132 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



refraction directly, and the difference between the refractive index for red and that for 

 violet rays is a measure of the dispersion. A comparison of the following determinations 

 by Walter, with similar constants for other precious stones, will show that the refraction 



and dispersion of diamond are in all cases the greater : 



n 



Red light {B line of the spectrum) .... 2-40735 



Yellow „ (Z) „ „).... 2-41734 



Green „ (£ „ „).... 2-42694 



Violet „ (fl- „ „).... 2-46476 



The dispersion co-efficient is thus — 



2-46476 - 2-40735 = 0-05741. 



For comparison, the following values of the refractive indices of a particular glass may 



be given : 



n 



Red light 1-524312 



Yellow „ 1-527982 



Green „ ... 1-531372 



Violet „ 1-544684 



The dispersion co-efficient is here — 



1-544684 - 1-524312 = 0-020372, 



less than half as great as that of diamond. The spectrum produced by a prism of this 

 glass will be only about half as long as one produced by a similar prism of diamond under 

 the same conditions. 



Anomalous double refraction. — Diamond, being crystallised in the cubic 

 system, should be singly refracting, that is, isotropic. This, however, is only strictly true 

 for such stones as are perfectly colourless, or of a yellowish colour, and are quite free from 

 enclosures of foreign matter, cracks, and other flaws. Such faultless stones when rotated 

 in the dark field of the polariscope remain dark. As previously mentioned, the stone under 

 examination should be immersed in methylene iodide, so as to diminish total reflection as 

 far as possible. 



Deeply-coloured stones, and those disfigured by cracks, enclosures, or other faults, 

 when placed in the dark field of the polariscope, allow the passage of light to the eye, but, 

 as a rule, to only a small extent. They have, under these circumstances, a greyish 

 appearance, brilliant polarisation colours being rarely seen. The feeble double refraction 

 possessed by such stones is not an essential character of the substance of the diamond 

 itself, but is due to disturbing influences ; hence it is distinguished as anomalous double 

 refraction. During its rotation in the polariscope it rarely happens that such a stone is 

 uniformly dark or uniformly light over its whole surface ; as a )-ule, certain areas are dark 

 while others are light, and vice versd. Frequently certain regularly bounded areas or 

 fields behave in a similar manner during rotation, while adjacent fields behave differently. 

 In most cases, however, the areas showing these differences in behaviour have no definite 

 arrangement relative to each other, and areas showing a feeble double refraction are often 

 enclosed in areas which are perfectly isotropic. 



The doubly refracting portions of the stone usually surround enclosures or cracks, and 

 it is in the immediate vicinity of these that double refraction is strongest and the 

 polarisation colours most brilliant. As the distance from a flaw of this kind increases 

 the double refraction becomes feebler, and at a certain distance disappears. Sometimes 

 a black cross, the arms of which consist of two dark brushes, is seen when a stone is. 



