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 Fig. 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 Mi- 

 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 produced 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, when it is made to 

 pass between them. 



