60 GENERAL CHARACTERS OF PRECIOUS STONES 



colour in different directions ; these precious stones behave in this respect like their glass 

 imitations. 



Taking refraction as the basis of classification, we have seen that all minerals can be 

 divided into two groups, namely, those which possess single refraction and those which 

 possess double refraction. The former group will include those minerals which are not 

 possessed of dichroism, while all dichroic minerals fall into the latter. Thus amorphous 

 substances and those which crystallise in the cubic system are characterised by single refrac- 

 tion and absence of dichroism, while all coloured minerals included in the remaining five 

 crystal systems are dichroic, and all without exception are doubly refracting. 



The phenomenon of dichroism then, furnishes us with additional aid in distinguishing 

 singly from doubly refracting stones. A body showing this character to even the feeblest 

 degree cannot be amorphous nor can it be a cubic mineral. The apparent absence of 

 dichroism however must be considered only as negative evidence in favour of single refrac- 

 tion, since dichroism may be present but so feeble as to be detected only with difficulty. It 

 has been shown above that the phenomena of double and of single refraction enables us 

 to distinguish a ruby from a red spinel, and this is made still more easy from the fact that 

 the hexagonal ruby is distinctly dichroic, while the cubic spinel does not possess this property. 

 In the same way an imitation ruby of red glass could not be confused with the genuine stone, 

 since the former, being amorphous, is not dichroic and shows the same colour in all 

 directions. 



The detection of dichroism usually requires the use of a special instrument. The most 

 convenient instrument for the purpose is that devised by the Viennese mineralogist Haidinger, 

 and known as a dichroscope. This instrument is inexpensive and easily used, and should 



be in the hand of every one who buys or sells precious 

 stones, since a single glance through it is sufficient to 

 establish the presence or absence of dichroism in a 

 stone. 



This instrument is shown in section, and of its 

 actual size, in Fig. 28. It consists essentially of a 



„ cleavage rhombohedron, C, of Iceland-spar (calcite), 



Fig. 28. The dichrosoope. (Actual size.) , , . , . i- • • , a 



which is longer in one direction than in others. At 



each of its oblique ends is cemented a glass prism or wedge, K, the outer surfaces of 



which are perpendicular to the long edge of the calcite rhombohedron. A brass tube, h, 



encloses these essential portions of the instrumeijt, and has at one end a small square 



aperture, b, and at the other a circular aperture, a. Between the circular aperture and 



the glass prism, K, is placed a lens, L, of such a focal length that on looking through the 



instrument in the direction ab a sharp image of the square aperture, 6, will be seen. This 



image will, however, owing to the intervention of the doubly refracting calcite, not be single, 



but double. The instrument is so proportioned that these two images, o and e, will appear 



side by side, in contact but not overlapping. The image o will be only slightly displaced 



from the axis of the instrument and will be quite colourless ,- the image e is rather more 



displaced, and shows a narrow red border on its inner edge and a narrow blue border on its 



outer edge, as indicated by striations in the small figure at the side, otherwise the image is 



colourless. The instrument is so constructed that the distance of the square aperture from 



the lens can be varied, which enables the images to be sharply focused and adjusted so that 



their edges are in contact. 



In using this instrument, the precious stone to be tested for dichroism is placed over the 



square aperture, b, and, the instrument being directed towards a clear sky, the observer 



