MICROSCOPIC OBJECTS. 127 



with a prism be screwed immediately behind the object- 

 glass of the microscope, (that is, between it and the 

 body,) the object wiU appear of the most brilliant colours 

 if affected by polarized light. 



"When an object under examii^ation exhibits the 

 colours by depolarizing the light, if the field of view 

 appear luminous, as in viewing transparent objects by 

 common hght, cause the lower prism tube or polarizer to 

 revolve, and in certain positions the field of view will 

 appear black. The objects now assume the colours com- 

 plementary to those which they had upon a bright ground ; 

 indeed their appearance is that of brilliant gems lying 

 upon black velvet. Many crystals exhibit these polarized 

 tints very intensely. The following crystallized on a slip 

 of glass I have found remarkably interesting both as 

 regards the elegance of then" form and the splendour of 

 their colours : — salicine, chlorate of potassa, oxalic acid, 

 prussiate of lime, nitrate of potash, and acetate of cop- 

 per." To these may be added the crystals and minerals 

 named in the Catalogue under this heading, especially the 

 xanthate of ammonia. "The great advantage of employ- 

 ing the microscope in viewing the polarized tints of bodies, 

 is, that very small specimens will answer equally weU with 

 larger and more expensive ones in the ordinary way, 

 while they do not require any troublesome process to 

 spht or cut them into different thicknesses for obtaining 

 the different tints, this being accomplished in the process 

 of the crystallization. 



"Sulphate of Copper. — This salt, which is of a fine blue 



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