148 ON CIRCULAR POLARISATION, AS EXHIBITED IN THE 



yellowish-white one. All these strata are sections of the 

 planes of the primitive rhomb, the third plane being per- 

 pendicular to the eye. These layers are crossed by the veins 

 a b, c d, turning away from the axis at their summits, so 

 as to fall more perpendicularly upon the faces AB, AC as 

 they approach to B and C. Each alternate vein is pink 

 where it traverses the pink layers, and of a deeper yellow 

 where it traverses the yellowish-white layers. I was now 

 anxious to ascertain whether there was any difference in the 

 mechanical state of those parts of the veins which gave the 

 black fringes, and those which produced circular polarisation, 

 as the veins were often visible in common light, an effect 

 which could arise only from a difference of mechanical or of 

 refractive density. With this view, I cut a plate about y^th 

 of an inch thick, out of a large amethyst, by planes passing 

 through the axis. Having divided this plate into two parts, 

 I placed the one above the other, so as to counteract its pola- 

 rising and doubly refracting forces, and exposing it to polari- 

 sed light, I had a system of rectilineal tints M, N, Fig. 14. of 

 opposite characters, separated by the black fringe AB, all of 

 which were perfectly free from the tints of circular polarisation*. 

 Upon examining them withan analysing microscope, I distinct- 

 ly observed, that they were crossed with the veins of the ame- 

 thyst, though these veins were entirely invisible either in ordi- 

 nary or polarised light previous to the super-position of the 

 plates. The tints produced by the ordinary polarising force were 

 always a minimum at the lines corresponding with the black frin- 

 ges 



* Plates of Rock-Crystal cut and arranged in this manner, form the best com- 

 bination for exhibiting the different orders of colours. 



