the Use of Nicol’s Prism. 331 
required, it will be sufficient to attach permanently to the 
Nicol a small reflecting surface, and make this always perpen- 
dicular to the incident light. The planes of polarization of 
both halves will then be perfectly definite. 7 
I have treated the method as applied to a Nicol used as 
polarizer, but it is by no means restricted to this case. The 
polarizer may be any kind of polarimeter, or if necessary the 
polarimeter may be the analyzer, while the auxiliary Nicol is 
used as polarizer. The method occurred to me when I was 
engaged on some observations on the refraction of polarized 
light at the surface of Iceland spar. I will describe the 
arrangements in so far as they bear on the matter in hand ; 
for, as it happened, they gave a very high degree of accuracy 
in the determination of the zero-reading. 
As my source of light I employed part of the filament of a 
Swan incandescent lamp. I selected a straight piece, and cut 
off the light from the rest of the filament with a diaphragm 
placed immediately in front of the lamp. The filament was 
placed at the principal focus of a lens, from which the light 
passed to the polarizing Nicol. Then came the spectrometer, 
from which the collimator had been dismounted ; so only the 
telescope remained. When the telescope was focussed for 
infinity and directed towards the light, a sharply defined 
image of the selected portion of the filament was seen. The 
top and bottom limits of the image were not hard lines, owing 
to the diaphragm not being quite at the principal focus of the 
collimating lens. They were, however, sufficiently definite 
for practical purposes. 
lt was necessary for my other observations that the axis of 
rotation of the spectrometer should be set at right angles to 
the light coming from the middle of the filament. I accom- 
plished this in the following manner. I first mounted a 
reflecting surface on the table of the spectrometer, and ad- 
justed it to be parallel to the axis of rotation. This may be 
done with accuracy although the telescope be tilted. Next I 
adjusted the axis of the telescope to be at right angles to this 
surface, by getting the cross wires to coincide with their 
image formed by reflection at the surface. Then I removed 
the reflecting surface, directed the telescope towards the 
light, and tilted the spectrometer till the cross wires coincided 
with the middle of the image. 
When the auxiliary Nicol was mounted on the spectrometer- 
table and the polarizer turned past the crossed position, the 
image of the filament by no means entirely disappeared ; but 
a patch of nearly complete extinction moved down the image 
_ from the top to the bottom. This was of course to be expected, 
since different positions of the image correspond to different 
