134 Dr. Brewster on the laws which regulate the 
let it be refracted into the line GB, before it is reflected from 
the surface MN.* A part of the ray GB will penetrate the 
surface MN, and be refracted into the line BF, while another 
part will be reflected in the direction BA, and again refracted 
at A into the line AS. Continue SA to C and FB to D. Then 
since half of the refraction is supposed to be performed before 
the ray reaches B, and half of it after it penetrates the 
medium MN, we have BAC = DBC = half the angle of devi- 
ation. But by Prop. V, ADB is a right angle, hence ABC 
is also a right angle, and the angles ABE, GBE, each half a 
right angle, or 45 0 . 
Cor. 1. At the instant of reflexion, when the refraction at 
B commences, the refracted ray sets off at right angles to the 
reflected portion. 
Cor. 2. The real angle of polarisation is 45°, the effect of 
the refractive force being merely to bend the ray of light so 
as to make it suffer reflexion at this particular angle. 
Cor. 3. The excess of the angle formed by the incident and 
the polarised ray, above a right angle, is equal to the angle of 
deviation. The angle PAB, Fig. 1, which is equal to the 
angle of deviation OAC, is obviously the excess of DAB 
above the right angle DAP. 
Sect. II. On the laws of the polarisation of light by reflexion from 
the second surfaces of transparent bodies. 
When a ray of light is incidenFupon a parallel plate of 
# In order to keep the figure from being complicated, I have supposed the reflexion 
to take place all at once when the ray reaches the surface MN. The demonstration 
would have been exactly the same if the ray had been represented as suffering a gra- 
dual reflexion in passing through the sphere of reflecting activity. 
1 
