182 POLARIZED LIGHT. SECT. XXI. 



reflected. Thus another property of polarized light is, that it 

 cannot be divided into two equal pencils by double refraction, in 

 positions of the doubly refracting bodies in which a ray of common 

 light would be so divided. 



Were tourmaline like other doubly refracting bodies, each of 

 the transmitted rays would be double ; but that mineral, when of 

 a certain thickness, after separating the light into two polarized 

 pencils, absorbs that which undergoes ordinary refraction, and 

 consequently shows only one image of an object. On this account 

 tourmaline is peculiarly fitted for analyzing polarized light, which 

 shows nothing remarkable till viewed through it or something 

 equivalent. 



The pencils of light, on leaving a double refracting substance, 

 are parallel ; and it is clear, from the preceding experiments, 

 that they are polarized in planes at right angles to each other 

 (N. 206). 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 common 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, analo- 

 gous to two parallel stretched cords, one of which performs its 

 undulations only in a horizontal plane, and the other in a vertical 

 or upright plane (N. 206). 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. By a simple mechanical law each 

 vibratory motion of the first is resolved into two vibratory 

 motions at right angles to one another. The ray of common 

 light may be assimilated to a round rod, whereas the two polar- 

 ized 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 



