160 THE MICROSCOPE. 



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 ; the 

 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. 



t( 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, 

 oeneach 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 



that tint which an analysing-plate of tourmaline would 

 produce when at right angles to the polarising-plate ; 



