526 Proceedings of Royal Society of Edinburgh. [sess. 
polarising Nicol as shown in the figure, and so arranged that the 
Nicol can be rotated without disturbing the glass (or else vice versa). 
The object of such relative rotation is to provide a means of 
altering the angle P 0 X of fig. 1, and therefore the angle POP' as 
explained already; and therefore the sensitiveness of the half- 
shade field. The alternative method of effecting such a change, 
by altering the angle QO Y, cannot be employed, because it would 
require the angle BAD of fig. 2 to be capable of alteration; which 
of course is impossible. With this type of rotator a “ triple-field,” 
a form that is preferred by some observers to the more usual double 
one, can of course be easily arranged by the provision of a second 
glass plate mounted on the opposite side of the Nicol. The middle 
part of the beam of light then passes unaffected, while both its 
side portions are rotated to an equal extent in the same direction. 
The author desires to express his best thanks to Professor 
MacGregor for the opportunity of carrying out the necessary 
experimental work in the Physical Laboratory of Edinburgh 
University. 
( Issued separately January 14 , 1907 .) 
