136 On the Laws of Crystalline 



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 Physi- 

 cal 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 investigation of M. Seebeck, who 

 has contributed greatly to the advancement of the subject. He 

 made very acurate 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 remark- 

 able formula (68), which gives the polarizing angle when the 

 axis lies in the plane of incidence, is due to him. The hypo- 

 theses 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 them f with the theory which I had origi- 

 nally given in my letter to Sir David Brewster. I have already 

 stated the correction which the theory underwent in conse- 



to it, but 75 out of it. The same surface, whon the light was reflected in air, 

 gave the polarizing angle more than two degrees below its proper value. 



To show that, in other respects, the general character of the phenomena is in 

 accordance with theory, we may observe that when N = S, and \ = or 90, if 

 common light be incident at 45 in the plane of the principal section of the crystal, 

 the whole of the reflected light will be polarized perpendicularly to that plane ; and 

 therefore if N be nearly equal to , while every thing else remains the same, the 

 reflected pencil will contain some unpolarized light, and will be only partially pola- 

 rized in a plane perpendicular tu the plane of incidence ; so that (as Brewster has 

 found by experiment) the crystal will then produce by reflexion the same effect 

 which is produced by ordinary fraction. This (as he also found) will not happen 

 when A. and the angle of inc.uience are each equal to 45, because the light is then 

 incident at the polarizing angi. 



* Poggendorff's Annals, VoT; xxi. p. 290 ; Vol. xxn. p. 126. 



t Ibid., Vol. xxxvin. p. 280. \ 



