polarisation of light by reflexion from transparent bodies. 131 
Prop. ir. 
The differently coloured rays which compose a beam of white light 
are polarised at angles of reflexion whose tangents are equal 
to their respective indices of refraction. 
This Proposition might have been admitted as a corollary to 
Prop. I.; but I have established it by the following experiment. 
A ray of light incident upon oil of cassia at an angle of 58° 38', 
suffers its maximum polarisation. When the angle is gf 
38' the blue rays predominate in the pencil that approaches 
to evanescence, while at an angle of 59 0 38' the red rays 
predominate. Hence it follows, that the polarisation of the 
red light is a maximum at an angle below the mean polaris- 
ing angle, and the polarisation of the blue light a maximum at 
an angle, above the mean polarising angle. See Sect. V. 
Prop. hi. 
When the refractive power of any body is infinitely small , its 
polarising angle will be 4,5°. 
The limit to which the index of refraction constantly ap- 
proaches is 1.000 which is the tangent of 45 0 
Prop. iv. 
When a pencil of light is polarised by 'reflexion, the sum of the 
angles of incidence and refraction is a right angle. 
Let MN, (Fig. 1, PL VI.) be the reflecting surface, and BA, 
a ray of light polarised by reflexion in the direction AD, and let 
AC be the refracted ray. Then since EF, the tangent of the 
polarising angle BAE, is equal to m, or the index of refraction, 
we have by the law of the sines, CL= — — -gp- But from 
