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: 

 i . 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. 



