OPTICAL PROPERTIES OF CRYSTALS 
253 
to their characteristic morphology, crystals exhibit certain physi¬ 
cal and optical properties according to the crystal system to 
which they are referred. Chief 
among these optical properties 
made use of by the chemist is 
the behavior of the crystals 
towards polarized light. 
Optically, crystals are either 
singly refractive (isotropic) or 
doubly refractive (anisotropic). 
If isotropic, they will show no 
change when rotated upon the 
stage of the microscope between 
crossed nicols. If anisotropic, 
they will appear alternately light 
and dark as the stage is turned. 
If, therefore, a crystal be placed 
upon the stage of a polarizing 
microscope near the center of 
the field between crossed nicols 
and the stage turned, the crystal 
will behave in one of two ways: 
1. It will remain dark throughout 
a complete rotation of the stage, 
that is, there is no change in its 
appearance in the dark field. 
2. As the stage is turned the 
crystal will alternately become 
bright or colored, and alternately 
disappear or become dark (ex¬ 
tinguish). In this case two pos¬ 
sibilities arise. Either the crys¬ 
tal disappears (extinguishes) when 
its long edges coincide with or are 
parallel to the cross-hairs, and is brightest midway between, or 
the position of extinction is not on the cross-hairs, but lies a 
little inclined to (is oblique to) the cross-hairs. In the former 
Fig. 142. Isotropic and Anisotropic 
Crystals between Crossed Nicol 
Prisms. 
