CRYSTALS UNDER THE MICROSCOPE 173 



In the triclinic system no axis of elasticity is parallel with a 

 crystallographic axis. 



For the relations between axes of elasticity and refractive 

 index, see Chapter IX, page 194. 



Observations with Converging Polarized Light. Strongly 

 converging polarized light offers one of the most valuable methods 

 of petrographic microscopic research, but it possesses only a 

 very restricted value for the chemist in microchemical quali- 

 tative analysis. Although it affords a means of differentiating 

 between crystal systems and thus yields information not ob- 

 tainable by parallel polarized light, useful optical phenomena 

 with converging polarized light are obtainable only when the 

 light is sent through crystals in the direction of the optic axis 

 in the case of uniaxial crystals or in a direction perpendicular 

 to the plane of the acute bisectrix l in the case of biaxial 

 crystals. 



Tiny uniaxial crystals will occasionally be found in a prepara- 

 tion lying in such a position as to be available for study with 

 converging polarized light; but in the case of biaxial crystals, 

 sections must be cut. It is obvious that cutting thin sections 

 of tiny individual friable crystals of the vast majority of the 

 salts met with in analytical work is impracticable or impossible. 

 Moreover, the information eventually obtained would not war- 

 rant the expenditure of time and patient labor required. 



Interference Figures. When a section of a uniaxial crystal 

 cut perpendicular to the optic axis is placed upon the stage of 

 the polarizing microscope, illuminated with strongly converging 

 polarized light and the observer looks into the microscope with 

 crossed nicols, but with no eyepiece in place, he will see a black 

 cross with a series of spectrum-colored concentric circles. 2 This 

 image is known as the interference figure. 



Biaxial crystals in sections normal to the acute bisectrix yield 

 (in typical cases) curved black bands or an asymmetric black 



1 The acute bisectrix is a line bisecting the angle formed by the intersection of 

 the two optic axes. 



2 Or in the case of circular polarization, the arms of the cross do not intersect 

 but leave a central light space. 



