SPECIFYING QUARTZ CRYSTAL ORIENTATION 259 



An immersion fluid for Rochelle salt can be made by mixing decalin with 

 any of the other substances. For example b mixture of 34 parts of dimethyl 

 phthalate and 66 parts of decalin should give the necessary index 1.495. 



2.10 APPENDIX 



THE RING ECCENTRICITY CORRECTION 



Referring to Fig. 2.41, we see that, at an angle a, from the optic axis to- 

 wards the plate thickness direction the phase relation is, by Equations (2.7) 

 and (2.9): 



.00917 /' sin' ai 



Ni = 



X cos (5 — ai) 



I I I I , I . 1 1 . I . . I I I I I I I II. I < . — : — I — I — . — . — I — I — I 



0° 10° 20° 30° 40° 50° 60° 



APPARENT ANGLE R 



Fig. 2.40 — Refraction correction nomograph 



while at an angle 0:2 away, it is: 



_ .00917 t' sin" aa 

 X cos (5 + a2) 



Whence, if these are equal, we have: 



sin ai sin az (7 w) 



cos(5 — ai) cos (5 + 02) 



