polarisation of light hy reflexion from transparent bodies . 149 
Prop. xxii. 
When a ray of light polarised by reflexion , is incident at any angle 
on the surface of a transparent body , so that the plane rf the 
second ref exion is at right angles to the plane of the first 
reflexion , and suffers successive reflexions in the plane of the 
second reflexion, it will lose its reflexibility when it has under- 
gone that number of reflexions which would have been necessary 
to polarise it, had it been direct light. 
This result was at first deduced a priori from Prop. xvii. 
and xix., and was afterwards established by experiment. The 
number of reflexions may be determined by the formulas in 
Prop. xvir. 
Prop. xxur. 
When a beam of light is emitted by the sup,, or by any other body 
which does not shine by reflected light, the particles which com- 
pose it are in every state of positive and negative polarisation 
from particles completely polarised to particles not polarised at 
all. 
This Proposition is an expression of the experimental results 
in Prop. xvi. and xvii., and may be illustrated in the follow- 
ing manner, the terms positive and negative polarisation being 
employed to denote the two kinds of polarisation by reflexion 
and refraction at the polarising angle, or by reflexion in two 
opposite planes. A ray of direct light, before it is incident 
upon glass, may therefore be represented as consisting of a 
number of particles p, p. See. of the following character, 
f _P_ P P JlJL P P P 
T c o ’ jo ’ 2 o » 3. 5 4 e » jo » 60 56° 45' 
_ P JL, ±., JL, JL, 1 
o° ’ 1° 2° 3° 4 0 5° 6° 56° 45' 
