between two Substances of different refractive Powers. 29 



I have not found any variation in these results from chang- 

 ing the position of the plane of reflexion on the diamond 

 surface. 



The result of these experiments and reasonings may be 

 thus stated. 



1. When the angle of incidence is less than the maximum 

 polarizing angle of the diamond, the nature of its reflexion is 

 similar to that of metallic reflexion : the phase of vibrations 

 in the plane of reflexion being more retarded than that of 

 vibrations perpendicular to the plane of reflexion, but per- 

 haps by a smaller quantity than in reflexion from metals. 



2. In the neighbourhood of the polarizing angle, the nature 

 of the reflexion is different from any that has hitherto been 

 described. The vibrations in the plane of reflexion do not 

 vanish, but on increasing the angle of incidence by three or 

 four degrees the phase of vibration is gradually retarded by 

 nearly 1 80°. In the reflexion of light whose vibrations are 

 perpendicular to the plane of reflexion, there is no striking 

 difference between the effects of diamond and those of glass. 



3. For angles of incidence greater than the polarizing angle, 

 there is no sensible difference between the effects of diamond 

 and those of glass. 



I may remark that the extent of vibration in the plane of 

 reflexion may be represented thus (the formula being purely 

 empirical and given only for illustration). The vibration in 



the incident light being a sin — (vt — x\ that in the reflected 



light is 



tan(7-i") . 2tt. 2tt. . 



— 77- a sin — ivt — x) — a a cos [vt — iu 



tan (V + 1") ^ : , h v 



where b is always small but never = 0, and is perhaps con- 

 stant. 



The conclusions at which I have arrived are at variance 

 with one of Sir David Brewster's (Phil. Trans. 1815). Sir 

 David Brewster's character as an experimental philosopher 

 stands deservedly so high, and my estimation of his accuracy 

 (as observed by myself in the repetition of many of his expe- 

 riments) is so great, that I think it necessary to point out 

 distinctly the nature of this disagreement. 



Sir David Brewster states that homogeneous light is com- 

 pletely polarized by the diamond at the proper angle. I have 

 made no experiments here with homogeneous light, and I 

 know that, on account of its extreme faintness, however ob- 

 tained, little confidence can be placed in results which depend 

 only on the evanescence of the reflected light. But the pha> 



