1. Double Refraction of Compressed Glass.—One of these confirmatory experiments was the production of double refraction by the compression of glass. Fresnel observes,[107] that though Sir D. Brewster had shown that glass under compression produced colors resembling those which are given by doubly-refracting crystals, “very skilful physicists had not considered those experiments as a sufficient proof of the bifurcation of the light.” In the hypothesis of moveable polarization, it is added, there is no apparent connexion between these phenomena of coloration and double refraction; but on Young’s theory, that the colors arise from two rays which have traversed the crystal with different velocities, it appears almost unavoidable to admit also a difference of path in the two rays.
[107] Ann. de Chim. 1822, tom. xx. p. 377.
“Though,” he says, “I had long since adopted this opinion, it did not appear to me so completely demonstrated, that it was right to neglect an experimental verification of it;” and therefore, in 1819, he proceeded to satisfy himself of the fact, by the phenomena of diffraction. The trial left no doubt on the subject; but he still thought it would be interesting actually to produce two images in glass by compression; and by a highly-ingenious combination, calculated to exaggerate the effect of the double refraction, which is very feeble, even when the compression is most intense, he obtained two distinct images. This evidence of the dependence of dipolarizing structure upon a doubly-refracting state of particles, thus excogitated out of the general theory, and verified by trial, may well be considered, as he says, “as a new occasion of proving the infallibility of the principle of interferences.”
2. Circular Polarization.—Fresnel then turned his attention to another set of experiments, related to this indeed, but by a tie so recondite, that nothing less than his clearness and acuteness of view could have detected any connexion. The optical properties of quartz had been perceived to be peculiar, from the period of the discovery [120] of dipolarized colors by MM. Arago and Biot. At the end of the Notice just quoted, Fresnel says,[108] “As soon as my occupations permit me, I propose to employ a pile of prisms similar to that which I have described, in order to study the double refraction of the rays which traverse crystals of quartz in the direction of the axis.” He then ventures, without hesitation, to describe beforehand what the phenomena will be. In the Bulletin des Sciences[109] for December, 1822, it is stated that experiment had confirmed what he had thus announced.
[108] Ann. de Chim. 1822, tom. xx. p. 382.
[109] Ib. Ann. de Chim. 1822, tom. xx. p. 191.
The phenomena are those which have since been spoken of as circular polarization; and the term first occurs in this notice.[110] They are very remarkable, both by their resemblances to, and their differences from, the phenomena of plane-polarized light. And the manner in which Fresnel was led to this anticipation of the facts is still more remarkable than the facts themselves. Having ascertained by observation that two differently-polarized rays, totally reflected at the internal surface of glass, suffer different retardations of their undulations, he applied the formulæ which he had obtained for the polarizing effect of reflection to this case. But in this case the formulæ expressed an impossibility; yet as algebraical formulæ, even in such cases, have often some meaning, “I interpreted,” he says,[111] “in the manner which appeared to me most natural and most probable, what the analysis indicated by this imaginary form;” and by such an interpretation he collected the law of the difference of undulation of the two rays. He was thus able to predict that by two internal reflections in a rhomb, or parallelopiped of glass, of a certain form and position, a polarized ray would acquire a circular undulation of its particles; and this constitution of the ray, it appeared, by reasoning further, would show itself by its possessing peculiar properties, partly the same as those of polarized light, and partly different. This extraordinary anticipation was exactly confirmed; and thus the apparently bold and strange guess of the author was fully justified, or at least assented to, even by the most cautious philosophers. “As I cannot appreciate the mathematical evidence for the nature of circular polarization,” says Prof. Airy,[112] “I shall mention the experimental evidence on which I receive it.” The conception has since been universally adopted.
[110] Ib. p. 194.
[111] Bullet. des Sc. 1823, p. 33.
[112] Camb. Trans. vol. iv. p. 81, 1831.