igo Dr. Brewster on the laws which regulate the 
of the mean index of refraction in computing the first 
column. 
The law of the polarisation of light by reflexion being thus 
experimentally established, we shall now proceed to point 
out its geometrical consequences, and to arrange, under sepa- 
rate propositions, the new truths to which it leads, as well as 
those which I have obtained from direct experiment. It will 
thus be seen, that the subject assumes a scientific form, and 
that we can calculate a priori, the result of every experiment, 
whether the light is incident upon the first or second surface 
of transparent bodies, or upon the separating surface of dif- 
ferent media, or whether it undergoes a series of successive 
reflexions in the same plane, or in planes at right angles to 
each other. 
Sect. I. On the laws of the polarisation of light, by reflexion 
from the first surfaces of transparent bodies. 
Prop. i. 
When a pencil of light is incident upon a transparent body at an 
angle , whose tangent is equal to the index of refraction, the 
reflected portion will be either wholly polarised, or the quantity 
of polarised light which it contains will be a maximum. 
This proposition is a repetition of the general law already 
established. In zvater, glass, and other bodies, whose refrac- 
tive power is less than 1.6, almost the whole of the pencil is 
polarised, at the polarising angle ; but in diamond, realgar, 
chromate of lead, oil of cassia, &c. whose refractive power ex- 
ceeds 1.6, the whole of the reflected pencil does not suffer 
polarisation, but the quantity of polarised light is a maximum 
at the angle indicated in the proposition. See Sect. V. 
