POLARISED LIGIIT. 



151 



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 hecome the test, par excellence, for quinine : 

 fig. 92, a and b. 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 properties of that remark- 

 able salt of quinine, the sulphate of iodo-quinine. 



" To test for quinidine, it is merely necessary to allow 

 the drop of acid solution to evaporate to dryness upon the 

 slide, and to examine the crystalline mass by two tourma- 

 lines, crossed at right angles, and without the stage. 

 Immediately little circular discs of white, with a well- 

 defined black cross very vividly shown, start into existence, 

 should quinidine be present even in very minute traces. 

 These crystals are represented in fig. 93. 



Fig. 93. 



" If we employ the selenite stage in the examination of 

 this object, we obtain one of the most gorgeous appear- 

 ances in the whole domain of the polarising- microscope : 

 the black cross at once disappears, and is replaced by one 

 which consists of two colours, being divided into a cross 



