114 THE MICROSCOPE. 



of the prism, and 5 to each other. If, therefore, we cut off a piece 

 from a prism of nitre with a knife driven by a smart blow of a hammer, 

 and polish the two surfaces perpendicular to the axis of the prism, so 

 as to leave the thickness of the sixth or eighth of an inch, and then 

 transmit a ray of polarised light along the axis of the prism, we shall 

 see the double system of rings shown in figs. 78 and 79. 



fig. 78. fig. 79. 



When the line connecting the two axes of the crystal is inclined 

 45 to the plane of primitive polarisation, the cross seen as first de- 

 scribed, on revolving the nitre, opens, and gradually assumes the form 

 of two hyperbolic curves, fig. 79. But if the tourmaline be revolved, 

 the black crossed lines will be replaced by white spaces, and the red 

 rings by green ones, 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 corresponds 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. 



A large number of crystals exhibit this curious and beautiful sys- 

 tem of coloured rings. Small specimens of salts may also be crystal- 

 lised 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 may 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 in the same fluid, some of which 

 are shown in fig. 80. 



Dr. W. B. Herapath has contributed an interesting addition to the 

 uses of polarised light, by applying it to discover the salts of alka- 

 loids, quinine, &c. in the urine of patients. He says : " It has long 



