Sir David Brewster ow the Compensations of Polarized Light. 391 



water. In other cases the light of the sky has its polarization increased by 

 reflexion from the water in the same plane in which it was itself polarized ; and 

 in other cases, the compensation is effected only in particular planes. At sun- 

 set, for example, the light reflected from the sea at a great obliquity in two vertical 

 planes inclined 45° to a vertical plane passing through the sun and the observer, 

 is compensated in these two planes, or the plane of its polarization is inclined 

 about 45° to the reflecting surface. The same observations apply to the light of 

 the two rainbows when reflected from the surface of water. 



6. When the light of the sky, or of the rainbow, is reflected from surfaces 

 not horizontal, such as the roofs of houses, sheets of falling water, or surfaces of 

 smoke and vapour, the compensations are more varied, and a perfect neutralization 

 of the light by the second reflexion is more frequently obtained. 



7. When the compensating rhomb, whose properties I have already described, 

 is made of glass not highly polished, light that has suffered total reflexion is seen 

 through the face ad. Fig. 1. As the faces ab, cd, are parallel, none of the light 

 regularly refracted by the face ab can suffer total reflexion from cd. Upon 

 examining this curious and unexpected phenomenon, I found that it was owing 

 to light radiated, or scattered from the surface ab, which falling upon cd at 



angles greater than that of total reflexion, whose sine is — , necessarily suffered 



total reflexion. That this was the cause of the phenomenon, I proved by covering 

 the surface ab with a film of dried milk, which radiated light from every part of 

 its surface, and produced a beautiful zone of totally reflected light, increasing 

 in brightness as the incidence upon ab became more oblique. In examining 

 this totally reflected light, I was greatly surprised to find, that it was partially 

 polarized, and exhibited an interesting example of compensation. 



Let MN, Fig. 8, be the luminous zone of totally reflected light with its blue 

 border. At the polarizing angle of the second surface of the rhomb, the polar- 

 ization is incomplete ; but at angles between that angle and 83°, the polarization 

 gradually diminishes, and at 83° it becomes common light, the rectilineal fringes 

 AB produced by the rock crystal passing into neutral light at cd, close to the 

 boundary mn of total reflexion. From 83° to 90°, which corresponds to a very 

 narrow space at cd, the light still appears compensated, though it is slightly 

 polarized, in a plane perpendicular to that of reflexion. 



