OPTICAL PROPKKTIKS OK CRYSTALS 



181 



supply has been exhausted. They all agree, however, in the 

 principle of totally reflecting either the onlinary or extraordinary 

 ray out of t he field. 



i-ually mounted, the polarizing nicol is under the microscope 

 with its plane of polarization crossing the field, from to 

 I sit on the scale, while the analyzer is mounted in the tube of the 

 microscope in such a way that it may be pushed in or out of the line 

 of vision as required ; its plane of polarization is at right angles to 

 that of the polarizer, or in the crossed position. 



Interference of polarized light. Whether we speak of light as 

 due to waves, or to the periodic vibrations or change in con- 

 ditions, or whether light is due to an electromagnetic disturbance 

 of the ether, it remains nevertheless true, that one disturbance is 



FIG. 331. 



influenced by another and may be added to or subtracted from the 

 other, according to the phase of each. If two waves of the same 

 length are vibrating in the same plane and phase, as the two waves 

 a and b in Fig. 331, but of different amplitudes or intensities, the 

 result is an entirely new wave c with an amplitude oc, or a wave 

 with an amplitude of a + b, and the illumination of the new wave is 

 equal to that of the other two combined. When the waves are in 

 opposite phases, the result is the difference of the two amplitudes, 

 the new wave a, with an amplitude oa equal to oc oa, and the 

 illumination is decreased. Should the amplitudes be equal and 

 the one wave be a half phase or wave length behind the other, their 

 difference would be zero and darkness would result, or one wave is 

 said to interfere with the other. 



When light of the same wave length or color and of the same 

 intensity, i.e., derived from the same source, is polarized in two 

 rays, these two rays will interfere if brought to vibrate in the same 

 plane. The result of this interference will depend upon the con- 

 ditions of vibration, or how much one wave has been retarded, or 

 is vibrating behind the other. 



