POLARIZA TION. 



431 



nearly so. The tourmaline analyzer previously used acts 

 in this way when turned a quarter way around. 



67O, Tourmaline Tongs. If these two frames, or 

 two tourmaline analyzers, be placed one over the other in 

 such a way that the bars of the second shall be perpen- 

 dicular to those of the 

 first, it will be seen that 

 the first will quench or 

 absorb part of the rays, FlG 



while the rays trans- 

 mitted by the first as polarized light will be quenched by 

 the second. But if the bars of the second be parallel to 

 those of the first, the polarized light transmitted by the 

 first will also be transmitted by the second. This partial 

 or total absorption of luminous rays is shown easily with 

 the " tourmaline tongs," which consist of two tourmaline 

 plates set in movable discs (Fig. 340). Light transmitted 

 by either plate is polarized (and colored by the accidental 

 tint of the tourmaline). When the 

 plates are superposed, polarized 

 light may be transmitted by both, 

 or all of the incident light may be 

 absorbed according to their relative 

 positions as above stated. 



FIG. 341. 



671. Polarization by Re- 

 flection. Light is polarized 

 when the rays whose vibrations lie in a particular plane are 

 alone allowed to pass. This effect may be produced by 

 causing a beam of light to be reflected by a non-metallic 

 mirror at a certain angle which depends upon the nature 

 of the reflecting substance. For glass, the ray must make 



