polarisation of light by reflexion from transparent bodies . 14,3 
of light is polarised by twenty-four plates at an angle of 6o°, 
then it is obvious that twelve plates will not polarise the 
whole pencil at the same angle. 
Let us suppose that the portion not polarised amounts to 
twenty rays out of a hundred. Then, if these twenty rays 
were absolutely un polarised, and in the same state as direct 
light, they would require to pass through twenty-four plates 
at an angle of 6o°, in order to be polarised : but the experi- 
ments show that they require only to pass through other 
twelve plates at that angle. It therefore follows, that the 
twenty rays have been half polarised by the first twelve plates, 
and the polarisation completed by the other twelve. Hence 
we see the mistake of Malus, who observes, that the light 
transmitted obliquely through glass consists 1st. of a quan- 
tity of polarised light, and “ 2d. of another portion not modified , 
“ and which preserves the characters of direct light ” 
Prop. xvii. 
If a ray of light is partly polarised by reflexion at any angle , it 
will be more and more polarised by every successive reflexion in 
the same plane , till its polarisation is complete , whether the re- 
flexions are made at angles all above or all below the polarising 
angle , or at angles some of which are above and some below 
the polarising angle. 
This proposition is deduced from numerous experiments 
which may be easily repeated. It is extremely difficult, how- 
ever, on account of the rapid attenuation of the light when it 
has undergone a few reflexions from glass, to determine satis- 
factorily the relation between the number of reflexions and the 
angles of incidence at which they polarise a pencil of light. 
The experiments which I have made are represented by the 
