CRYSTALS UNDER THE MICROSCOPE 171 



to their characteristic morphology, crystals exhibit certain 

 physical and optical properties according to the crystal system 

 to which they are referred. Chief among these optical proper- 

 ties 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 (aniso tropic) . 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 (extinguish). In this case two possibilities arise. Either 

 the crystal disappears (extinguishes) when its long edges coin- 

 cide 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 case we speak of the crystal as having parallel 

 extinction, and in the latter as having oblique extinction. Crystals 

 exhibiting a lozenge or equilateral rhomb outline and which 

 extinguish when the cross-hairs bisect the acute and obtuse 

 angles of the lozenge (a variant of parallel extinction) are some- 

 times said to exhibit symmetrical extinction. 



Anisotropic or doubly refracting crystals further fall into two 

 groups: I. Those which exhibit no double refraction in one di- 

 rection through the crystal uniaxial crystals. II. Those which 

 exhibit no double refraction in two directions biaxial crystals. 



Those directions parallel to which there is no double refraction 

 have been designated as the optic axes. The directions vary 

 slightly according to the wave length of light but for all practical 

 purposes may be considered as constant for white light. 



Crystals belonging to the tetragonal and hexagonal systems 



