the Use of Nicol’s Prism. 325 
and at right angles to the incident light, turned the plane of 
polarization through 11/+1’. 
In a paper on Polarizing Prisms (Phil. Mag. 1883) Mr. 
Glazebrook has suggested a new form of flat-ended prism in 
which the axis of the spar is at right angles to the length of 
the prism. In this case XN=90°, and w=1°49. 
lO =" 0, 
palates bE 
So instead of equation (8) we have 
6+ 8, 
9 —p='237" sin W cos yr, 
and the outstanding error is substantially the same as before. 
Hquation (9) appears to be inconsistent with a result ob- 
tained by Mr. Glazebrook (Phil. Mag. Oct. 1880, p. 252). 
For the sake of simplicity, he supposed the ordinary ray only 
to emerge from the crystal. He examined two positions of the 
Nicol. First the emergent ray lay in the principal plane, so 
that the plane of polarization coincided with the principal plane. 
Next he supposed the Nicol to be turned through 90° about 
an axis, lying in the principal plane, but inclined at 5° to the 
emergent ray, and he found that now the plane of polarization 
was inclined at 5° 3/ to the principal plane. But this does 
not show that the plane of polarization has been turned through 
90°+5° 3/ about the direction of the ray, as is evident on 
examination of the annexed figure. 
Fig. 4, 
P’ is, as before, the emergent light ; N, N’, X, X’ are the 
two positions of the normal to the face and the axis. 
