1 62 ELEMENTS OF APPLIED MICROSCOPY. 



and pleochroism. Absorption is the power of absorbing 

 all or part of the light which vibrates in certain planes, 

 and pleochroism the similar property of removing the 

 rays of certain colors in particular planes. If a rock 

 section be examined with parallel rays of polarized light 

 and the stage be rotated, minerals which exhibit absorp- 

 tion will show a change in intensity of light from the 

 usual shade almost to black, while pleochroic minerals 

 show a change in color, or at least in shade. 



We have now data sufficient to identify any one of the 

 six crystallographic systems, whose optical characters 

 may be briefly summarized as follows: Isometric system; 

 isotropic, exhibiting no interference figures in conver- 

 gent light. Tetragonal system: anisotropic, uniaxial, 

 extinction symmetrical, sections giving uniaxial inter- 

 ference figures, often with rectangular cleavage. Hex- 

 agonal system: anisotropic, uniaxial, extinction symmet- 

 rical, sections giving uniaxial interference figures, three- 

 or six-sided, or show cleavage lines intersecting at angle 

 of 60°. Orthorhombic system: anisotropic, biaxial, ex- 

 tinction symmetrical in all sections. Monoclinic sys- 

 tem: anisotropic, biaxial, extinction symmetrical in one 

 zone, while other sections show extinction angles. Tri- 

 clinic system: anisotropic, biaxial, extinction angles in 

 all sections. By application of these criteria, com- 

 bined with observation of the more obvious optical 

 characters, color, aggregation, etc., it is possible to identify 

 any of the commoner minerals with ease and certainty. 



4. The Structure and Behavior of Alloys. — Metal- 

 lography, or the study of the structure of metals with the 



