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 Royal 
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 Ritz, 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. JI 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 léther, 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.” 
