46 Presidential Address 



is it a concrete physical entity on which 

 we can experiment? 



Now it has to be freely admitted that 

 it is exceedingly difficult to make experi- 

 ments on the ether. It does not appeal 

 to sense, and we know no means of getting 

 hold of it. The one thing we know 

 metrical about it is the velocity with 

 which it can transmit transverse waves. 

 That is clear and definite, and thereby 

 to my judgment it proves itself a physical 

 agent; not indeed tangible or sensible, 

 but yet concretely real. 



But it does elude our laboratory grasp. 

 If we rapidly move matter through it, 

 hoping to grip it and move it too, we 

 fail: there is no mechanical connection. 

 And even if we experiment on light we 

 fail too. So long as transparent matter 

 is moving relatively to us, light can be 

 affected inside that matter; but when 

 matter is relatively stationary to matter 

 nothing observable takes place, however 

 fast things may be moving, so long as 

 they move together. 



Hence arises the idea that motion with 



