204 THE OPTIC AXES OF CRYSTALS. [SECT. xxi. 



stance, are parallel ; and it is clear, from the preceding ex- 

 periments, that they are polarized in planes at right angles 

 to each other (N. 201). But that will be better understood 

 by considering the change produced in common light by the 

 action of the polarizing body. It has been shown that the 

 undulations of ether, which produce the sensation of com- 

 mon light, are performed in every possible plane, at right 

 angles to the direction in which the ray is moving. But the 

 case is very different after the ray has passed through a 

 doubly refracting substance, like Iceland spar. The light 

 then proceeds in two parallel pencils, whose undulations are 

 still indeed transverse to the direction of the rays, but they 

 are accomplished in planes at right angles to one another, 

 analogous to two parallel stretched cords, one of which per- 

 forms its undulations only in a horizontal plane, and the 

 other in a vertical or upright plane (N. 201). Thus the 

 polarizing action of Iceland spar and of all doubly refracting 

 substances is to separate a ray of common light, whose 

 waves or undulations are in every plane, into two parallel 

 rays, whose waves or undulations lie in planes at right angles 

 to each other. The ray of common light may be assimilated 

 to a round rod, whereas the two polarized rays are like two 

 parallel long flat rulers, one of which is laid horizontally on 

 its broad surface, and the other horizontally on its edge. 

 The alternate transmission and obstruction of one of these 

 flattened beams by the tourmaline is similar to the facility 

 with which a card may be passed between the bars of a 

 grating or wires of a cage, if presented edgeways, and the 

 impossibility of its passing in a transverse direction. 



Although it generally happens that a ray of light, in 

 passing through Iceland spar, is separated into two polarized 

 rays, yet there is one direction along which it is refracted in 

 one ray only, and that according to the ordinary law. This 

 direction is called the optic axis (N. 202). Many crystals 

 and other substances have two optic axes, inclined to each 

 other, along which a ray of light is transmitted in one pencil 



