1 54 Da*. Brewster o» the laws which regulate the 
of incidence. When the pencil is incident at the maximum 
polarising angle, or at an angle whose tangent is equal to the 
index of refraction for the mean refrangible rays, these rays 
alone will be polarised. Neither the red rays, which are in- 
cident at an angle above their polarising angle, nor the blue 
rays which are incident at an angle below their polarising 
angle, will be completely polarised ; and when the reflected 
pencil which contains them is viewed through a doubly re- 
fracting crystal, the mean refrangible rays will vanish, while 
the red and blue rays will compose a beam nearly white, and 
will not vanish, in consequence of its not being completely 
polarised. 
Prop. xxv. 
If a pencil of white light polarised by reflexion is incident at the 
polarising angle upon any transparent surface, so that the plane 
of the second reflexion is at right angles to the plane of its primi- 
tive polarisation, a portion of the pencil consisting oj the mean 
refrangible rays will lose its reflexibility , and will entirely pene- 
trate the second surface, while another portion of the beam, com * 
posed of the blue and red rays, will not lose its reflexibility, but 
will suffer reflexion and refraction like ordinary light . 
This proposition founded also on experiment may be proved 
by the same reasoning as the preceding, for since the angle 
at which polarised rays lose their reflexibility is the same as 
the angle at which they are polarised, only one set of the rays 
which compose a white beam can lose their reflexibility at the 
same angle. 
