CRYSTALS UNDER THE MICROSCOPE 167 



Thus prepared, this polarizer is pushed into the opening in the 

 horseshoe base of the microscope until the center of the plate 

 falls in the optic axis of the microscope, the mirror of the instru- 

 ment having been removed or swung aside. Light thrown upon 

 the plate will be polarized and reflected in the line of the optic 

 axis of instrument. 



FIG. 103. Obtaining Polarized Light by Reflection. 



To further assist the student in the application of the polarizing 

 microscope, the following brief synopsis is given to refresh his 

 memory relative to his crystallographic knowledge. 



Fundamental Crystallographic Concepts. According to the 

 viewpoint of the crystallographer, crystals are polyhedra, 

 bounded by plane surfaces, whose forms are dependent upon 

 physical and chemical properties and governed by the correla- 

 tion of certain internal forces or attractions which we may call 

 a definite internal grouping or arrangement of molecules or 

 atoms. 



It must be remembered, however, that the chemist in recent 

 years has discovered a number of substances, appearing when 

 illuminated with ordinary light as thick syrupy liquids, yet 

 which yield optically most of the phenomena observed in solid 

 crystalline bodies. To this interesting class of compounds the 

 terms liquid crystals, crystalline liquids, or flowing crystals have 

 been given. 



It appears probable that only chemical elements and their 

 definite compounds form crystals. 



Crystals may form when a solid phase separates from a liquid. 



