Effect of Bodies on Light and Heat. 393 



polarizer and an analyzer ) the light is colored with the tints of the 

 spectrum. The analyzer and polarizer used are generally two Nicol 

 prisms placed a little apart so that a film or lamina of crystalline min- 

 eral may be inserted between them. A nicol prism has a length three 

 times its width, and is formed by splitting a rhombohedral crystal of 

 Iceland spar along its optic axis, then polishing the cut faces and re- 

 joining them with transparent Canada balsam, fig. 164, A D. In ac- 



FIG. 164. Nicol Prism formed 

 from Crystal of Iceland Spar. 



A D. Optic axis. 



R E. Path of the extraordinary 

 ray. 



X O G. Path of the ordinary 

 ray. 



tion the prism allows the ex- 

 traordinary ray to pass 

 FIG. 164. through from E, to E, the or- 



dinary ray being refracted to and totally reflected thence toward Gl- 

 and lost. The other ray is the one used as "polarized light." An an- 

 alyzer can also be made with a transparent mirror, placed at the polar- 

 izing angle. The reflected ray being polarized in one plane, it follows 

 that the refracted ray is made up of all the other planes or azimuths. 

 If the mirror is not transparent this light is absorbed, if it is trans- 

 parent this light is transmitted. By passing this through several 

 plates of glass, "a bundle, "it becomes polarized too. If two nicol 

 prisms be used, the light passing through each is polarized the extra- 

 ordinary ray in each being transmitted and the other refracted to the 

 side of the prism. If the prisms be parallel, that is, the plane of the 

 extraordinary ray in one correspond with that in the other, the light 

 that gets through the first will also go through the second, but if one is 

 turned around 45 some light gets through about -J- of it. But if it is 

 turned around 90 so they are at right angles, the light that gets through 

 the first will be cut off entirely by the second, and extinguished. If, 

 however, a crystalline plate of tourmaline, for example, or mica, be in- 

 troduced, having its principal plane at an angle intermediate to that of 

 the two prisms, the light will pass, on the theory that it is partly turned 

 in passing from the polarizer into the crystal plate, and finished up in 

 the passage thence to the analyzer. As stated above, a single tourma- 

 line plate cut parallel with its optic axis, transmits light in a single 

 polarized plane. Two such plates together, with their axes parallel, 

 will still transmit the light, since the azimulh of vibration into which the 

 movement of the light is turned in the first is also permitted in the 

 second. But if the plates be crossed, the light passing the first is to- 

 tally extinguished by the second. But if a third tourmaline or a plate 

 of mica be interposed with its axis oblique to both, the plane of the 

 polarized ray from the first is twisted to a position at right angles from 

 that with which it started. 



