Sib David Brewster on the Compensations of Polarized Light. 379 



Angles of Incidence. No. of Rays, out of 1000 polarized by Reflection and Refraction. 



24° 10.5 



80° 158 



83f 139.3 



Hence, it is obvious that the compensation is not produced by equal quantities of 

 light polarized in opposite planes ; and it would be absurd to suppose that the 

 portions of common light existing in each of the partially polarized pencils per- 

 formed any part in the compensation. But even if it did, it could act only by its 

 quantity — that is, by the relation which it bore to the polarized portion of the 

 beam. Now, in the three cases which we have noticed, the ratio of the common 

 to the polarized portion of the pencil is not the same, although the compensation 

 is perfect, as the following numbers show : 



Ratio of common and polarized Light. 



Hence, we are forced to the conclusion, that the compensation is produced neither 

 by an equality of oppositely polarized rays, nor by a proportional admixture of 

 common light, but by equal and opposite physical states of the whole pencil, whe- 

 ther reflected or refracted. 



Let us now consider what takes place at the polarizing angle, or 56° 45', in 

 glass. The whole of the reflected light, or 792- rays, is here wholly polarized, 

 and the same quantity of oppositely polarized light, viz. 792- ^^Y^' exists in the 

 refracted beam. Now, this refracted beam is not capable of compensating the 

 reflected one, notwithstanding their equality in point of polarized light, and 

 though the reflected beam is not mixed with common light ; so that, upon the 

 old hypothesis, the refracted beam can owe its deficient power of compensation 

 only to the large quantity of common light which it contains. 



But though in the compensations already mentioned the proportions of com- 

 mon to polarized light are different ; yet, in other cases of compensation, such 

 as the following, the proportion is pretty nearly equal ; but this equality is acci- 

 dental, and is not the cause by which the compensation is produced. 



3 c 2 



