On the other hand, this method of presentation, by appealing to the propagation of light, is likely to confuse the beginner, who is apt to assume that Einstein postulated the invariant velocity of light as a hypothesis ad hoc for the sole purpose of accounting for Michelson’s negative experiment. In this way the entire theory is supposed to hinge on Michelson’s experiment, and the critic assumes that could Michelson’s experiment be explained in some other way, Einstein’s theory would be obviated. This assumption appears all the more natural to the critic as Michelson’s experiment is, nine times out of ten, the only negative experiment he is acquainted with. The result is that he assumes Einstein’s theory to be nothing but a wild guess grafted on one of those highly delicate experiments where the chances of error are always great. As a matter of fact, by reasoning in this way, the critic loses sight of the entire raison d’être of the theory. It is safe to say that even had Michelson’s experiment never been performed, Einstein’s theory would have been forthcoming just the same (though, of course, had Michelson’s experiment given a positive result, enabling us to measure our velocity through the ether, the theory of relativity would have been untenable). This explains why, in presenting the theory, we started by showing how it arose as a necessary consequence of the irrelevance of absolute velocity in all electromagnetic experiments, hence sprang from the invariance of the equations of electrodynamics, which expresses [Pg 148] mathematically the aggregate of all the negative results. If the reader has grasped the significance of these Lorentz-Einstein transformations, we may proceed to examine certain of their particular consequences, and to show how, quite apart from the negative experiments, the theory of relativity has cleared up a number of obscure points in our understanding of electromagnetics. For instance, we mentioned that a charged body at rest was surrounded by an electric field of force, whereas the same body in motion (or an electric current) was surrounded both by an electric and a magnetic field. Ampère had given a formula describing the distribution and intensity of the electromagnetic field surrounding an electric current of any given intensity. But this formula was purely empirical, and it was felt that we should have been able to anticipate the existence of this electromagnetic field and derive an exact expression of its disposition and magnitude by purely rational methods. These hopes were disappointed; for there appeared to be no rational connection between the field developed by an electrified body when at rest and its field when set in motion.