328 Mr. J. C. M‘Connel’s Notes on 
Then the table of the spectrometer is turned through two 
right angles—the axis of rotation having been previously set 
perpendicular to the incident light—the light quenched, and 
the reading taken again. The plane of polarization now leans 
as much to one side of the axis as it did before to the other. 
So the mean of the two readings is the reading when the 
plane of polarization is parallel to the axis of rotation of the 
table. 
The above is of course only a general explanation. The 
statement in italics requires fuller examination, and we shall 
find that it is only strictly true when the Nicol 1s symmetrical. 
Let us, then, first suppose that the Nicol is perfectly symme- 
trical—that is, that the two faces are parallel and the optic 
axes of the two halves coincident. We shall assume through- 
out the investigation that the axis of rotation is accurately 
perpendicular to the incident light. In the previous note we 
have carefully determined the position of the plane of pola- 
rization in terms of the direction of the emergent light, and 
we have shown that, to a very close approximation, it is at 
right angles to that position of the plane of polarization of 
light incident along the same path which gives complete 
extinction in the Nicol. The discrepancy was shown to be 
negligible. So there is no ambiguity in speaking of the 
plane of polarization of either half of the Nicol for a particular 
direction of the light. 
In the first position of our Nicol on the spectrometer, the 
emergent light is parallel to the incident, and the planes of 
polarization of the two halves are also parallel. The axis of 
rotation, too, is at right angles to the incident light. Imagine, 
then, a line fixed relatively to the Nicol to be drawn from the 
second face parallel to the emergent light. When the Nicol 
has been rotated through two right angles, this line becomes 
again parallel to the incident light, and the plane of polari- 
zation of the second half of the Nicol now leans exactly as 
much to one side of the axis of rotation as the plane of pola- 
rization of the first half leant before to the other. The process 
therefore is so far strictly accurate. 
Secondly, let us suppose that the Nicol, though not sym- 
metrical, yet produces no angular deviation on light traversing 
it. We have seen in the previous note that the plane of pola- 
rization of a Nicol is nearly at right angles to the principal 
plane, and that to a first approximation the small deviation is 
proportional to the angle between the external light and the 
principal plane. 
Let Xn, X,n, be the principal planes of the two halves. 
B, B, their poles. 
P the direction of incident and emergent light. 
