DR. BREWSTER ON THE LAWS OF 
134 
have already applied to the polarization of light by reflexion, and to establish 
on the basis of actual experiment the true laws of the phenomena. 
The first step in this inquiry is to ascertain the law according to which the 
polarizing force of the refracting surface changes the position of the planes of 
polarized light, — a subject which, in as far as I know, has not occupied the 
attention of any other person. 
If we take a plate of glass deviating so slightly from parallelism as to throw 
off from the principal image the images formed by reflexion from its inner 
surfaces, we shall be able to see, even at great obliquities, the transmitted light 
free from all admixture of reflected light. Let this plate be placed upon 
a divided circle, so that we can observe through it 
two luminous discs of polarized light A, B (Fig. 1.) 
formed by double refraction, and having their planes 
of polarization inclined + 45° and — 45° to the plane 
of refraction. At an angle of incidence of 0°, when 
the light passes perpendicularly, the inclination of 
the planes of polarization will suffer no change ; but 
at an incidence of 30° they will be turned round 
40' ; so that their inclination to M N or the 
angle aac will be 45° 40'. At 45° their inclination 
will be 46° 47'- At 60° it will be 50° 7 f ; and it will 
increase gradually to 90°, where it becomes 66° 19'. 
Hence the maximum change produced by a single 
plate of glass upon the planes of polarization is 
66° 19' — 45° = 21° 19', an effect exactly equal to 
what is produced by reflexion at angles of 39 or 70°. 
It is remarkable, however, that this change is made in the opposite direction, 
the planes of polarization now approaching to coincidence in a plane at right 
angles to that of reflexion. This difference is exactly what might have been 
expected from the opposite character of the resulting polarization, the poles of 
the particles of light which were formerly repelled by the force of reflexion, 
being now attracted by the refracting force. 
In this experiment the action of the two surfaces is developed in succession, 
so that we cannot deduce from the maximum rotation of 21° 19', the real 
Fig. 1. 
