160 



chemical rays are also transversal; and we may confidently 

 assert, that the three species of rays — those of light and 

 heat, and the chemical rays, — are produced not only by vi- 

 brations of the same medium, but by the same kind of vibra- 

 tions, propagated with nearly the same velocities. If, there- 

 fore, the third ray of MM. Cauchy and Neumann has any 

 existence, it must be referred to " some other agent," the 

 nature of which it is impossible to conjecture. 



Enough has now been said to show that the optical theory 

 which we have examined, and which has passed current in 

 the scientific world for a considerable period, is quite inade- 

 quate to explain the leading phenomena of light, and that it 

 is based upon principles which are altogether inapplicable to 

 the subject, M. Cauchy states, in the memoir so often 

 quoted {Mem. cle VInstitut, Tom. x. p. 294), that the first ap- 

 plication which he had made of his principles was to the 

 theory of sound, and that the formulas which he had deduced 

 from them agreed remarkably well with the experiments of 

 Savart and others on the vibrations of elastic solids. As I 

 have already intimated, it is in the solution of such ques- 

 tions (which, however, have long been familiar to mathemati- 

 cians) that the fundamental equations of M. Cauchy may be 

 most advantageously employed ; and had he pursued his re- 

 searches in this direction, his labours would doubtless have 

 been attended with more success, and with greater benefit 

 to science. 



II. 

 On FresneVs Formula for the Intensity of Reflected 

 Light, with Remarks on Metallic Reflexion. 



When Mr. Potter discovered, by experiment, that more 

 light is reflected by a metal at a perpendicular incidence 

 than at any oblique incidence (at least as far as 70°), the fact 

 was looked upon, by himself and others, as contrary to all re- 

 ceived theories ; and certainly the universal opinion, up to 

 that time, was, that the intensity of reflexion always increases 



