54 Presidential Address 



time taken by light to travel to and fro 

 on a measured interval fixed on a rigid 

 block of matter is independent of the 

 aspect of that block with respect to any 

 motion of the earth through space. A 

 definite and most interesting result: but 

 it may be, and often is, interpreted 

 loosely and too widely. 



It is interpreted too widely, as I think, 

 when Professor Einstein goes on to as- 

 sume that no non-relative motion of 

 matter can be ever observed even when 

 light is brought into consideration. The 

 relation of light to matter is very curious. 

 The wave front of a progressive wave 

 simulates many of the properties of 

 matter. It has energy, it has momentum, 

 it exerts force, it sustains reaction. It 

 has been described as a portion of the 

 mass of a radiating body, which gives 

 it a curiously and unexpectedly corpuscu- 

 lar ' ' feel. ' ' But it has a definite velocity. 

 Its velocity in space relative to the ether 

 is an absolute constant independent of 

 the motion of the source. This would 

 not be true for corpuscular light. 



