POLARIZED LIGHT. 



103 



visible ; as the analyzer passes from a to 5, the 

 brightness of the image gradually diminishes ; from 

 b to c the brightness increases. The positions 

 marked 1, 2, 3, 4, are called the neutral axes, only 

 half the amount of light being transmitted. 



In order to analyze a polarized beam it is not 

 necessary that the analyzer should be of the same 

 material as the polarizer ; a reflected ray may be 

 examined by a tourmaline or crystal of Iceland 

 spar, and a refracted or transmitted ray can be 

 reflected from the surface of a mirror. 



The student will have gathered from what we 

 have stated in the preceding pages, that the effect 

 of an analyzer on a polarized ray is the alternate 

 transmission and stoppage of that ray. The most 

 gorgeous effects are, however, obtained when a 

 doubly refracting film is interposed between the 

 polarized ray and the analyzer, producing what is 

 termed " chromatic polarization." 



This doubly refracting film receives the polar- 

 ized ray, and doubly refracts it ; in other words, 

 the series of undulations of which the ray is com- 

 posed on entering the film (sometimes called the 

 depolarizer) is broken into two systems within it, 

 forming the ordinary and extraordinary rays. 



Piq. 22. 



If a polarized ra> is allowed to enter a film of 

 selenite, it becomes refracted, and forms two dis- 



