148 Dr. Brewster on the laws which regulate the 
Prop. xx. 
If the reflecting plane upon which the polarised ray is received , is 
made to deviate in the slightest degree from the position which 
deprives the maximum portion of the ray of its reflexibility, a 
part of the light that had formerly lost its reflexibility will now 
suffer reflexion , and will be polarised in the plane of the second 
reflexion, whereas before the deviation took place, this portion 
of light was polarised in the plane of the first reflexion. 
When the plane of the second reflexion is perpendicular to 
the plane of primitive polarisation, every particle of the ray 
that suffers reflexion will be polarised in the last of these 
planes, but when the least deviation takes place, a part of the 
polarised ray is depolarised, so that it receives its character 
from the second reflexion, and is polarised in the plane of that 
reflexion. It is very interesting to observe two such opposite 
effects produced by the most minute change in the position 
of the second reflecting plane. 
Prop. xxi. 
If a ray of light reflected from a transparent body at any angle , 
excepting the polarising angle, is reflected from another body in 
a plane at right angles to that of its first reflexion , the reflected 
portion will be polarised by the second reflexion in the same 
manner, and at the same angle as if it had been direct light. 
This proposition is deduced from experiment. The por- 
tion of light that is polarised at the first reflexion, will lose 
its reflexibility at the second reflexion, in proportion as the 
angle of reflexion approaches to the polarising angle. 
