OPTICAL PROPKRTIKS OF CRYSTALS 



201 



Fio. 349. Interference Figure of Ara- 

 gonite with the Plane of the Optic Axea 

 Parallel to the Plane of one of the 

 Nicols. 



t\vo optic :i\rs will emerge an equal distance on either side of the 

 :i\i> uf the microscope, and the interference figure as a whole will 

 lit- -y imnet rieally placed in the field of the microscope. The line 

 drawn through the two points 

 O and O' will be the trace of 

 the plane of the optic axes on 

 the plane of the section. 



The optic axes will be the 

 axes of cones of rays which pass 

 through the section; the in- 

 clination or path of each will 

 vary with the distance OO'. 

 At some distance from O and 

 O', depending upon the double 

 refraction and thickness of the 

 section and the inclination of 

 the ray, there will emerge two 

 rays with a phasal difference of 

 a whole wave length. These 

 two rays will be made to vibrate in the same plane in passing 

 the analyzer and will interfere. The point where one ray is 

 retarded behind the other one wave length will appear dark if 

 monochromatic light is used, as the analyzer adds a phasal 



difference of 1/2 X. As 

 the section of this cone of 

 rays, in the plane of the 

 mineral section, is ellipse- 

 like, elliptical shadows or 

 dark areas will appear 

 around each optic axis as 

 indicated. Alternating 

 concentric areas of light 

 and darkness will appear 

 as indicated in the dia- 

 gram, according to the 

 phasal difference of the 

 emerging rays. When 

 white light is used, the concentric areas will be colored as in 

 the interference figure of uniaxial crystals. In order to determine 

 what portion of the field will be dark in crossed nicols, due to the 

 light extinguished by the nicols, it is necessary to determine the 



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