Similar models have been invented, with more or less success, by other physicists, in such quantities, even, that nearly all that could be done has been tried. The outcome of these various attempts is, in my opinion, this: that we must admit the possibility of mechanical representations, a possibility that is already shown by the fact that the formulae of the electromagnetic field can be given the form of the general equations of dynamics. On the other hand it cannot be denied that, when we desire them to be applicable to a comparatively wide range of phenomena, the theories that have been proposed become so complicated that they can give us but little satisfaction. So, I think, the majority of physicists will agree to attach less importance to mechanical models and even to the general dynamical analogies, however helpful these may be to us, than to Maxwell's equations that summarize so admirably all that is essential.
Will it be possible to maintain these equations? I am not thinking here of the comparatively slight modifications that have been found necessary in the theory of relativity; and by which we explain, for instance, the curvature of a ray of light in a gravitation field. A greater and really serious danger is threatening from the side of the quantum theory, for the existence of amounts of energy that remain concentrated in small spaces during their propagation, to which several phenomena seem to point, is in absolute contradiction to Maxwell's equations. However this may be, even if further development should require profound alterations, Maxwell's theory will always remain a step of the highest importance in the progress of physics.
But I must not trespass any longer on your patience. Let me rather thank you for the attention with which you have listened to what I had to say, which was mainly intended as a tribute to the memory of one of the greatest Masters of Science.