614 
ME. E. T. GLAZEBEOOK ON THE EEFEACTION OF PLANE 
set screws all the adjustments requisite for accurate measurement of the position of 
the plane of polarization of the light emerging from the Nicol can he accomplished. 
Before asking permission, therefore, to lay the preceding results before the Boyal 
Society I waited until these circles were ready, and have repeated the experiments 
described above, using one of them instead of the sugar-cell to produce the rotation 
in the plane of polarization of the light. 
The only additional adjustment necessary was to make sure that the axis of rotation 
of the polarizing Nicol was parallel to the incident light. The Nicol was placed at 
about a metre behind the slit of the collimator and adjusted by looking along the axis of 
the collimator until the centre of the face of the Nicol appeared to be in the axis. In 
the figure (7), which gives the apparatus in plan, S is the slit of the collimator, and 
7, 
N the Nicol. Between the slit and the Nicol was placed a screen, A B, with another 
slit, P, adjusting it, so that P lay also in the axis of the collimator, and between P 
and S a lamp, L, so as to illuminate both slits. The lamp was so adjusted that the 
light from it fell along the axis of the collimator in one direction, while hi the other 
after passing through the slit P, it fell on the face, N, of the Nicol and was there 
reflected. A convex lens was placed between P and N in such a position that the 
rays reflected from the face of N, formed an image of the slit on the screen A B. On 
rotating the Nicol round its axis this image moved over the screen, and if the axis of 
rotation is parallel to the axis of the collimator, as it should be, the centre of the image 
will describe a circle round P as centre. A horizontal scale was affixed to the screen 
along A B and the position of the Nicol adjusted until the image of the slit, as the 
Nicol was turned, cut this scale in points equidistant from P on either side of it. 
This made it certain that the axis of the Nicol was in the same vertical plane as 
that of the collimator. But the axis of the collimator had been levelled with a spirit 
level, and by altering one of the feet on which the circle carrying the Nicol rested its 
axis was rendered horizontal. Thus the axis of the Nicol was rendered parallel to 
that of the collimator. 
