498 ADDRESS DELIVERED BEFORE THE BRITISH ASSOCIATION 



transmitted through a fine grating, it is turned aside, or dif- 

 fracted, according to laws which the wave-theory has explained. 

 Now Professor Stokes has shown that, when the incident light 

 is polarized, the plane of vibration of the diffracted ray must differ 

 from that of the incident, the two planes being connected by a 

 very simple relation. It only remained, therefore, for observa- 

 tion to determine whether the planes of polarization of the incident 

 and refracted rays were similarly related, or not. The experiment 

 was undertaken by Professor 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. Holtzmann, on 

 repeating the experiment, the question must be regarded as still un- 

 determined. The difference in the experimental results is ascribed 

 by Professor Stokes to the difference in the nature of the gratings 

 employed, the substance of the diffracting body being supposed to 

 exert an effect upon the polarization of the light, which is diffrac- 

 ted by it under a great obliquity. I learn from Professor Stokes 

 that he proposes to resume the experimental inquiry, and to test 

 this supposition by employing gratings of various substances. If 

 the conjecture should prove to be well founded, it will, unfor- 

 tunately, greatly complicate the dynamical theory of light. In 

 the meantime 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 interference-refractor of M. Jamin, which 

 the inventor has already applied to the study of the effects upon 

 light* produced by grazing a plate of any soluble substance in- 

 closed in a fluid. 



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 overthrown by the decisive experimentum cruets 

 of MM. Fizeau and Foucault. It was, therefore, probable, a 

 priori, that this law the only one peculiar to the theory would 

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

 mental test by M. Jamin. Water, it is known, has its maximum 

 of density at about 40 of Fahrenheit ; so that, if Newton's law 

 were true, its refractive index should also have a maximum value 

 at the same temperature. This has been disproved by M. Jamin, 

 by observing the interference of two rays, one of which has passed 



