THE PARTIAL POLARIZATION OF LIGHT BY REFLEXION. 
71 
zation, the plane of polarization being a tangent to the curve at the incidence 
which corresponds to any particular point of it. 
When we employ a surface of diamond in place of glass, the inclination of 
the axes ab,c d 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 natural 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 diamond, the axes of all the particles are brought into 
a state of parallelism 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 known: 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 ab,c d 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 pencil, 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 principal section in the plane of reflexion, and viewing through 
it the images A and B at 80° of incidence. As the axis of A is inclined 33° to 
MN or the section of the rhomb, the ordinary image of it will be much brighter 
