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lliat the hypothesis of ethereal molecules symmetrically 

 distributed had led, in the hands of M. Caiichy, to a 

 complete theory of rectilinear polarization in crystals (see 

 his Exercices de Maihematiques, Cinquieme Annee, Paris, 

 1830, and the Memoires de VJnstitut, torn x. p. 293), the 

 notion of endeavouring to account for the phenomena of 

 elliptic polarization, by freeing the hypothesis from any re- 

 striction as to the distribution of the ether, would naturally 

 occur to any one ^yho was thinking on the subject, no less 

 than to M. Cauchy himself. And though, for my own part, 

 I never was satisfied with that theory, which seemed to me to 

 possess no other merit than that of following out in detail 

 the extremely curious, but (as I tliought) very imperfect, 

 analogy which had been perceived to exist between the vi- 

 brations of the luminiferous medium and those of a common 

 elastic* solid (for it is usual to regard such a solid as a rigid 



• The analogy was suggested by the hypothesis of transversal vibrations, which, 

 when viewed in its physical bearing, was considered by Dr. Young to be "perfectly 

 appalling in its consequences," as it was only to solids that a " lateral resistance" 

 tending to produce such vibrations had ever been attributed. (Supplement to the 

 Encyclopedia Brilannica, vol. vi. p. 862: Edinburgh, 1824). He, admit;, how- 

 ever, that the question whether fluids may not " transmit impressions by lateral ad- 

 hesion, remains completely open for discussion, notwithstanding the apparent diffi- 

 culties attending it.'' As far as I am aware, Fresnel always regarded the ether as a 

 fluid. M. Poisson affirms that it must be so regarded, and attributes its apparent 

 peculiarities to the immense rapidity of its vibrations, which does not allow the law 

 of equal pressure to hold good in the state of motion (Annates 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, ditfering from all ponder- 

 able bodies, whether solid or fluid, in this respect, that it absolutely refuses, in any 

 case, to change its density, and therefore propagates to a distance transvetsal vibra- 

 tions oii/y; while ordinary elastic fluids transmit only normal vibrations, and ordi- 

 nary solids admit vibrations of both kinds. This hypothesis also includes the 

 supposition that the density of the ether is unchanged by the presence of pondera- 

 ble 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 /ain/es/ existence; but it is neces- 

 sarily introduced by his identilicalion of the vibrations of light with those of an in- 

 definitely extended elastic solid. 



