102 



A HALF-HOUR WITH 



Let a a' represent two plates of tourmaline 

 with their angles coincident, a is the polarizer and 

 of the analyzer ; with the plates in this position, 

 the polarized ray c passes through to d (Fig. 19). 



Fig. 20. 



6 &', two slices of tourmaline crossed ; 6, beam of common 

 light ; c, polarized ray stopped by &'. 



If we now cross the plates, the ray c is no longer 

 transmitted. If the analyzer is now revolved 

 another 90, the ray is again transmitted. Re- 

 volve it 90 more, the ray is stopped ; and, on the 

 completion of the circle, the ray again becomes 

 visible. 



The following diagram illustrates the effect of 



the various positions of the analyzer. At a the 

 ray is visible, at 6 invisible, at c visible, at d in- 



