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the Use of Nicol’s Prism. 327 
of the lens HK, or rather on a scrap of paper attached to it. 
The image a of course remains at the same point on the paper, 
however the lens E is moved laterally in its own plane. On 
rotating the Nicol circle I found that a did not move ; so I 
knew that the mirror was at right angles to the axis of rota- 
tion. I then adjusted the Nicol circle till the middle point of 
the image of the slit fell on the optical centre of the lens, 
which | had previously marked on the paper. If I had 
chosen a wrong point as the optical centre, the error would 
have been at once evident on turning the lens E through 180° 
in its own plane. This method gave without much difficulty 
an accuracy of 1°, with a lens E of only 4 inches focal length. 
2. On anew Method of obtaining the Zero-reading of a 
Nicol Cirele. 
In a large class of experiments on polarized light, it is 
necessary to know the reading of the Nicol circle when the 
plane of polarization is parallel to the axis of rotation of some 
part of the apparatus, e. g. of the table of a spectrometer. The 
usual method of obtaining this reading depends on the pola- 
rizing power of a glass reflecting surface. It is easy to fix 
the surface on the spectrometer-table parallel to the axis of 
rotation. Ifthe angle of incidence be made that of maximum 
polarization, and the Nicol turned till the reflected light is 
reduced to a minimum, the plane of polarization is then per- 
pendicular to the plane of incidence. This method is simple, 
but it is not very sensitive. Hven with poor illumination the 
minimum reflecied light is, with glass at any rate, by no 
means evanescent; and as the Nicol is turned the intensity 
remains sensibly constant for some distance on either side of 
the minimum point. The sensitiveness, too, does not increase, 
but rather falls off with a more powerful light. 
The method I am about to describe is nearly as simple ; 
and, as it depends on the crossing of two Nicols, its sensi- 
tiveness is only limited by the power of the source of light. 
In its simplest form the process is as follows :—An auxiliary 
Nicol is fixed on the table of the spectrometer, the polarizer 
turned till the light is quenched, and the reading taken. 
