PROPKKTIKS <)F CRYSTALS 



177 



If in any cose, or by any means, one ray, cither the o or e ray. 

 could In- absorbed, light passing out of the fragment would ! vi- 

 brating in one plane only. It is a property of plates of tourma- 

 line that when cut parallel to the c axis they absorb one ray, the 

 ordinary, and the extraordinary ray, vibrating parallel to the c axis 

 only, is transmitted. All light tran-inittcd by such a section of 

 tourmaline is vibrating in one plane, that parallel to the c axis. 

 Such a section of tourmaline, or any other device, used to produce 

 polari/cd light is termed a polarizer, Fig. 324. 



When the light, as transmitted by the polarizer, is viewed 

 through another similar section of tourmaline, it will be observed 



-a 



- o 



FIG. 324. Tourmaline Polarizer. 



at once that the intensity depends upon the relation of the two 

 sections of tourmaline. When the two c axes of the sections are at 

 90, as in Fig. 325, no light will be transmitted by the second section 

 in this crossed position, and the condition will be that of darkness ; 

 the amount of light, transmitted by the second section, termed the 

 analyzer, constantly increases from zero in the crossed position to a 

 maximum, when the axes of the polarizer and analyzer are 

 parallel. The analyzer allows no light to pass, the vibrations of 

 which are at right angles to its vibration plane; as all light pass- 

 ing the polarizer is thus vibrating, no light can pass when the 

 two sections are in the crossed position, and darkness is the result. 

 As the analyzer is rotated, the amount of light passing increases to 



