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British Association. 42 1 



■explained. Now, Prof. Stokes has shown that, when the incident liglit 5s 

 polarized, the plane of vibration of the diffracted ray must differ fj'om 

 that of the incident, the two planes being connected by a very simple re- 

 lation. It only remained, therefore, for observation to determine whether 

 i\iQ planes of polarization of the incident and refracted rays were similarly 

 related or not. The experiment was undertaken by Prof. Stokes himself, 

 and he has inferred from it that the original hypothesis of Fresnel is the 

 true one. • But, as an opposite result has been obtained by M. Iloltzinann, 

 on repeating the experimentj the question must be regarded as still un- 

 determined. 



The difference in the experimental results is ascribed by Prof. Stokes 

 to the difference in the nature of the gratings employed by himself and 

 by the German experimentalist, the substance of the difiracting body 

 being supposed to exert an effect upon the polarization of the light, which 

 is diffracted by it under a great obliquity. I learn from Prof. Stokes that 

 he proposes to resume the experimental inquiry, and to test this supposi- 

 tion by employing gratings of various substances. If the conjecture 

 should j^rove to be well founded, it will greatly complicate the dynamical 

 theory oi light. In the m(d?in tizne the hypothesis is one of importance 

 in itself, and deserves to be verified or disproved by independent means. 

 I would venture to suggest that it may be effectively tested by means of 

 the beautiful Interfcnncc-refracior of M. Jamiu, which the inventor has 

 already applied to study the effects upon light produced by grazing a 

 plate of any soluble substance inclosed in a fJuid. It is well known that 

 the refractive index of bodies increases with their density; and the theory 

 of emission has even expressed the law of their mutual dependence. 

 That theory, it is true, is now completely overthroM^n by the decisive ex- 

 perimentum crucis of MM, Fizeau and Foucault. It was, therefore, prob- 

 able, a priori J that this law — the only one peculiar to the theory — should 

 be found wanting. Its truth has recently been put to an experimental 

 test by M. Jamin. Water, it is known, has its maximum of density at 

 about 40° of Fahrenheit; and accordingly, if Newton's law were true, 

 its refractive index should also have a maximum value at the same tem- 

 perature. This has been disproved by M. Jamin, by observing the inter- 

 ference of two rays, one of which has passed through air, and the other 

 through water; and thus the last conclusion of the emission-theory has 

 been set aside. 



It would occupy too much of your time were I to touch, even lightly, 

 Tipon the subject of the chemical action of light, and the many beau- 

 tiful and important discoveries of the art to which it has given rise. 

 I nniy, however, mention, as one of the latest of the marvels of pho- 

 tography, that M. Poiteviu has succeeded in producing plates in relief, 

 for the purposes of engraving, by the action of light alone. The process 

 depends upon the change in the affinity for water, produced by the 

 action of light upon a thin plate of gelatine, which is impregnated with 



bichromate of potash. 



