378 Sir David Brewstkr on the Compensations of Polarized Light. 



mixture of polarized and common light ; and I have accordingly endeavoured at 

 different times, though without success, to obtain such a test. While studying, 

 however, the polarizing structure of the atmosphere, where it became desirable 

 to ascertain the degree and kind of polarization which light reflected from diffe- 

 rent parts of it experienced, I was led to a series of experiments, which furnished 

 me with the test of which I had been in search. 



The comparative brightness of the two images in Iceland spar, directed to 

 different parts of the sky, afforded a very imperfect indication of its state of pola- 

 rization ; and I had, therefore, been in the practice of employing the uniaxal or 

 biaxal system of rings for this purpose.* Upon placing such a system between 

 light partially polarized in one plane, and light partially polarized in an opposite 

 plane, I found that the rings disappeared, the direct system being seen on one 

 side of the plane of disappearance, and the complementary system on the other side. 

 In this experiment, the polarization of the light in one plane was compensated 

 by the polarization of the samd light in the opposite plane, and consequently both 

 of the pencils that had undergone the two successive polarizing actions, had re- 

 ceived the same degree of polarization in opposite planes. In virtue of these two 

 equal and opposite polarizations, the light at the point of compensation, where the 

 system of rings disappeared, had been restored from partially polarized to com- 

 mon light, and the light on each side of this point of compensation was in oppo- 

 site states of partial polarization. 



In order to have a more distinct idea of the nature of this experiment, let us 

 suppose that light reflected once, at 24° of incidence, from glass, whose index of 

 refraction is 1.525, is afterwards made to suffer one refraction at 80° by another 

 surface of the same glass.f In this case, the partial polarization produced by re- 

 flexion is exactly compensated by the equal and opposite partial polarization pro- 

 duced by refraction. In like manner, a second reflexion at 83^°, in an opposite 

 plane, will compensate the first reflexion at 24°, or the refraction in the same 

 plane at 80°. 



Now, in these three cases of compensation, the quantity of polarized light in 

 the three pencils is very different, as appears from the following table : 



• See my Treatise on New Philosophical Instruments, 1813, p. 349. 



I The action of one refraction is obtained by using a prism of well annealed glass, as shown in 

 the Philosophical Transactions, 1830, p. 135, fig. 2. 



