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BELL SYSTEM TECHNICAL JOURNAL 



makes a confused pattern but good orientations can be made anyway. A 

 "raw" crystal can be mounted adjustably in a jig that is lowered into the 

 conoscope, the optic axis lined up, the jig transferred to a saw, and sections 

 sawed directly. 



Let us turn now to the quantitative analysis of the ring pattern seen in 

 the eye piece when examining a uniaxial crystal. We wish to know the size 

 of the smallest ring in the field, or rather the corresponding angle in the 

 crystal. This first dark ring (analyzer and polarizer crossed) is the result 

 of the slow wave falling one wave length behind the fast one. If the plate 

 thickness (Fig. 2.34) is /' the path length in the crystal is 



/ = 



cos 6 



(2.8) 



Fig. 2.34 — The angle of the smallest ring 

 This is to be substituted in Eq. 2.7, namely: 



iV = - (w/ — n,) 

 A 



(2.7) 



Now it can be shown that, quite accurately, at the angle 9 from the optic 

 axis: 



tis - n/ = .00917 sin^^ 



(2.9) 



where .00917 is the difference in the refractive indices for the ordinary ray 

 and the extraordinary ray for green mercury light traveling at right angles 

 to the optic axis. (These are generally given the symbols Ug and He or n^ 

 and Hf respectively.) 



i' 



Ni = 



X .00917 sin 6 = 1 



X cos 6 



and since X = .000546 mm. for green mercury light this may be written 

 /' sin etsine = 0.0595 mm. (2.10) 



