OPTICAL STRUCTURE OF THE AMETHYST. 141 



and some perfectly colourless, I cut various plates of them per- 

 pendicular to the axis, and examined them when exposed to 

 polarised light. A structure of a very extraordinary kind pre- 

 sented itself; but in most cases it was so minute, that I was 

 obliged to analyse the emergent light by microscopes of Agate 

 and Tourmaline *. When the structure was regular, three sepa- 

 rate sets of veins were seen, as shewn in Plate X. Fig. 1. which 

 represents a plate cut from the pyramidal summit of a pink- 

 coloured amethyst. 



The veins resembled a number of V's inclosed in one another, 

 and each set was opposite an alternate face of the prism, the apex 

 being directed to the centre. Upon examining these alternate 

 veins, I found that the series distinguished by a faint blue 

 tint, produced the succession of colours, by turning the prism 

 from right to left ; while the series with a faint yellow tint pro- 

 duced the same succession, by turning the prism from left to 

 right f. 



Each of these fringes was placed between two of an oppo- 

 site character, and separated from them by a black fringe , 

 where the crystal produced none of the tints of circular polari- 

 sation, 



* After I discovered that the Agate gave a single polarised image, in conse- 

 quence of the dispersion, and partial absorption, of the rays which form the other 

 image (See Phil. Trans. 1813, p. 102. and 1814, p. 189.), I employed it constant- 

 ly as a part of my apparatus, as may be seen in the Phil. Trans. 1814, p. 203, 

 206, 208, &c. &c, and when the aid of a microscope was necessary, I cemented 

 a thin plate, with Canada Balsam, upon the plain side of a Plano-convex Lens. 

 By the method described in the Phil. Trans. 1819, p. 146, I have extinguished 

 one of the images of Calcareous Spar so completely, that the place where it should 

 have been could not be distinguished, even in the strongest lights ; and I have ac- 

 cordingly used it as an analysing prism, in preference to the agate and the tourma- 

 line. Epidote, Mica, and other substances which absorb one of the pencils, may 

 be employed for the same purposes. 



f See the description of the figures at the end of the paper. 



