150 



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 j 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 



Fi;?. 92. — 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 ; b, red stage of selenite employed, 



that tint which an analysing-plate of tourmaline would 

 produce when at right angles to the polarising -plate ; 



