162 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 
