POLARISATION OF LIGHT. 



43 



-i led pyramid. The shaded veins 

 turn the planes of polarisation from right 

 to left, while all the intermediate un- 

 shaded veins, and the three unshaded 

 sectors turn the planes of polarisation 

 from left to right. The line or narrow 

 space where these two structures meet, 

 has no action on the planes of polarisa- 

 tion ; and the action increases on each 

 side of it to the centre of the shaded veins 

 where it is a maximum. Hence it is 

 obvious, that the amethyst is not, in the 

 ordinary sense of the woVd, a compound 

 mineral, in which dissimilar faces of the 

 crystal are brought into mechanical con- 

 tact : for in this case we should have a 

 dark line of junction, and the intensity of 

 the polarising force would be the same in 

 every part of the combined layers. In 

 the present case, the right-handed passes 

 gradually into the left-handed polarisa- 

 tion, and there is no appearance whatever 

 of cleavage in the direction of these 

 planes. "We must, therefore, consider 

 the amethyst as a mineral in which, like 

 the analcime, the ether is distributed in 

 an unusual manner, or in which the 

 structure has been regulated by laws of 

 crystallisation which have not yet been 

 recognized by mineralogists. In* some 

 specimens, these opposite layers are so 

 minute, that the maximum intensity of 

 the force which turns the planes of 

 polarisation is nearly reduced to no- 

 thing:, so that the black cross in the 

 centre of its rings is seen with nearly 

 the same distinctness as in calcareous 

 spar. Hence, in the veins of amethyst 

 1 millimetre thick (l-25th of an inch) we 

 find them of very different intensities, 

 the arc of rotation for the mean yellow 

 ray varying from up to 23.9945. 



"The colouring matter of the amethyst 

 is arranged in a very remarkable man- 

 ner in relation to these veins. The lilac 

 tints often reside in the veined structure ; 

 and in some specimens Dr. Brewster 

 found the red colouring matter arranged 

 in veins corresponding with the dark 

 spaces where the two structures meet. 

 In another specimen the right-handed 

 veins were lilac, the left-handed veins 

 brownish red, and their lines of junc- 

 tion yellowish white. In other crystals, 

 the colouring matter affects the largest 

 masses of the crystal, such as those left 

 white in fig. 47. 



When an amethyst is perfectly formed, 

 its structure is symmetrical, as shown in 

 Jig. 48, which represents the section of 

 part of the pyramid and part of the 



Fig. 48. 



prism of an amethyst in Dr. Brewster's 

 possession, measuring: nearly two in- 

 ches and three-tenths across. " On 

 the three alternate sides of the prism," 

 says he, " viz. M N, O P, and Q R, 

 are placed sectors M c N, O d P, 

 Q a R, which are divided into two parts 

 by dark lines, c c', dd ", a d ', which 

 separate the direct structures of A, C, 

 and E from the retrograde structures of 

 B, D, and F. On the other three alternate 

 faces of the prisms are placed the three 

 veined sectors M c 6 a R, N cb dO, and 

 Pdba Q, which meet at b in angles of 

 120, and consist of veins of opposite 

 structures, alternating with each other, 

 and so minute, that in many places the 

 circular tints are almost wholly extin- 

 guished by their mutual action. The 

 direct sectors, A, C, and E, are all con- 

 nected together by the three radial veins 

 b a, be, bd, and are therefore to be con- 

 sidered as the expanded terminations of 

 those veins. The retrograde sectors B, 

 D, and F, are expansions of the first 

 retrograde veins next to b d c, d b a, and 

 a b (' ; and the lines c c', d d', and a of 

 are continuations of the dark or neutral 

 lines which separate the first retrograde 

 vein from the direct radial veins. 



All the sectors A, B, C, D, E, and F, 

 are of a yellowish brown colour, and all 

 the rest of the crystal is of a pale lilac 

 colour, the lilac tints being arranged in 

 the manner previously described. The 

 phenomena which I have now mentioned 

 as existing in this specimen are very 

 common in the amethyst; and I have 

 never yet found a specimen in which the 

 yellow tints were not confined to those 

 portions which formed the expanded ter- 

 minations of veins; a fact which indi- 

 cates that this would have been the 

 colour of the crystal, whether its actions 

 were direct or retrograde, and that the 

 lilac colour affects in general those por- 



