NOTES. 



501 



other. Fig. 65 shows the image formed by a crystal of Iceland spar which has 

 one optic axis, The colours in the rings are exactly the same with those of 

 Newton's rings given in Note 194, and the cross is black. If the spar be turned 

 round its axis, the rings surfer no change; but if the tourmaline through which 

 it is viewed, or the plate of glass, B, be turned round, this figure will be seen at 

 the angles 0, .90, 180, and 270 of its revolution. But in the intermediate 

 points, that is, at the angles 45, 135, 225, and 315, another system will 

 appear, such as represented in fig. 66, where all the colours of the rings are 

 complementary to those of fig. 65, and the cross is white. The two systems 

 of rings, if superposed, would produce white light. 



Fig. 65. fig. 66. 



NOTE 208, p. 208. Saltpetre, or nitre, crystallises in six-sided prisms having 

 Fig. 67. Fig.. 68. 



