396 



F. K Wright — Bi-quartz Wedge Plate. 



the intensity I' of the light emerging from the small nicol 

 will not be precisely V — I cos 2 a, hut I' = *9 I cos 2 a. 



If the two fields are matched to show equal intensity, the 

 plane of the analyzer will not in general coincide with the 

 bisector of the angle a, but will include with it a small angle 

 h (fig. 4). 



Fig. 4. 



An equation from which the angle h can be figured when 

 a is given has been derived by Bates, but his expression as 

 noted above does not take cognizance of the loss of light in the 

 extra prism and accordingly requires slight modification. 

 Let OP, = direction of plane of large polarizer 

 OP 2 = direction of plane of small polarizer 



a = angle between two polarizers; angle between the 

 normal (OC) to the plane of analyzer AA„ and I, intensity of 

 light in the field covered by the large polarizer alone when 

 viewed through analyzer; and I 2 intensity of light of the 

 field covered by the small polarizer. The angle P a OA angle 

 between large polarizer and analyzer is 



< P.OA 



while 



<C 



P 2 OA 



a 



- + 



From the intensity formula of the preceding article 



1 1 = cos 2 < P.OA = sin 2 (" - $\ 



1 2 = -9 cos 2 a cos 2 < P 2 OA = -9 cos 

 The condition of matched fields is 



a si 



-•&+>) 



I, = I. 



or 



