polarisation of light by reflexion from transparent bodies. 14,1 
incident at 8o°, the angle of incidence at the separating surface 
is 47° 2 9', while at an incidence of 90° it is no more than 
48“ 28', differing only 59' from the other. 
Prop. xv. 
When light is polarised at the separating surface of two media , the 
sum of the angles of incidence and refraction is a right angle , and 
the reflected ray forms a right angle with the refracted ray. 
This proposition is demonstrated in the same manner as 
Prop. IV and V, the separating surface producing always the 
same phenomena as the first surface of any body, whose index 
of refraction is equal to the quotient of the indices of refrac- 
tion for the two contiguous bodies. 
It would be a waste of time to extend the application of the 
general law to other cases where the reflecting surfaces are 
inclined at different angles, or where the incident pencil tra- 
verses a number of different media, and receives particular 
changes at each successive reflexion. We shall, therefore, go 
on to another branch of enquiry, and consider the laws which 
regulate the phenomena when a pencil is polarised by several 
successive reflexions, a subject to which Malus has not even 
alluded. 
