POLARISING CRYSTALS. 



149 



assumes the form of the two hyperbolic curves, fig 88a. But 

 if the tourmaline be revolved, the black crossed lines will 



Fig. SSa. 



be replaced by white spaces, and the red rings by green, 

 the yellow by indigo, and so on. These systems of rings 

 have, generally speaking, the same colours as those of 

 thin plates, or as those of a system of rings round one 

 axis. The orders of the colours commence at the centres 

 of each system; but at a certain distance, which corre- 

 sponds to the sixth ring, the rings, instead of returning 

 and encircling each pole, encircle the two poles as an 

 ellipse does its two foci. When we diminish or increase 

 the thickness of the plate of nitre, the rings are diminished 

 or increased accordingly. 



Small specimens of salts may also be crystallised and 

 mounted in Canada balsam for viewing under the stage of 

 the microscope ; by arresting the crystallisation at certain 

 stages, a greater variety of forms and colours will be 

 obtained : we may enumerate salicine, asparagine, acetate 

 of copper, phospho-borate of soda, sugar, carbonate of 

 lime, chlorate of potassa, oxalic acid, and all the oxalates 

 found in urine, with the other salts from the same fluid, a 

 few of which are shown at fig. 89. 



Dr. W. B. Herapath contributed an interesting addi- 

 tion to the uses of polarised light, by applying it to discover 

 the salts of alkaloids, quinine, &c. in the urine of patients. 



