Continuity 59 



lishes is that there have so far always been 

 compensations; so that the attempt to 

 observe motion through the ether is being 

 given up as hopeless. 



Surely, however, the minute and curi- 

 ous compensations cannot be accidental, 

 they must be necessary. Yes, they are 

 necessary ; and I want to say why. Sup- 

 pose the case were one of measuring 

 thermal expansion; and suppose every- 

 thing had the same temperature and the 

 same expansibility; our standards would 

 contract or expand with everything else, 

 and we could observe nothing ; but expan- 

 sion would occur nevertheless. That is 

 obvious, but the following assertion is not 

 so obvious. If everything in the Uni- 

 verse had the same temperature, no 

 matter what that temperature was, 

 nothing would be visible at all ; the exter- 

 nal world, so far as vision went, would not 

 appear to exist. Visibility depends on 

 radiation, on differential radiation. We 

 must have differences to appeal to our 

 senses, they are not constructed for uni- 

 formity. 



