THE STATUS OF THE ETHER 113 



is to be reconciled with the hypothesis of the ether or the differential 

 equations at all. In fact, the views of Sir Joseph are to me in many 

 places incomprehensible. In his lectures recently delivered at the Koyal 

 Institution on the electromagnetic theory of light, however, Sir Joseph 

 categorically expresses himself as of the opinion that the electromag- 

 netic theory of light is one of the great achievements of modern science. 

 To me this means that he approves of the ether. To take the extreme 

 argument of Bitz, who employs as a fundamental necessity the retarded 

 potential, seems to me to be exactly the same thing as to say that the 

 ether exists, for since nothing whatever is propagated with finite 

 velocity, this is the same to me as saying that it is propagated in the 

 ether. In the first part of this paper, I have defined what I mean by 

 the ether in very guarded form. This definition I see no reason to 

 change. Whether we begin with the retarded potential and find that 

 it satisfies a differential equation, or whether we begin with the differ- 

 ential equation and find that it is satisfied by a retarded potential is to 

 me a matter of utter indifference and implies an ether. I admit that 

 we still have to find a hypothesis for the ether which makes it give rise 

 to this differential equation. The hypothesis of Maxwell seems to me 

 the easiest one yet proposed. I will therefore close by stating my 

 present opinion, that the ether is as good to-day as it ever was, but that 

 apparently the notions of time and space have had to be modified in the 

 method suggested by Lorentz and splendidly developed by Einstein and 

 Minkowski. At the same time, we can not deny that there exists to-day 

 what we may call la crise de V ether, and we are far from being able to 

 say with Lord Kelvin, " It is absolutely certain that there is a definite 

 dynamical theory for waves of light, to be enriched, not abolished, by 

 electromagnetic theory." 



