Cadmium, and Mercury. 385 



o) L1 = 2'330, e L1 = 2-350, w Na =2-356, e Na =2'378. An independent 

 measurement of e — a> on the prisms gave "019 for Li-light 

 and "020 for Na-light. 



Observations on natural Sphalerite and Wurtzite. 



Samples from more than twenty different occurrences of 

 natural zinc sulphide minerals were studied optically. With a 

 prism of the light amber-colored sphalerite from Sonora the 

 refractive indices for Na-light and Tl-light were accurately 

 determined on a spectrometer. The values obtained were 

 2-3688 ± -0001 and 2"3990 ± -0002. Although this material 

 is the purest natural sphalerite obtainable (containing only 

 0*22 per cent FeS) the impurity is sufficient to raise the 

 refractive index -0006 for Na-light and -0007 for Tl-light 

 (page 386). The refractive index of pure sphalerite, probably 

 correct within ±'0002, therefore, is ?? Na = 2 3682, v n = 2*3983. 



The dispersion of the Sonora sphalerite was determined from 

 measurements on two prisms, a goniometer and monochromatic 

 illuminator being used. The illuminator was standardized by 

 observations on the following lines: Li, Na, Tl, Sr (blue). 



Wave-length Eefractive Index 



420 2-517 



434 2-493 



486 2-436 



535 2399 



589 2-369 



630 2-353 



671 2-340 



760... 2-320 



' The effect of dissolved FeS upon the refraction constants of 

 Sphalerite and Wurtzite. — Natural blendes may contain as 

 much as thirty per cent of FeS. Ferriferous blendes are 

 deeply colored, but in very thin flakes they are a clear orange- 

 brown by transmitted light. The blende from Saxony con- 

 taining 28 - 2 per cent of FeS absorbs light in gradually 

 increasing amounts from the red end of the spectrum to about 

 450/x//,, where it is practically opaque. The wurtzite formed 

 by heating this blende is somewhat lighter in color, but is not 

 distinctly pleochroic in any part of the spectrum. Its average 

 refractive index is very near the refractive index of the blende 

 — was observed to be -005 less — and its double refraction about 

 •02. The mutual optical relations of sphalerite and wurtzite 

 are thus not sensibly altered by the presence of large amounts 

 of FeS. 



The absolute values of the refractive indices are greatly 

 increased by FeS. The following table shows the character of 



