290 
DR. BREWSTER ON THE PHENOMENA AND LAWS 
If we now take two plates of each of these metals and examine the light 
which has undergone more than one reflexion, we shall find that the quantity 
of light which each polarizes in the plane of reflexion increases with every 
reflexion, and that in several of them the whole incident pencil is completely 
polarized. 
When the luminous object is a wax-candle placed at the distance of ten feet, 
eight reflexions from a plate of steel at angles between 60° and 80° polarize 
the whole of the light, while at angles above 80° and below 60° a greater num- 
ber of reflexions is required. With galaena, lead, cobalt, and antimony, a 
much smaller number of reflexions polarizes the whole pencil ; whereas with 
pure and highly polished silver a very great number is necessary: the light 
reflected from the silver becomes redder and redder, indicating an increasing 
absorption or dispersion of the less refrangible rays. 
By the use of common light it would be in vain to attempt to discover the 
law according to which the polarization of the incident pencil is effected in 
different metals ; but by another mode of analysis we shall be led to the ma- 
thematical law for computing the exact proportion of the reflected pencil which 
is polarized at certain angles when the number of reflexions exceeds one. 
Sect. II. On the action of metals upon polarized light . 
If a pencil of polarized light is received on a polished metallic surface 
placed so as to have a rotatory motion round the polarized ray, the reflected 
light will receive no modification (excepting what arises from its property of 
apparently polarizing a portion of light in the plane of reflexion) when the 
plane of incidence is inclined 0°, 90°, 180°, and 2/0° to the plane of primitive 
polarization ; but in every other azimuth of the plane of incidence the reflected 
pencil will be found to have suffered a remarkable change, which gradually 
increases as the azimuth of that plane varies from 0° to 45°, from 90° to 135°, 
from 180° to 225°, and from 2/0° to 315°. At the azimuths of 45°, 135°, 225°, 
and 315°, the effect is a maximum, and it gradually diminishes from 45° to 90°, 
from 135° to 180°, from 225° to 2/0°, and from 315° to 360°. 
In order to investigate the nature of this change, we shall suppose the plane 
of reflexion from the metal to be inclined —45°, or to the left of the plane of 
