154 



the rotatory fluids — if their laws were essentially or very 

 considerably different — his theory would be inconsistent 

 with a wide range of well known facts, and, notwith- 

 standing its so-called explanations of other laws, should be 

 finally abandoned. Under these circumstances, therefore, 

 he vei'y naturally supposed that his new results must be 

 in complete harmony with the phenomena discovered by 

 M. Arago, and analysed 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 opposed 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 po- 

 larization would, as we have seen, be absolutely impos- 

 sible. 



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 tohe 

 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 ISriO, 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 perpendicu- 

 lar to the direction of their propagation, thought it right to 

 remark 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 



