472 PHYSICAL SCIENCES. 



polarised ray r s may pass through it, exhibits the system of rings re- 

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



Fig. 67. Fig. 68. 



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

 successively to those given in figs. 68, 69, and 70. The colours of the 



Fig. 69. Fig. 70. Fig. 71. 



rings are the same with those of thin plates, but they vary with the 

 thickness of the nitre. Their breadth enlarges or diminishes also with the 

 colour, when homogeneous light is used. 



NOTE 214, p. 189. Fig. 71 represents the appearance produced by 



