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ROYAL IRISH ACADEMY. 



May 8, 1843. — Sir Wm. R. Hamilton, LL.D., President, in the 

 Chair. 



Professor MacCullagh read the following communication : — On 

 the Laws of Metallic Reflexion, and on the Mode of making Expe- 

 riments upon Elliptic Polarization. 



Several years ago, as the Academy are aware, I made an attempt 

 to investigate the laws according to which light is reflected at the 

 surfaces of metals, and I then proposed certain formulae which repre- 

 sented, with sufficient accuracy, all the facts and experiments which 

 I was able to collect upon the subject (see the Proceedings of the 

 Academy, vol. i. p. 2, October 1836 ; Transactions, vol. xviii. p. 71, 

 note). But in order to test these formulae satisfactorily, it was ne- 

 cessary to obtain measurements far more exact than any that had 

 previously been made ; and for this end I devised an instrument, 

 which was constructed for me by Mr. Grubb, and of which a brief 

 description has been given in the Proceedings, vol. i. p. 159. I regret 

 to say, however, that nothing of much consequence has yet been done 

 with the instrument. Some preliminary 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 suf- 

 fered, 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, 

 respecting 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 alterations necessary; and I now re- 

 solved 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 employ- 

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

 opportunity which the nature of the proposed experiments 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 com- 

 puting the effects of total reflexion, when it takes place at the 

 common surface of two ordinary media. I wrote therefore to Mu- 

 nich for several 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 mean- 

 time I got sight of a new theory, which, from its connexion with my 

 former researches, possessed more immediate interest, and the pur- 

 suit of which, in conjunction with other studies and various engage- 

 ments, caused me again to suspend the inquiry respecting the laws 

 of metallic reflexion. I allude to the Dynamical Theory of Crystal- 

 line Reflexion and Refraction, communicated to the Academy in 



