376 



tiling of much consequence has yet been clone with the 

 instrument. Some prehminary trials of its performance were 

 indeed made in the summer of 1837, and the results of one 

 of these shall presently be given; but an accidental strain 

 which it suffered, while I was preparing to undertake a series 

 of experiments, caused me to discontinue the observations 

 at the time ; and being then obliged to superintend the 

 printing of my essay on the Laws of Crystalline Reflexion 

 and Refraction (Transactions R. I. A., vol. xviii. p. 31), 

 my attention was drawn afresh to this latter subject, respect- 

 ing which some new questions suggested themselves, which 

 I thought it right to discuss in notes appended to the essay. 

 I was not afterwards at leisure to take up the experimental 

 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 alte- 

 rations 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 

 sufficiently precise could not be obtained without employing 

 light of definite refrangibility. I wished, moreover, to take 

 the opportunity which the nature of the proposed experi- 

 ments presented, of verifying the theory of Fresnel's rhomb, 

 or rather of verifying, 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 se- 

 veral articles which I wanted ; among others, for a set of 

 rhombs cut at different angles, out of different 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, pos- 

 sessed 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 



