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 electromagnetic 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 differential 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 l'éther, 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."
