186 



MINERALOGY 



examined between crossed nicols, it will be found, on rotation of the 

 section between the nicols, that the light will be entirely extin- 

 guished, or decrease to a minimum illumination, at every 90, and 

 the section will be dark. From the point of darkness the illumi- 

 nation increases constantly upon further revolving the section, 

 until a maximum is reached at a point 45 from the point of dark- 

 ness, and then decreases to a minimum after a revolution of the 

 section through another angle of 90 ; these conditions are repeated 

 four times in the complete revolution of 360. 



Interference of polarized light in passing mineral sections. 

 Let Fig. 337 be such a section ; then light entering the section 

 will be transmitted as two rays vibrating in planes at right angles 



to each other. Let ee'oV 

 represent the elliptical sec- 

 tion of the indicatrix; 

 the two rays will leave the 

 section vibrating in the 

 planes ee' and oo' ; also let 

 PP' and AA' be the vibra- 

 tion planes of the polar- 

 izer and analyzer. If RO 

 represents the amplitude 

 and the direction of the 

 vibrations of the plane 

 polarized ray passing the 

 polarizer, then on entering 

 the section this ray will be 

 resolved into two rays, 001, vibrating parallel to o', and oe, vibrat- 

 ing parallel to ee'. When the ray oo' enters the analyzer one com- 

 ponent oo u vibrating parallel to the vibration plane AA' of the 

 analyzer passes, and passes without diminution, while the other 

 component, vibrating parallel oiOn at right angles to A A' having 

 no component in the plane A A' is extinguished by the analyzer. The 

 two rays oon and oe n , vibrating parallel to AA' and therefore in 

 position to pass the analyzer, are also in position to interfere, 

 and the resultant light depends upon this interference. When 

 white light is used, the resulting interference color will depend 

 upon the double refraction of the substance ; upon the direction 

 of the section in the crystal; and upon the thickness of the 

 section. 



When monochromatic light is used and one ray is retarded behind 



