Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/142

 undulations of sound, consisting simply in the direct and retrograde motions of the particles in the direction of the radius, with their concomitant condensation and rarefactions. And yet it is possible to explain in this theory a transverse vibration, propagated also in the direction of the radius, and with equal velocity, the motions of the particles being in a certain constant direction with respect to that radius; and this is a polarization."

In an article on "Chromatics," which was written in September of the same year for the supplement to the Encyclopaedia Britannica, he says: "If we assume as a mathematical postulate, on the undulating theory, without attempting to demonstrate its physical foundation, that a transverse motion may be propagated in a direct line, we may derive from this assumption a tolerable illustration of the subdivision of polarized light by reflexion in an oblique plane," by "supposing the polar motion to be resolved" into two constituents, which fare differently at reflexion.

In a further letter to Arago, dated April 29th, 1818, Young recurred to the subject of transverse vibrations, comparing light to the undulations of a cord agitated by one of its extremities. This letter was shown by Arago to Fresnel, who at once saw that it presented the true explanation of the non-interference of beams polarized in perpendicular planes, and that the latter effect could even be made the basis of a proof of the correctness, of Young's hypothesis: for if the vibration of each beam be supposed resolved into three components, one along the ray and the other two at right angles to it, it is obvious from the Arago-Fresnel experiment that the components in the direction of the ray must vanish: in other words, that the vibrations which constitute light are executed in the wave-front.

It must be remembered that the theory of the propagation of waves in an elastic solid was as yet unknown, and light was