FLUATE 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-yellow into a yellowish-white. When 
the cube was turned 90° round, the b/we was changed into a 
pale yellow-green, and the complementary orange-yellow into a 
yellowish-purple. 
This change of colour was consonant to the Jaws 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, 0, 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 
