74 Mr. Mac Cullagh on Crystalline Reflexion. 



The experiments with oil of cassia must be very difficult, on account of the 

 great feebleness of the reflected light. Sir David Brewster, however, resumed 

 them at different times ; and he laid an extensive series of his results before the 

 Physical Section of the British Association at its late meeting in Bristol. 



It was not until the latter end of November, 1836, that I became acquainted 

 with the investigations of M. Seebeck, who has contributed greatly to the 

 advancement of the subject. He made very accurate experiments on the light 

 reflected in air from Iceland spar. He detected the deviation, notwithstanding 

 its smallness, and measured it with great care. He also made the first step in the 

 theory of crystalline reflexion; and the remarkable formula (68), which gives the 

 polarising angle when the axis lies in the plane of incidence, is due to him. The 

 hypotheses which he employed were similar to those of Fresnel, and they enabled 

 him to solve the problem of reflexion in the case just mentioned, but not to 

 attempt it generally. The date of his first papers* is the year 1831 ; but he did 

 not publish his experiments on the deviation until a recent occasion, when he was 

 led to compare themt with the theory which I had originally given in my letter 

 to Sir David Brewster. I have already stated the correction^ which the theory 

 underwent in consequence of those experiments, and by which it was brought to 

 its present simple form. 



* PoggendorfF's Annals, vol. xxi. p. 290 ; vol. xxii. p. 126. f Ibid. vol. xxxviii. p. 280. 



% Two or three months after this correction had been published in the Philosophical Magazine, 

 a notice of it was inserted in PoggendorflPs Annals, vol. xl. p. 462. Up to that time, I believe, 

 nothing had been published in Germany on the general theory of crystalline reflexion ; at least the 

 writer of the notice (whom I take to be M. Seebeck) does not seem to have heard of any other 

 theory, or any other principles, than mine. But in the next number of PoggendorfF, vol. xl. p. 497, 

 there appeared a letter from M. Neumann, in which the writer speaks of a theory of his own, 

 founded on principles exactly the same as those which I had already announced, and refers to a 

 paper which he had communicated on the subject to the Academy of Berhn. The paper has been 

 printed in the Transactions of that Academy for the year 1833; and through the kindness of the 

 author I have received a copy of it, just in time to acknowledge it here. On casting my eye over 

 it, I recognise several equations which are familiar to me ; in particular, the equations (vii.), (viii.), 

 (ix.), (x.), which I discovered independently in November last. M. Neumann's paper is very 

 elaborate, and supersedes, in a great measure, the design which I had formed of treating the 

 subject more fully at my leisure ; nor can I do better than recommend it to those who wish to 

 pursue the investigations through all their details. 



Trinity College, Dublin, March, 1838. 



