2OO Notes on some Points in the Theory of Light. 



it is usual to regard such a solid as a rigid system of attracting 

 or repelling molecules, and M. Cauchy has really done nothing 

 more than transfer to the luminiferous ether both the constitu- 

 tion of the solid and differential formulas of its vibration), still 

 I should have been glad, in the absence of anything better, to 

 find my equations supported by a similar theory, and their form 

 at least countenanced by the like mechanical analogy. Besides, 

 I recollected that Fresnel himself, in his Memoir on Double Re- 

 fraction, had indicated a "helicoidal arrangement," or something 

 of that sort, as a probable cause of circular polarization* and as 

 this was an hypothesis of the same kind as the other, only not so 

 general, I was prepared to find that the supposition of an arbitrary 

 arrangement, whatever might be thought of its physical reality, 

 would lead to equations of the same form as those which I had 

 assumed. Upon trial, however, the very contrary proved to be 

 the case ; for though it was possible to obtain additional terms, con- 

 taining differential co-efficients of the third order, multiplied by 

 the same constant (7, yet this constant always came out with 

 the same sign in both equations, whereas a difference of sign 

 was essential for the expression of the phenomena. I had no 

 sooner arrived at this result than I perceived it to be fatal to 

 the theory of M. Cauchy, and to afford a demonstration of its 

 insufficiency, not only in the particular application which I had 

 made of it, but in all its applications. For the hypothesis 



allow the law of equal pressure to hold good in the state of motion (Annales de 

 Chimie, torn. xliv. p. 432). M. Cauchy calls the ether a fluid, though he treats it 

 as a solid. My own impression is, that the ether is a medium of a peculiar kind, 

 differing from all ponderable hodies, whether solid or fluid, in this respect, that it 

 absolutely refuses, in any case, to change its density, and therefore propagates to 

 a distance transversal vibrations only ; while ordinary elastic fluids transmit only 

 normal vibrations, and ordinary solids admit vibrations of both kinds. This hypo- 

 thesis also includes the supposition that the density of the ether is unchanged by 

 the presence of ponderable matter. As to M. Cauchy's third ray, with vibrations 

 nearly normal to the wave, there is no reason to believe that it has even the 

 faintest existence; but it is necessarily introduced by his identification of the 

 vibrations of light with those of an indefinitely extended elastic solid. 

 * Memoires de V Institut, torn. vii. p. 73. 



