166 Dr. Herapath on the Optical Properties of 



the lozenge, like lithic acid. The crystals deposited from alcohol 

 are of this character. 



At other times it forms small stellse, composed of acicular 

 crystals radiating from a centre like the spokes of a wheel. 



A more cautious crystallization will produce short pyramids 

 like the ammoniaco-magnesian phosphate from alkaline urine. 

 This is the case when a solid plate of iodine is suspended in a 

 solution of the disulphates in acetic acid. Some days elapse ere 

 they form, in consequence of the very slow solution of the iodine. 

 (See fig. 2.) 



The primary form of these crystals appears to be derived from 

 the rhombic prism, but it is very possible that the substance may 

 be dimorphous. 



One remarkable fact is evident throughout the whole of this 

 crystalline metamorphosis, — the optical properties remain the 

 same ; and the merest film of this remarkable substance possesses 

 decided power over the rays of light. 



In the following examination of their optical properties I made 

 use of Oberhauser's achromatic microscope, with half an inch 

 object-glass and No. 2 eye-piece; a low power, certainly under 

 100 diameters. 



A. Their brilliant emerald-green colour reflected to the eye 

 has been already noticed. This beam of green light produced 

 by reflexion is decidedly a polarized ray when the plane of the 

 crystal is inclined 41° to the plane of the incident ray. 



B. Their perfectly transparent and almost colourless appear- 

 ance when examined by transmitted light has also been noticed. 



C. The production of complementary colours, when examined 

 by means of a slightly polarized light, has also been spoken of. 



D. The action of a single tourmaline upon them is very de- 

 cided. 



E. The action of two tourmalines must also be investigated. 



F. The action of one tourmaline and a selenite stage is also 

 very peculiar, and will be minutely examined. 



G. The action of two tourmalines and a selenite stage must 

 also be explained. 



H. The phenomena exhibited by these crystals, when used as 

 polarizers and analysers, is also worthy of remark, and of course 

 permit of various crystalline substances being used as tests of 

 their remarkable polarizing properties. 



I. The phenomena of depolarization by these crystals will be 

 touched upon under sections C. and E. &c. 



(B.) The perfect polarizing powers which these crystals exhibit 

 must now be proved and illustrated. 



When two crystals of the prismatic form are examined in a 

 superimposed condition, the following effects will be apparent : — 



