POLARISING CEYSTALS. 



151 



solution will take place ; then, upon the tip of a very fine 

 glass-rod let an extremely minute drop of the alcoholic 

 solution of iodine be added. The first effect is the produc- 

 tion of the yellow or cinnamon-coloured compound of iodine 

 and quinine, which forms as a small circular spot; tho 

 alcohol separates in little drops, which by a sort of repul- 

 sive movement, drive the fluid away ; after a time, the acid 

 liquid again flows over the spot, and the polarising crystals 

 of sulphate of iodo-quinine are slowly produced in beautiful 

 rosettes. This succeeds best without the aid of heat. 



" To render these crystals evident, it merely remains to 

 bring the glass-slide upon the field of the microscope, with 

 the selenite stage and single tourmaline, or Nicol's prism, 

 beneath it ; instantly the crystals assume the two comple- 

 mentary colours of the stage ; red and green, supposing 

 that the pink stage is employed, or blue and yellow, pro- 

 vided the blue selenite is made use of. All those crystals 

 at right angles to the plane of the tourmaline, producing 



Fig. 90. In this figure heraldic lines are adopted to denote colour. The 

 dotted parts indicate yellow, the straight lines red, the horizontal lines blue, 

 and the diagonal,-or oblique lines, green. The arrows show the plane of the 

 tourmaline, a, blue stage ; 6, red stage of selenite employed. 



that tint which an analysing-plate of tourmaline would 

 produce when at right angles to the polarising-plate ; 



