208 Notes on some Points in the Theory of Light. 



facts, and, notwithstanding its so-called explanations of other 

 laws, should be finally abandoned. Under these circumstances, 

 therefore, he very naturally supposed that his new results 

 must be in complete harmony with the phenomena discovered 

 by M. Arago, and analyzed so successfully by MM. Biot and 

 Fresnel ; although, had he taken the precaution of acquiring 

 such a clear notion of the phenomena as would have enabled 

 him to translate them into analytical language, he must have 

 perceived that they were entirely opposite to his results, and 

 that this opposition furnished an argument which swept away 

 the very foundations of his theory. For, if the constitution 

 of the luminiferous medium were such as M. Cauchy sup- 

 poses, the well-known phenomena of circular and elliptic 

 polarization would, as we have seen, be absolutely impossible. 

 Thus the argument which overturns the particular theory 

 of elliptical polarization destroys at the same time all the 

 other optical theories of M. Cauchy, because they are all 

 built on the principles which we have now demonstrated to be 

 false. But though the principles of M. Cauchy are now, for 

 the first time, formally refuted, they were objected to, on 

 general grounds, so long ago as the year 1830, by a person 

 whose opinion, on a question of mechanics, ought to have had 

 considerable weight. This was M. Poisson, who, having de- 

 duced from the equations of motion of an elastic solid the con- 

 sequence that such a body admitted vibrations perpendicular 

 to the direction of their propagation, thought it right to re- 

 mark that this conclusion could not be supposed to account 

 for transversal vibrations in the theory of light, because (as 

 he expressed himself) "the same equations of motion could 

 not possibly apply to two systems [of molecules] so essen- 

 tially different from each other" as the ethereal fluid and 

 an elastic solid.* (See the Annales de Chimie^ torn. xliv. 



* As the theory of M. Cauchy (Mem. de Vlnstitut, torn, x.) had been communi- 

 cated to the Academy of Sciences some months before the period (October, 1830) at 

 which M. Poisson wrote, there can be no doubt that M. Poisson' s remark was di- 

 rected against that theory, though he did not expressly mention it. 



