PLUATE OF LIME, AND THE DIAMOND. 159 



Both of these crystals seemed to inclose a number of cubes 

 of different tints, having their faces parallel to the external 

 cube. 



When polarised light was transmitted through the faces 

 ADC, BEG, it was distinctly depolarised, the neutral axes AD, 

 AC being coincident with the sides, and the depolarising axes 

 AE, DC with the diagonals of the square faces ; and, what 

 was still more remarkable, in all the specimens there were 

 portions of the crystal where the polarised light suffered no 

 change. In these experiments, the tint polarised by the 

 spar was a blue of the first order, having a pale red for its 

 complementary colour. 



In order to examine the tints with more correctness, I com- 

 bined the cube of fluor-spar with a plate of sulphate of lime, 

 which polarised a brilliant blue of the second order, having an 

 orange-yellow for its opposite colour. The blue was changed 

 into a scarlet-red, and sometimes into a purple-red, and the 

 complementary orange-yellorv into a yellowish-white. When 

 the cube was turned 90° round, the blue was changed into a 

 pale yellow-green, and the complementary orange-yellow into a 

 yellowish-purple. 



This change of colour was consonant to the laws which regu- 

 late the action of all crystals upon light ; but I was surprised 

 to observe, that when the cube of fluor-spar remained station- 

 ary, there was one portion of it at m, Fig. 3. which made the 

 blue colour red, and the orange-yellow a yellowish-white, while 

 another portion, at n, made the blue colour green, and the 

 orange-yellow, purple. In another specimen, I found the same 

 opposition in the effects produced by two different portions, 

 m, o, which were separated by a third portion that had no ac- 

 tion upon light, the part o producing the same effect that 

 m would have done when turned 90° round, and m the same 



effect 



