278 - . On Polarization of Light by Reflexion. 
exactly like common light ; so that we are entitled to assume such a 
pencil as the representative of natural light, and to consider every ’ 
thing that can be established respecting the one, as true respecting 
the other. : * 
In applying this principle to the analysis of the phenomena pro- 
duced by reflexion, I placed the planes of polarization of the com- 
pound beam in the plane of reflexion; but though this led to some 
interesting conclusions, it did not develope any general law. I then 
conceived the idea of making the plane of reflexion bisect the right 
angle formed by the planes of polarization ; and in this way I observ- 
ed a series of symmetrical effects at different angles of incidence, 
which threw a broad light over the whole subject. 
In order to explain these 
results, let AB (Fig. 1) rep- 
resent the two pencils of op- 
positely polarized light as sep- 
arated by double refraction ; 
let a6, cd be the directions of 
their planes of polarization, 
forming a right angle aec, 
and let the plane of reflex- 
ion MN, of a surface of plate 
glass, bisect the angle aec, 
so that the planes ab, cd 
form angles of + 45° and—45° 
with the plane MN. Let 
arhomb of calcareous spar ee 
have its principal section now placed in the plane of reflexion. 
At an incidence of 90°, reckoned from the perpendicular, the re- 
flected images of A and B suffer no change, the angle aec is stilla 
right angle, and the four pencils formed by the calcareous spar are 
all of equal intensity. As the incidence However diminishes, the angle 
aec diminishes also, and the ordinary and extra ordinary images of 
A and B differ in intensity. At an incidence of 80° for example, 
the angle aec is reduced from 90° to 66°; at 70° it has been reduced 
to 40°, and at 56° 45’, the maximum polarizing angle, it has been 
reduced to 0°; that is, the planes of polarization ab, cd are now 
parallel. Below the polarizing angle, at 50°, the axes are again in- 
clined to each other, and form an angle of 22°. At 40° they form 
Fig. 1. 
