REPORT ON PHYSICAL OPTICS. 353 



The principle of transversal vibrations, thus deduced from the 

 phenomena of interference of polarized light, is easily extended 

 to the case of common or unpolarized light. For when a ray of 

 such light falls perpendicularly upon a double- refracting crystal, 

 it is divided into two polarized pencils, neither of which, it ap- 

 pears from the preceding, can contain vibrations normal to the 

 surface of the wave. If, then, there were any such in the inci- 

 dent ray, they would be destroyed by refraction, and there 

 would ensue a loss of vis viva, and consequently a diminution in 

 the intensity of the light ; in other words, the sum of the in- 

 tensities of the two refracted pencils would be less than that of 

 the incident, which is contraiy to observation. In unpolarized 

 light therefore, as in polarized, the vibrations are only on the 

 surface of the waves ; and we must conceive such light to consist 

 of a rapid succession of systems of waves polarized in every pos- 

 sible plane passing through the normal to the front of the wave. 

 The phenomenon of polarization then, in this theory, consists 

 simply in the resolution of the vibrations into two sets, in two 

 rectangular directions, and the subsequent sepai'ation of the two 

 systems of waves thus produced. 



The erroneous views of mathematicians on this subject, ac- 

 cording to Fresnel, have arisen from the imperfect physical 

 conceptions which they have made the basis of their reasoning. 

 Elastic fluids have been represented as composed of particles in 

 contact, capable only of condensation and dilatation ; and ac- 

 cordingly the accelerating forces have been conceived to arise 

 solely from the difference of density of the consecutive shells of 

 the fluid. In this case, it is evident that if any row of particles 

 is displaced in the direction of the connecting line, this row will 

 slide upon the succeeding one, and the motion will be resisted 

 by no elastic force. But when we regard these bodies as they 

 really are, composed of molecules separated by intervals which 

 are probably considerable as compared with their magnitude, 

 and acting on one another according to some law varying with 

 the distance, the whole question is altered. When any row or 

 line of such molecules is similarly displaced, and through a 

 space which is small compared with the separating intervals, the 

 molecules of the succeeding row will be moved in the same di- 

 rection by the forces which are thus developed with the change 

 of distance ; so that the vibrations of the particles composing 

 the first row will be communicated to those of the second, and 

 thus the vibratory motion will be propagated in a direction per- 

 pendicular to that in which it takes place*. The rapidity of the 



• The existence of transversal vibrations has been fully establislied in other 

 cases of vibratory motion. M. Savart and Mr. Wheatstone have shown that in 

 1834. 2 A 



