176 DOUBLE REFRACTION. SECT. XXI. 



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 un- 

 dergoes ordinary refraction, and consequently shows 

 only one image of an object. On this account, tourma- 

 line 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 prece- 

 ding experiments, 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 common light, are performed 

 in every possible plane, at right angles to the direction 

 in which the ray is moving. But the case is veiy dif- 

 ferent after the ray has passed through a doubly refract- 

 ing substance, like Iceland spar. The light then pro- 

 ceeds 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 an- 

 other, analogous 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. 

 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 un- 

 dulations 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 edge- 



