104 



V HALF-HOUR WITH 



tinct rays, a is a polarized ray, b the film of 

 selenite, c is the extraordinary ray, d the ordi- 

 nary ray; but one of these rays is retarded. If 

 they are analyzed by a double-image prism, the 

 ordinary and extraordinary rays will again be 

 divided into c d, c c and d d, d c; and if the 

 original ray be passed through a circular aperture, 

 two coloured discs will be observed, the colour 

 depending upon the thickness of the selenite film. If 

 one disc is red, the other will be green, the colours 

 being complementary to each other. 



When a plate of tourmaline or a NichoFs prism 

 is used, one of these rays is alternately suppressed. 

 If the analyzer is revolved, we shall find that 

 when the angles of the polarizer and analyzer 

 coincide, and supposing a red and green selenite is 

 used, the colours will appear in the following 

 order : 



Fig. 23. 



At a the ray would be green, at b red, at c 

 green, at d red ; as the analyzer approached 1, the 

 colour fades ; when it reaches that position the 

 colour will disappear ; as it approaches 6, the red 

 increases in brilliancy until it reaches 6, when ib 

 will have reached its maximum brightness. In 

 the positions 2, 3, and 4, no colour will be found. 



