352 FOURTH REPORT — 1834. 



SO sagaciously divined. When sound is propagated through air 

 or tvater, the vibrations of the particles of the fluid are per- 

 formed in the direction in which the wave advances; and if the 

 vibrations of the ether, which are supposed to constitute light, 

 were of the same kind, the objection would seem to be insuper- 

 able. But the case is altered, if, as is now assumed, the vi- 

 brations of the ethereal paiticles be trcmsverse to that of the 

 ray's progress. And though we were unable to render any ac- 

 count of this hypothesis, or even to show that it is consistent 

 with mechanical principles, yet the numerous classes of phe- 

 nomena which it has explained, and the striking and exact 

 manner in which its predictions have been verified on trial, com- 

 pel us to admit, that if the law to which we have thus reduced so 

 various and such complicated facts be not itself a law of nature, 

 it is at least coordinate with it, in such a sense that we may 

 take it as the representative of actual existence, and reason 

 from it as we would from an established physical law. 



The hypothesis of transversal vibrations first occurred to Dr. 

 Thomas Young, who illustrated it by the propagation of undu- 

 lations along a stretched cord agitated at one of its extremities. 

 Young seems to have been led to this principle while consider- 

 ing the results arrived at by Sir David Brewster, in his re- 

 searches on the laws of double refraction in biaxal crystals. 

 The principle was soon after raised above the rank of a mere 

 hypothesis, and shown to be a necessary consequence of the 

 laws of interference of polarized light, if the theory of waves 

 be admitted at all. It follows, in fact, from the laws of com- 

 position of vibrations, that the intensity of the light resulting 

 from the union of two rays oppositely polai'ized vrill be con- 

 stanf, and independent of the phase (as was proved to be the 

 case in the experimental researches of MM. Arago and Fresnel,) 

 only when the vibrations normal to the wave are evanescent. 

 It appears from the same investigation that the actual vibra- 

 tions are either parallel or perpendicular to the plane of polariza- 

 tion. As far as the phenomena of interference are concerned, 

 it is indifferent which of these results be assumed to be the fact. 

 But the theory of transversal vibrations itself, when applied to 

 the laws of double refraction, leads to the conclusion that the 

 vibrations which constitute the ordinary ray in uniaxal crystals 

 are perpendicular to the principal platie ; and this being its 

 plane of polarization, Fresnel concluded that the vibrations of a 

 polarized ray are on the surface of the wave, and perpendicular 

 to the plane of polarization *. 



* " Memoire sur la Double Refraction," Mem. Inst., torn. vii. 



