polarisation of light by reflexion from transparent bodies. 155 
Prop. xxvi. 
The imperfectly polarised portion of light described in Prop. XXIV, 
and the portion which does not lose its reflexibility as described 
in Prop. XXV., increase with the dispersive and the refractive 
power of the reflecting surface, so that in substances where the 
dispersive and refractive forces are very great, these portions 
constitute in the one case almost the whole of the reflected pencil, 
and in the other almost the whole of the pencil that would have 
been reflected under ordinary circumstances. 
When the dispersive power of the reflecting surface is so 
high as to throw the blue and red rays to a great distance 
from the mean ray, the quantity of polarised light at the mean 
polarising angle must be very small, and must obviously di- 
minish as the dispersive power increases, the quantity of im- 
perfectly polarised light consisting of the blue and red rays 
increasing in the same proportion. When the refractive 
power is high, the polarising angle increases, and the quan- 
tity of reflected light becomes very great, being, in the case 
of diamond, about one half of the incident beam. Hence in 
rock crystal, which has a higher refractive power, and a lower 
dispersive power than water, the image does not wholly 
vanish at the polarising angle. 
