OPTICAL INSTRUMENTS. 197 



Fig. 715. Fig. 716. 



Selenite Designs. Numerous splendid and beautiful 

 effects are produced by arranging plates of selenite of differ- 

 ent thicknesses and designs, on plates of glass. These, in 

 ordinary light, will appear perfectly transparent ; but when 

 viewed in polarized light, will exhibit the most gorgeous 

 coloring, and may be made to undergo the most extraor- 



Fig.717. Fig. 718. Fig. 7 19. 



dinary changes, by viewing them through a tourmaline 

 plate, Nichol's prism, or any other analyzer, and causing 

 the analyser to revolve before the eye. They are mounted 

 between circular glass plates, one and three quarter 

 17* 



