338 SCIENCE PROGRESS 



For they are not mechanical waves, they are electromagnetic ; 

 who is to say what the actual mechanism of such waves really 

 is? Why try to mix up ideas appropriate to the elastic solid 

 theory of light with more modern conceptions ? 



Moreover, every attempt to bring the ether to book, to 

 force it out of the realm of conception into that of demon- 

 strable physical fact, to exhibit its presence and function by 

 direct experiment, has hitherto been found impossible ; experi- 

 ments directed to that end have always yielded a negative 

 result. True there is astronomical aberration but the inter- 

 pretation of aberration is simplest on the corpuscular theory ; 

 ideas connected with an ether appear only to complicate 

 it — just as the propagation of light in straight lines or rays 

 is far simpler on a corpuscular theory. But then a wave 

 theory is true; and waves have seemed to demand a medium 

 in which those waves exist — such a medium cannot possibly 

 be ordinary matter ; moreover, an ether of finite properties 

 has seemed to be demanded for the known very definite 

 velocity of wave propagation. 



But is velocity as definite and unalterable as we had imagined 

 or is its apparent constancy to be otherwise explained when 

 moving bodies are taken into account ? So there arises the 

 question how the transmission of light is affected by motion, 

 motion of source relative to medium — if there be one — as well as 

 the apparent effect due to motion of receiver relative to source. 



Innumerable attempts have been made to detect some result 

 of ether drift past the earth — some result, that is, due to the 

 motion of source and receiver together through a stationary 

 wave-conveying medium — some influence on frequency or 

 wave-length or direction or intensity due to the motion ot 

 the earth through an ether: and all fail. 



One positive result there is, one case of speed of light 

 being affected — but then this occurs only when matter is 

 artificially moved — the great result of Fizeau. 



But that is no case of absolute motion; matter is being moved 

 relatively to the earth. The Fizeau effect concerns the change 

 in the velocity of light produced by a stream of water up or 

 down which the light is travelling ; and no wonder the result 

 is positive. It demonstrates that the motion of matter affects 

 the speed of light inside that matter but it does not tell us 

 by what mechanism that effect is produced. Something of 



