64 



CHEMICAL PHYSICS. 



of this substance, just as in the case of Iceland spar; but tourmaline 

 possesses the peculiar property of absorbing the ordinary ray, while 

 the extraordinary one passes through. 



If two plates, cut parallel to the axis of the crystal, are laid upon 

 each other in a crossed position, as AB in Fig. 24, it is found that 



FIG. 24. 



Tourmaline plates. 



light from the first plate is cut off by the second one, and darkness 

 results. If one plate be turned round upon the other, it will be 

 found that the combination is most transparent in two positions dif- 

 fering by 180 degrees, one of them, ab, being the natural position 

 which the plates originally occupied in the crystal. The combina- 

 tion is most opaque in the two positions at right angles with these, 

 while intermediate positions, such as a'b', show intermediate be- 

 havior. These facts show that light which has passed through one 

 such plate is in a peculiar condition. It is said to be plane-polarized. 

 In order to understand polarization of light, we should bear in 

 mind that the particles of ether undulate in a variety of planes per- 

 pendicular to the line of propagation. We assume that in polarized 

 light the undulations of the ether particles take place in a single 

 plane. These ether undulations may be compared to those taking 

 place in a cord fastened at one end and shaken by the hand at the 



FIG. 25. 



Undulation in a cord. 



other end, as in Fig. 25. According to whether we move the hand 

 horizontally, obliquely, or vertically, the undulations will lie in a 

 horizontal, oblique, or vertical plane, as represented at A. 



