POLARISED LIGHT. 



117 



ties of that remarkable salt of quinine, so fully described by me in the 

 Philosophical Magazine for March ] 852, and the chemical analysis of 

 which was published in the number for September in the same year. 

 In fact, these crystals are perfectly identical with the sulphate of iodo- 

 quinine in every respect. 



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 tourmalines, 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 

 shown in fig. 82. 



fig. 82. 



If we employ the selenite stage in the examination of this object, 

 we obtain one of the most gorgeous appearances 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 having a red and green fringe, whilst the four intermediate 

 sectors are of a gorgeous orange-yellow. These appearances alter upon 

 the revolution of the analysing-plate of tourmaline; when the blue 

 stage is employed, the cross will assume a blue or yellow tint, accord- 

 ing to the position of the analysing-plate. These phenomena are ana- 

 logous to those exhibited by certain circular crystals of boracic acid, 

 and to those circular discs of salicine (prepared by fusion) ; the differ- 



