OPTICAL PROPERTIES OF CRYSTALS 259 



Observations with Converging Polarized Light. — Strongly con- 

 verging 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 obtain- 

 able by parallel polarized light, easily interpretable optical phe- 

 nomena 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 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 

 it is rare that a crystal will lie in a position such that a beginner 

 will be able to properly interpret the phenomena he may observe, 

 moreover it is seldom possible to change the orientation of the 

 tiny crystals with which he usually has to deal. Since, however, 

 the information which may be gained through the use of con- 

 verging polarized light may be of the greatest value in the identi- 

 fication of a compound, it is well worth while to always make 

 such examinations whenever a suitably equipped microscope is 

 available. 



Interference Figures. — When a section of a uniaxial crystal 

 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. 1 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 cross 

 superimposed upon spectrum-colored lemniscates or hyperbolas. 2 



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

 but leave a central light space. 



2 For a very comprehensive discussion of interference figures see Weinschenk - 

 Das Polarizations Mikroskop, or Weinschenk-Clark, 1. c, Chapter V. See also 



