1 72 On the Optical Prope>-ties of a newly-discovered Salt of Quinine. 



of these microscopically thin compound plates ; the slightest 

 touch is sufficient to disrupt them. The slightest movement 

 in the liquid will at times destroy the connexion existing between 

 the edges of the component prisms ; they thus lose their uniform 

 and parallel arrangement. Wherever they cross, polarization and 

 obstruction of the rays of light necessarily occur, and the plate 

 is of course useless for the purpose designed. 



But although it is not possible to obtain one large enough to 

 surmount the eye-piece, yet it is perfectly easy to procure plates 

 of sufficient size to act as analysing crystals upon the field or 

 stage of the instrument, of course used superimposed on the 

 tourmaline, or artificial tourmaline attached to the stage; and 

 when these are placed at right angles, the phenomena of polari- 

 zation may be exhibited with great splendour. (Vide fig. 13.) 



When these crystals have been thus arranged, and the selenite 

 stage interposed, the field becomes coloured according to the 

 thickness of the plate of selenite ; and the extent of the field so 

 coloured will depend on the magnitude and breadth of the su- 

 perior artificial tourmaline employed ; frequently the whole field 

 of seven or eight inches, or even eleven inches in diameter, has 

 been coloured with an uniform tint. 



Fig. 13 exhibits a polyhedral compound crystal of the new sub- 

 stance employed as an analysing plate, the artificial tourmaline 

 being placed beneath it. The radiating crystals are those of 

 disulphate of quinine, which crystallized upon the plate of glass 

 used to mount the analysing crystal, in consequence of the eva- 

 poration of the mother-liquid from which the plate was formed, 

 and, depositing the excess of disulphate beneath the plate, thus 

 produced the splendid specimen now attempted to be depicted. 

 The crystals of course depolarize the light, and it is transmitted 

 by the superior or analysing plate ; and if the crystals are thin 

 enough, the prismatic colours are shown. But when the selenite 

 stage is placed upon the polarizing plate on the stage of the micro- 

 scope, and therefore inferior to the analysing plate with the disul- 

 phate of quinine beneath it, the appearances exhibited are of the 

 most gorgeous character — it is in vain to attempt to depict them. 

 The analysing plate of course developes the colour of the stage 

 employed ; its whole breadth therefore assumes the colour of the 

 stage if at right angles to the plane of the plate below, or the 

 complementary tint if parallel to it. The radiating crystals of 

 disulphate of quinine assume every hue and tint of the spectrum : 

 the experiment must be witnessed to be fully understood and 

 properly appreciated. 



It will be recollected that in this experiment it is not at all 

 necessary to employ a tourmaline ; the whole phenomenon may 

 be exhibited with equal brilliancy by using the two plates of 



