REPORT ON PHYSICAL OPTICS. 361 



the first, except tlie perpendicular one, experience proves that 

 a portion of the light is reflected and another portion refracted. 

 According to this theory, then, some of the molecules obey the 

 polarizing force and have their axes brought into the plane of 

 reflexion, while others do not. To account for this diversity of 

 eff"ect there must be some diversity of condition in the molecules 

 themselves. The theory of M. Biot supplies this by attributing 

 to them an oscillatory movement round their axes of polariza- 

 tion, the molecules yielding to the polarizing force or not, 

 according to the phase of the oscillation in which they are 

 found at the moment they reach the surface. 



The force which impresses the property of polarization upon 

 the refracted pencil is supposed by M. Biot to act also in the 

 plane of incidence, its operation however being to turn the axes 

 of polarization of the luminous molecules in a direction perpen- 

 dicular to that plane. Thus, when a ray of light traverses the 

 surface of a plate of glass at the polarizing angle, it is subjected 

 to the action of two forces, one tending to bring the axes of 

 polarization of the molecules into the plane of incidence, the 

 other to turn them at right angles to it; and the molecules them- 

 selves 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 TraiU de Phy- 

 sique*. The whole quantities of light oppositely 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 common 

 light. The whole quantities of light polarized by the two 

 forces being then equal, the remaining portions effectively polar- 

 ized 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 re- 

 quired, 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 expla- 

 nation of the phenomena ? The assumed forces and the known 

 laws have not been connected, in any one instance, by the 

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



