On Polarization of Light by Reflexion. 279 
an angle of 50°, and at 0°, or a perpendicular incidence, they are 
again brought back to their primitive inclination of 90°. Taking 
MN to represent the quadrant of incidence from 90° at M, to 0° at 
N, the curves, 90°, 0°, show the progressive change which takes 
place in the planes of polarization, the plane of polarization being a 
tangent to the curve at the incidence which corresponds to any par- 
ticular point of it. 
When we employ a surface of diamond in place of glass, the in- 
clination of the axes ab, cd is reduced to 46° at an incidence of 80°, 
to 8° at an incidence of 70°, and at 67° 43’ the axes become parallel. 
Such being the action of the reflecting forces upon A and B taken 
separately, let us now consider them as superposed and forming natu- 
ral light. At 90° and 0° of incidence, the reflecting force produces 
no change in the inclination of their axes or planes of polarization ; 
but at 56° 45/ in the case of glass, and 67° 43/ in the case of dia- 
mond, the axes of all the particles are brought into a state of paral- 
lelism with the plane of reflexion; and consequently when the image 
which they form is viewed by the rhomb of calcareous spar, they will 
all pass into the ordinary image, and thus prove that they are wholly 
polarized in the plane of reflexion. 
All this is entirely conformable to what has been long lena but. 
we now see that the total polarization of the reflected pencil at an 
angle whose tangent is the index of refraction, is effected by turning 
round the planes of polarization of one half of the light from right to 
left, and of the other half from left to right, each through an angle 
of 45°.. Let us now see what takes place at those angles where. 
the pencil is only partially polarized. At 80° for example, the angle 
of the planes a6, cd is 66°, that is, each plane of polarization has 
been turned round in opposite directions from an inclination of 45° 
to one of 33° with the plane of reflexion. The light has therefore 
suffered a physical change of a very marked kind, constituting now 
neither natural nor polarized light. It is not natural light, because its 
planes of polarization are not rectangular; it is not polarized light, 
because they are not parallel. It is a pencil of light having the 
physical character of one half of its rays being polarized at an angle 
of 66° to the other half. It will now be asked, how a pencil thus 
characterized’ can exhibit the properties of a partially polarized pen- 
cil, that is, of a pencil part of whose light is polarized in the plane 
of reflexion, while the rest retains its condition of natural light. 
This will be understood. by replacing the analysing rhomb with its 
