l\V4 



Mr. P. T. Glazebrook on JS r icoVs Prism. 



through which the Nicol has been turned, as read by the circle 

 attached, is greater than that through which the plane has 

 been moved. . 



Let /S be the angle between the axis of rotation and tins 

 principal plane. Owing to the rotation, the plane moves as 

 tangent to a cone of semi- vertical angle /3. 



Let OP, Q be perpendicular to 

 two positions of the plane, C the 

 axis of rotation. Draw great circles 

 C PA, C Q B, making C A and C B 

 each quadrants. P Q is the angle 

 the plane has been turned through ; 

 A OB is that through which the 

 Nicol has moved. Also 



A0P = /3. 



Draw C M L bisecting the angle 

 ACB. Let 



A0B= 7 , POQ = <£ ; 



AL: 



1 

 2> 



PM=^ 



r 



PC=J-/5. 



For the right-angled triangle, PCM, 



sin PM= sin PC sin PCM, 



A . 7 r* 



sin | = sin~ cos p. 



(12) 



Let us suppose, as before, that u = 5°, and that the rotation 

 given by the circle is 90°, then 7 = 90°. And from the above 

 formula, 



0=89° 32'; 



•snd the error introduced is about one half per cent. Moreover 

 the sign of the error is always positive : that is, the rotation 

 as measured by the circle may be greater than its actual value 

 by one half per cent. This, of course, is very small compared 

 with the optical error introduced from obliquity between the 

 axis of rotation and the direction of the incident light. 



