430 



JSOTilS. 



The 



rings are complementary to those of fig. 65, and the cross is white, 

 two systems of rings, if superposed, would produce white light. 



NOTE 908, p. 182. Saltpetre, or nitre, crystalizes in six-sided prisms 

 having two optic axes inclined to one another at an angle of 5. . A slice 



Fig. 67. 



of this suhstance about the 6th or 8th of an inch thick, cut perpendicu- 

 larly to the axis of the prism, and placed very near to s, fig. 64, so that 

 the polarized ray rs may pass through it, exhibits the system of rings 

 represented in fig. 67, where the points C and C mark the position of the 

 optic axes. When the plate B, fig. 64, is turned round, the image 



Fig. 69. 



Fig.lQ. 



