92 REPORT ON PHYSICAL OPTICS. 



molecules into the plane of incidence, the other to turn them at 

 right angles to it ; and the molecules themselves yield to one or 

 other of these forces according to the phases of their fits. For the 

 manner in which this may be supposed to take place we must refer 

 to the Traite de Physique* The whole quantities of light oppo- 

 sitely polarized by the two forces M. Biot supposes to be equal ; 

 but he conceives that the force which polarizes the reflected pencil 

 is exerted on a much greater number of molecules than those 

 which actually undergo reflexion. These molecules, thus polarized 

 in the plane of incidence, enter into the transmitted beam 

 neutralize an equal number of molecules polarized by refraction in 

 the opposite plane, and compound with them a beam of com- 

 mon light. The whole quantities of light polarized by the two 

 forces being then equal, the remaining portions effectively polarized 

 will still be equal, conformably to the law discovered by M. Arago. 



I have endeavoured to present the theory of M. Biot as fully 

 as the limits of the present paper will permit, because it appears to 

 me that the number and the nature of the hypotheses required, in 

 order to render any account of the phenomena of polarization in 

 the theory of emission, furnish in themselves a sufficient argument 

 against it. But let all these be admitted, and how far can we be 

 said to have advanced towards an explanation of the phenomena ? 

 The assumed forces and the known laws have not been connected, 

 in any one instance, by the sure processes of mathematical deduc- 

 tion ; and we are therefore unable to state how far the explanation 

 offered is competent to express even the general facts, far less can 

 we calculate them numerically, and compare the results with those 

 of observation. 



The first attempt to connect the modifications of reflected light 

 with the theory of waves was made by Dr. Thomas Young. This 

 sagacious philosopher succeeded in solving the problem of reflexion 

 in the case of perpendicular incidence, and showed that the inten- 

 sity of the reflected light in that case was represented by a simple 

 function of the refractive index.f This formula was afterwards 

 reproduced as the result of a more refined analysis by M. Poisson, 

 in a memoir on the simultaneous motions of two elastic fluids in 

 contact, read to the French Academy in 18174 In that memoir, 



* Book vi. chap. i. vol. iv. 



t Encyc. Brit. Supp., Art. CitaoMATics. 



% M<im. Inst., torn. ii. 



