214 Notes on some Points in the Theory of Light. 



which we have examined, and which has passed current in the 

 scientific world for a considerable period, is quite inadequate 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,* that the first 

 application 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 questions (which, 

 however, have long been familiar to mathematicians) that the 

 fundamental equations of M. Cauchy may be most advanta- 

 geously employed ; and had he pursued, his researches in this 

 direction, his labours would doubtless have been attended with 

 more success, and with greater benefit to science. 



II. On Fresnel's 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 received 

 theories ; and certainly the universal opinion, up to that time, 

 was, that the intensity of reflexion always increases with the 

 incidence. It may therefore be worth while to remark, that the 

 formula given by Fresnel for reflexion at the surface of a trans- 

 parent body, though not of course applicable, except in a very 

 rude way, to the case of metals, would yet lead us to expect, for 

 highly refracting bodies, as the metals are supposed to be, pre- 

 cisely such a result as that obtained by Mr. Potter. For when 

 the index of refraction exceeds the number 2 + \/3, or the tan- 

 gent of 75, the expression for the intensity of reflected light 

 will be found to have a minimum value at a certain angle of in- 



* Mem. de t'Institut, torn. x. p. 294. 



