116 



THE MICKOSCOPE. 



which forms as a small circular spot; the alcohol separates in little 

 drops, which, by a sort of repulsive movement, drive the fluid away ; 

 after a time, the acid liquid again flows over the spot, and the polar- 

 ising crystals of sulphate of iodo-quinine are slowly produced in beauti- 

 ful rosettes. This succeeds best without the aid of heat. 



To render these crystals evident, it merely remains to bring the 

 glass-slide upon the fleld of the microscope, with the selenite stage and 

 single tourmaline, or Nicol's prism, beneath it ; instantly the crystals 

 assume the two complementary colours of the stage; red and green, sup- 

 posing that the pink stage is employed, or blue and yellow, provided the 



fig. 81. In this figure the heraldic Knes are adopted to denote colour. The dotted 

 parts indicate yellow, the straight lines red, the horizontal lines blue, and the dia- 

 gonal, or oblique lines, green. The arrows show the plane of the tourmaline, 

 a, blue stage ; b, red stage of selenite employed. 



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 ; whilst those at 90° to these educe the complementary tint, as 

 the analysing-plate would also have done if revolved through an arc 

 of 90°." 



This test is so ready of application, and so delicate, that it must 

 become the test pwr excellence for quinine : fig. 81, a and 6. 



Not only do these peculiar crystals act in the way just related, but 

 they may be easily proved to possess the whole of the optical proper- 



