OPTICAL STRUCTURE OF THE AMETHYST. 151 



paratus for conducting experiments on the polarisation of 

 light. If we wish to place the principal section of the analy- 

 sing prism exactly in the plane of primitive polarisation, we 

 have only to interpose a thin plate of amethyst, like that 

 shewn in Yig. 1., and if the tints of both sets of veins are ex- 

 actly similar, the analysing prism will have the required posi- 

 tion. If the one set of tints is bluer or whiter than the other, 

 or if there is the slightest difference between them, the posi- 

 tion of the prism must be altered, till that difference is no 

 longer perceptible. 



If we wish to place a plate of sulphate of lime, or any other 

 crystal, so as to have its principal section in the plane of pri- 

 mitive polarisation, the interposition of the amethyst plate will 

 give us the same assistance, by indicating that the circular 

 tints are not affected by it ; whereas if we wish to place the 

 axis of the sulphate of lime at an angle of 45' to the primitive 

 plane, the amethyst will point out this position, when the op- 

 posite circular tints suffer an equal change. 



The observations contained in the preceding pages are the 

 results of an immense variety of experiments, which acciden- 

 tal circumstances put it in my power to make upon this inte- 

 resting mineral. Having had access to whole bagfuls of ame- 

 thystine pyramids from the Brazils, in the possession of Mr 

 Alexander, lapidary in Edinburgh, 1 have examined some 

 hundred specimens ; and though I have not been able to re- 

 present one-tenth part of the varieties of arrangement assu- 

 med by the colouring matter and the veins, yet I have given 

 the most general, and, I trust, the most interesting of them. 

 I have purposely omitted the different appearances which 

 are |)i;oduced by crossing the veins of different specimens, be- 

 cause they are deducible from established principles, and like- 

 wise other phenomena of colour, which arise from the action 

 of a number of minute strata upon light, wh^n it is made to 

 pass between them. 



