WRIGHT: MEASUREMENTS OF REFRACTIVE INDICES 541 



In this instance tiie refractive index was computed from equation 

 y for k = \ and k = 2 and from equation g for k = \ and k = 3. 

 Tlie agreement is satisfactory. 



(2) Uniaxial. Plate parallel to axis. (Equation h) 



Calcite plate (V. and H. collection No. 23.) Opticallv - ; rf = 0.082 

 mm.; X - 0.000589 mm.; oj = 1.658; e = 1.480. 



k 24 23 22 21 20 19 



i 16?5 23?5 29?8 33?2 37?5 



6 1.486 1.487 1.487 1.485 1.487 1.486 



Average e = 1.4863. On this section the path-difference in wave 

 lengths for the different interference lines was ascertamed by compu- 

 tation, the plate being too thick for the determmination of the path- 

 differences b}^ ordinary methods of compensation. 



(3) Biaxial. Plate normal to a bisectrix. (Equations e, f) 

 Anhydrite. Plate approximately normal to the acute bisectrix 



t; X = 0.000589 mm.; a = 1.570; ^ = 1.575; 7 = 1.613. 



(a) Plane of incidence 7 a plane. (Equation /) 



k 1 -1 



i 17?0 35?3 43?5 

 7 = 1.615 (computed from equations for k = 1 and k = —1); 7 = 

 1.609 (computed from equation for k =0). 



(b) Plane of incidence 7 (8 plane. (Equation e) 



fc 2 3 4 



i 25?4 39?3 48?5 



7 = 1.608 (computed from equations for A; = 2 and A; = 3); 7 = 1.607, 

 (computed from equations ior k = 2 and k = A). 



The values of 7 thus obtained vary within relatively large limits. 

 This may be due to the fact that the section was not exactly normal 

 to the bisectrix and also that the values of a and )3 are slightly in 

 error. In this instance 7 is considerably larger than a and /J and in 

 computing 7 a slight variation in either a or /3 produces a relatively 

 large variation in 7. The mean of the values listed above is 1.610 

 or 0.003 less than the refractive index 7 given for anhydrite. 



(4) Biaxial. Plate perpendicular to the optic normal. (Equations c, d) 

 Selenite. Plate parallel* to cleavage plane (010). Normal to /S; 



d = 0.700 mm. (measured by micrometer); X = 0.000589 mm.; a = 

 1.5204; /3 = 1.5227; 7 = 1.5296. 



(a) Plane of incidence ^a plane. (Equation c) 



k 11 12 



i 27?4 57?2 



/3 1.523 1.523 



(b) Plane of incidence /S 7 plane. (Equation d) 



fc = 10; i = 51?2; /3 = 1.522. 

 Average /3 = 1.5227 

 In this instance the path-difference, in terms of wave lengtli, for the 

 different interference lines was computed. 



