120 MICROSCOPIC ILLUSTRATIONS.' 



the one being elicited by the ordinary, and the other by the 

 extraordinary ray. Hence they are termed dichromatic. 

 To display dichroism in crystals, the lower prism, or pola- 

 rizer, should alone be used ; the eye-piece being allowed 

 to remain as when objects are viewed by common light. 

 If, whilst the dichromatic body is under examination, the 

 polarizing prism be turned a quarter round, the colour 

 will also be changed. In conducting these experiments, 

 the illumination need not be so intense. In large crystals, 

 dichroism may be readily seen without the aid of a micro- 

 scope, simply by placing them between the eye and a 

 japanned reflector inclined at the polarizing angle. 



In order to convert the microscope into a complete 

 polariscope, I employ, instead of the mirror n, a pola- 

 rizing reflector, formed of several parallel plates of 

 thin glass fixed at the angle of polarization. A stage 

 must be constructed with a large condensing lens, and 

 made to slide upon the same stem, c, as is used for the 

 mirror. A double object-glass must be attached to the 

 body for obtaining a proper magnifying power, and the 

 analyzer should be made of tourmaline, in order to pro- 

 cure an expansive field of view. With this arrangement, 

 the phenomenon of polarized light, as exhibited in unan- 

 nealed glass, plates of amethyst, as also the coloured 

 rings and black crosses in crystals cut across their optical 

 axes, are seen in the most perfect manner. Professor 

 Amici was the first to exhibit these curious effects by 

 means of the microscope, although I do not know the 

 r xact construction of his apparatus. 



