CHAPTER II.
MAXWELLIAN EQUATIONS FOR MOVING MEDIA AND FRESNEL'S DRAGGING COEFFICIENT. LORENTZ'S EQUATIONS.
The modern principle of relativity arose on the ground of Lorentz's electrodynamics and optics of moving bodies. Einstein's work, in fact, consisted mainly in deducing logically, on the basis of plausible and sufficiently general considerations, certain formulae of space and time transformation, which in Lorentz's theory had partly a purely mathematical meaning and partly the character of an hypothesis invented ad hoc ('local time' and the contraction hypothesis, respectively). In a word, Einstein has given a plausible support to, and a different interpretation of, what appeared already in the theory of the great Dutch physicist. In its turn, the theory of Lorentz, based on the macroscopic treatment of a crowd of electrons (though later supported and made vital by physical evidence of an entirely different kind), was constructed by its author chiefly with the purpose of accounting for optical phenomena in moving bodies, which may be best grouped summarily under the head of Fresnel's 'dragging coefficient' and with which the equations of Maxwell and of Hertz-Heaviside have proved to be in complete disagreement.
Now, it seems to me that the best, most natural and most efficient way of propagating new ideas (if indeed there is such a thing arising in the collective mind of humanity) is to show their intimate connexion with older ones, and the more so when the new ideas have the reputation, widespread but partly unjustified in our case, of being of a very revolutionary character. It will be advisable, therefore, before entering upon our proper subject, to turn back to Lorentz and Maxwell. In doing so, I must warn the reader at the outset that the new Relativity, though grown on electromagnetic soil, does not—in spite of a current opinion—require us at all to adopt an electro-
