OF ELLIPTIC POLARIZATION. 
291 
primitive polarization. In this position let a plate of highly polished steel 
receive the polarized ray of ordinary intensity. At 89°, 88°, and 87° of inci- 
dence, almost no change is produced upon it by the action of the metal. We 
can easily see that the plane of polarization of the ray is turned from right to 
left, exactly as it would be by a transparent surface. In like manner at all 
angles of incidence from 0° to about 40° no decided effect is produced, except 
the change in the plane of polarization. At angles less than 87° the change 
begins to appear, reaches its maximum at about 75°, and diminishes gradually 
to 40°. By means of the analysing rhomb, it is easily seen that a great portion 
of the original pencil has had its plane of polarization changed from +45° to 
0°, as the incidence diminishes from 75° to 0°. If, indeed, we measure the ro- 
tation of the principal section of the rhomb when the extraordinary pencil is 
a minimum at different angles of incidence, we shall find it to correspond 
• COS ( 2 -f“ t ) ) 
with 45° — < p, <p being calculated from the formula tan <p = CQS ^ ~ _~^ in 
which 
sin i 
sin i 
3.732, the index of refraction for steel. The value of <p will be 
found to be nearly the same at 87° and 40°, which shows why at these two 
angles the change under our consideration is just beginning to appear with 
light of ordinary intensity. 
The physical effect of the metallic surface being a maximum at 75°, we shall 
now examine the character of the pencil reflected at that angle. 
1 . The pencil thus reflected is not polarized light, because it does not vanish 
during the revolution of the analysing rhomb. 
2. It is not common light, because when we reflect it a second time at 75° 
from another steel surface, it is restored to light polarized in one plane. 
In order to discover its nature, let it be transmitted along the axis of calca- 
reous spar. The system of rings is changed into the form shown in Plate X. 
fig. 1 . almost exactly in the same manner as if a thin film of a crystallized body 
which polarizes the pale blue of the first order had crossed the system. If we 
substitute for the calcareous spar films of sulphate of lime, which give different 
tints, we shall find that these tints are increased according as the metallic 
action coincides with, or opposes that of the crystal. 
