196 



Bow en — Birefringence of T orb emit e. 



sections and in parallel light they are absolutely iso- 

 tropic. In convergent light very thick plates are uni- 

 axial and appear to be optically positive for white light. 

 The prismatic cleavage is sufficiently good so that occa- 

 sional grains represent prismatic sections though always 

 elongated parallel to the base and in these the relative 

 values of the two indices compared with that of the 

 liquid show that the index of the ordinary ray is the 

 lesser for white light. The interference colors, are how- 

 ever, very abnormal. There is no suggestion of the 

 normal succession of colors as the grains increase in 

 thickness. The color of all the grains is not far from 

 that of a sensitive tint plate, though it is sometimes the 









Fig. 1. 





U650 









.IX) 



k 1-640 



1 



- 









^ 1-630 



\mo 



- 









- 



1 , 1 



■ I . i ... f 



i . i 



700 660 620 SSO S40 SOO 

 Wave Length yLLAl 



460 420 



Fig. 1. — Eefraction indices of " torbernite ' ' from Cornwall. 



blue of an overthick plate and again the red of a thin 

 plate. It was suspected, therefore, that the mineral was 

 isotropic for a color in the middle of the spectrum and a 

 detailed measurement of the indices for various colors 

 was undertaken. This was done in immersion liquids 

 under the microscope and a monochromator was used as 

 a light source. The indices of the liquids for various 

 colors was known, and, therefore, their dispersion curves 

 also, so that it was only necessary to find the wave- 

 lengths for which the index matched three or four adja- 

 cent liquids and repeat the determinations for each ray. 

 In this manner, it was found that the mineral was posi- 

 tive for the red end of the spectrum, negative for the 

 blue end and isotropic in the green at about 515/*/*. The 



