Laws of Metallic Reflexion. 231. 



subject, respecting which SOKLG new questions suggested them- 

 selves, which I thought it right to discuss in notes appended to 

 the essay. I was not afterwards at leisure to take up the ex- 

 perimental inquiry, until the beginning of the year 1839, when 

 I began to think of putting the instrument in order for that 

 purpose. The strain which it had suffered rendered some slight 

 alterations necessary ; and I now resolved to make additions to 

 it also, with the view of operating upon the fixed lines of the 

 spectrum, as a few trials had convinced me that measures suffi- 

 ciently precise could not be obtained without employing light 

 of definite ref rangibility. I wished, moreover, to take the oppor- 

 tunity which the nature of the proposed experiments presented, 

 of verifying the theory of Fresnel's rhomb, or rather of verify- 

 ing, by means of the rhomb, the formulae which Fresnel has 

 given for computing the effects of total reflexion, when it takes 

 place at the common surface of two ordinary media. I wrote 

 therefore to Munich for several articles which I wanted ; among 

 others, for a set of rhombs cut at different angles, out of dif- 

 ferent kinds of glass. But while I was waiting for these some 

 months elapsed, and in the meantime I got sight of a new 

 theory, which, from its connexion with my former researches, 

 possessed more immediate interest, and the pursuit of which, in 

 conjunction with other studies and various engagements, caused 

 me again to suspend the inquiry respecting the laws of metallic 

 reflexion. I allude to the " Dynamical Theory of Crystalline 

 Reflexion and Refraction," communicated to the Academy in 

 December, 1839.* This was followed soon after by a general 

 " Theory of Total Reflexion,"! founded on the same principles. 

 The latter theory, forming a new department of physical optics, 

 and involving the solution of questions not previously attempted, 

 was analytically complete when it was communicated to the 

 Academy in May, 1841 ; but its geometrical development has 

 since required my attention from time to time, and has not yet 

 been brought to that degree of simplicity of which it appears to 



* Proceedings^ VOL. i. p. 374, (supra, p. 145). 

 f Ibid. VOL. it. p. 96 (xttpra, p. 187). 



