDS 



covered. Transmission of light? Who knows but 

 that the interstellar luminiferous ether may thin out 

 and finally forsake us ! The law of gravitation ? 

 Men like Wallace and Whittaker are willing to 

 abandon it before they have got as far as the 

 galaxy ! Yet it seems as if these two agencies must 

 be our chief, perhaps our only, guides in exploring 

 ultra-galactic space. We must, therefore, deal with 

 them at some further length. 



Transmission of Light. One of the most powerful 

 arguments against the existence of ultra-galactic 

 universes is based upon the laws of the transmission 

 of light. This argument, already referred to, dis- 

 proves the existence of luminous stars throughout 

 infinite space, by pointing out that the sky would 

 then have a uniform brightness equal to that of the 

 sun in every part. The value of this argument 

 depends upon the truth of the following assump- 

 tions : 



1. That the luminiferous ether pervades all space. 



2. That the number of dark bodies is compara- 

 tively small. 



3. That the stars are irregularly distributed. 



4. That luminous stars have an eternal existence. 



The first assumption certainly appeals to our ima- 

 gination, and no valid argument has been brought 

 up against it. The third assumption is sometimes 

 overlooked. It is obvious that an infinite line of 

 stars might exist behind every star in the heavens 



