the Use of Nicol’s Prism. 329 
Draw Bm at right angles to B P and Bym at right angles 
to Bm. We may assume that BB, is very small and BP 
Fig. 6, a 
B 1 
B in 
nearly a right angle. The angle between the planes of 
polarization of the two halves is composed of two parts, of 
which one, B P B,, is approximately equal to Bm, and the other 
is approximately proportional to the difference between Pn, 
and Pn and therefore to Bym.’ So the angle is independent 
of small variations in the position of P, and is therefore a 
constant for each particular Nicol. In some Nicols, as we go 
from one half to the other, the plane of polarization is turned 
in the direction of a right-handed screw. These we may call 
right-handed Nicols with a certain rotation-angle. Supposing, 
then, our auxiliary Nicol is right-handed, the plane of the 
second half leans at first too much to the right, but, when the 
Nicol has been rotated through 180°, it leans too much to the 
left. The correct reading in the case of a right-handed Nicol 
is therefore that obtained by turning the Nicol circle from 
the mean reading through half the rotation-angle in the right- 
handed direction. 
The question now arises, how we are to determine the rota- 
tion-angle. The following process explains itself. Place the 
auxiliary Nicol B on the path of the light from the polarizer 
A, which is mounted in a graduated circle. Turn A till the 
light is quenched and take a reading. Then turn A so as to 
allow the light to pass through B, and place in the path of 
the light from B a third Nicol C. Adjust C till the light _ 
from B is quenched. Remove B, cross C with A, and read 
again. 
