If we regard the FitzGerald contraction as physically real—and we must do so in order to explain Michelson’s negative result—we must recognise that it is certainly compensated in a miraculous way; since no experiments, whether optical or electrical, appear to be capable of revealing it. Furthermore, in the last experiment, that of Trouton and Noble, we are not dealing primarily with the FitzGerald contraction; we are here in the presence of an experiment of electrodynamics, and we are endeavouring to detect a purely mechanical force arising as a result of the motion of our electrified bodies through the ether. The laws of electrodynamics require that such a force be generated, and yet no force can be observed; so we must assume that the mysterious compensations that conceal our motion through the ether are of so perfectly general a nature that they affect mechanics as well as electromagnetics and optics. True, none of the experiments we had been able to execute exceeded the second order in precision; and there was nothing in Lorentz’s theory, even when the FitzGerald contraction was taken into consideration, to suggest that all experiments, whatever their nature and however precise they might be, would fail to detect our velocity through the ether. A very simple hypothetical experiment will make this point clear. Suppose, for instance, that the earth were rushing through the ether with a speed approximating that of light; and suppose that we suddenly stepped in front of a mirror. All that the FitzGerald contraction hypothesis would ensure would be that the time required by the light rays to leave our body and be reflected back to our eyes should be independent of the orientation of the mirror with respect to the earth’s motion. But the actual duration which the light rays would require, hence the time it would take for our image to appear to us, would depend essentially on the earth’s speed through the ether; so that theoretically, at least, a measure of duration should reveal this mysterious speed. However, in view of the fact that the highly precise experiments that had been performed were incapable of detecting this velocity through the ether, it appeared legitimate to extrapolate, to assume that something deeper was at stake, and that in all probability this velocity would remain undetected, however far we might extend the precision of our electrodynamic or optical experiments, and however much we might vary their nature. Lorentz accepted what appeared to him as the inevitable, and asserted that the time had come to recognise that nature seemed to have entered into some giant conspiracy to defraud us of a knowledge of our velocity through the ether. Accordingly, he laid down his celebrated principle of [Pg 135] correlation, according to which adjustments were so regulated in nature that the velocity of our planet through the ether could never be detected, however precise our experiments.