Now, experiment suggests that the propagation of light in vacuo furnishes us with a concrete example of this critical velocity. It follows that a measure of the speed of light propagation will yield us the precise numerical value of the critical velocity. For this reason, the propagation of light assumes a position of vast theoretical importance. Needless to say, this importance of light is due solely to its physical behaviour in vacuo, solely to the fact that it moves with the invariant speed, not to the fact that because of some accidental circumstance it happens to be visible to the human eye. For light passing through glass or water is also visible, but its speed, being reduced, is no longer invariant. Hence light propagation through matter can no longer serve to define the invariant velocity directly. Besides we can get away from light as a visible propagation by considering an invisible electromagnetic propagation in its stead. If we wish, we can do better still and obviate electromagnetic waves entirely. For when account is taken of the negative experiments in electrodynamics, we realise that the invariant velocity is none other than the constant which enters into Maxwell’s equations, whence any experiment capable of measuring would automatically yield us the invariant velocity. In particular, we might measure the ratio of an electric charge when computed in terms of electrostatic, then of electromagnetic units; this again would yield us the value of Maxwell’s constant , hence of the critical invariant velocity. We might also appeal to the necessary consequences of the existence of a critical velocity, for instance, to the peculiar Lobatchewskian law which it would entail for the additions of velocities. We should then check these consequences by experiment, discover whether they were justified by facts, and deduce therefrom the existence and value of the invariant velocity. [Pg 205]