142 Dr. Brewster on the laws which regulate the 
Sect. IV. On the law of the polarisation of light by successive 
reflexions. 
Prop. xvi. 
When a ray of light is incident at any angle except a right angle 
upon the surface of a transparent body , a certain portion of the 
reflected light is completely polarised , while the remaining por- 
tion has suffered a physical change, or has acquired , in various 
degrees , a character approaching to complete polarisation. 
This proposition has been established by direct experiments 
made with glass, whose polarising angle is 56° 45'. 
If a pencil of light is reflected from glass at an angle of 
6 2 0 30', or 50° 20', i. e. either above or below the polarising 
angle, the portion of light which is not completely polarised, 
has so far received this character, that it will be completely 
polarised by a second reflexion at the same angle, whereas 
had it been absolutely unpolarised light, it could not have 
been polarised at any angle different from 56° 45', the real 
angle of polarisation. 
In like manner three reflexions at an angle of 65° 33' or 
t 46° 30', or four reflections at an angle of 6j° 33', or 43° 31' 
will polarise the whole pencil, while at angles above 82° or 
below i8°more than 100 reflexions are necessary to produce 
complete polarisation. 
The truth of the proposition for the transmitted rays is 
established by the experiments which I have already published 
' on the polarisation of light by oblique refraction.* If a pencil 
* See Pbil. Trans. 1814, Part I.p. 219. 
