OPTICAL PROPERTIES l CRYSTALfi 



I'.H 



Interference figures in uniaxial crystals. In a -Action parallel to 

 the base, it has been shown that when t he light ray is passing parallel 

 (o the c axis, or optic axis, the section is dark between crossed nicols 

 and ivmaiiis so during a complete revolution. Sections in any ot her 

 direction when viewed with parallel rays are of uniform color, pro- 

 vided the section is of uniform thickness. This color effect, caused 



FIG. 339. 



by interference, is quite different when viewed with converging 

 light. The path of each ray varies with the inclination of the ray 

 to the section and will be proportionally retarded. The difference 

 between the slow and fast ray will increase with the length of the 

 path, yielding different interference colors when white light is 



Used. 



The petrographic microscope is fitted with a strongly condensing 

 lens, which is capable of being easily slipped in the line of vision, 

 directly under the stage, where it is almost in contact with the 



