158 



INSTRUMENTATION 



wave retarding plate, the X direction of which is set at 45° with respect 

 to El and £'2. X is the direction of vibration of the slow ray in the 

 quarter-wave plate. As explained by Osterberg (1947), the X direction 

 is the natural line of reference for measuring the angle 6 of rotation of 

 the polarizer. 6 is the angle between the X direction and the direction 

 of the electric vector transmitted by the polarizer, d is positive in 

 the clockwise direction with respect to the observer as he looks into 

 the microscope. The light passes through the quarter-wave plate 

 before entering the zonal polarizers. Rotating the polarizer through 

 the angle 6 serves only to introduce a phase difference 8 = 26 between 



Slide 



Polarizer 



V 



Objective 



Analyzer 



n 



^Condenser 

 diaphragm 



^Micoid disk 

 I / 



/X 



Eyepiece 



Conjugate area 



-El 



Complementary area 



Fig. III. 14. A polanret system which permits the ampHtude ratio h and the phase 

 difference 5 to be varied continuously and independently. 



the undeviated and deviated rays. It suffices to limit d to the range 

 — 7r/2 ^ ^ 7r/2. Let a denote the angle between £'2 and the direc- 

 tion of the electric vector transmitted by the analyzer. Rotation of 

 the analyzer varies only the amplitude ratio h between the undeviated 

 and deviated rays such that h~ = T = tan" a. In principle, therefore, 

 the phase difference can be varied from —w to tf (one complete wave- 

 length) and the amplitude ratio can be varied from zero to infinity. 



The zonal polarizers preferably are made of thin polarizing film of 

 high optical ciuality, but they may be constructed of left- and right- 

 handed elements of quartz which rotate the plane of polarization 45°. 

 The zonal polarizers can also be constructed of two half-wave retarding 

 plates. Thin polarizing films are preferred because they give rise to 

 fewer complications from troublesome phenomena due to birefringence 

 and because the polarizer may be omitted whenever it is unnecessary 

 to vary the phase difference, provided that the zonal polarizers are 

 polarizing sheets. On the other hand, thin polarizing films are difficult 



