178 



MINERALOGY 



a maximum when the two vibration planes are parallel, and all the 

 light passing the polarizer is transmitted by the analyzer. 



Intermediate positions are explained as in Fig. 326 ; let PP' be the 

 vibration plane of the polarizer and AA' that of the analyzer, be the 



amplitude and plane of vibration of 

 light passing the polarizer ; according 

 to the parallelogram law in mechanics 

 this wave may be divided into two 

 waves vibrating at right angles to 

 each other, one ce parallel to the 

 vibration plane of the analyzer and 

 with an amplitude ce, the other vi- 

 brating at right angles to it and with 

 an amplitude eb ; the ray represented 

 by ce passes the analyzer, while eb 

 at right angles to it is extinguished. 

 The amplitude of the transmitted 

 ray bf, which illuminates the field of the analyzer, increases from 

 zero in the crossed position, where all light is absorbed, to cb in 

 the parallel position when all the light is transmitted. 



Two such sections when mounted in a holder are known as the 

 tourmaline tongs, and may be used to test the double refraction 

 and the vibration planes of light in any mineral section placed 

 between them. 



If the light reflected from a polished table top is viewed through 



one of the tourmaline sections, as an analyzer, on revolving the 

 section the intensity of the transmitted light will be greatest when 

 the long or c axis of the tourmaline section is parallel to the table 



