15S ELEMENTS OF APPLIED MICROSCOPY. 



isometric system be mounted on a slide and placed on 

 the stage of the microscope, the stage still remains dark. 

 Conditions are unchanged, since amorphous substances 

 like glass and crystals of the isometric system produce 

 no polarizing effects. When calcium-carbonate crystals, 

 or those of any other substance of the last five systems, 

 are examined with crossed Nicols, the crystals become 

 visible as shining bodies on a black ground. They exer- 

 cise a polarizing action of their own upon the plane polar- 

 ized light which passes through them, breaking it up 

 into two rays polarized at right angles to each other and 

 not coincident with the plane in which the light is sup- 

 pressed by the analyzer. Thus, when viewed with 

 crossed Nicols, polarizing or anisotropic bodies become 

 luminous, while non-polarizing or isotropic bodies do 

 not. 



One other phenomenon must be noted in the most 

 rudimentary account of the polariscope, the production 

 of color effects by thin plates of anisotropic substances. 

 If a plate of mica be placed upon the stage of the micro- 

 scope, the entering ray of plane polarized light, which we 

 may call A, is resolved into two rays with vibrations at 

 right angles to each other, A' and B'. Of these, the or- 

 dinary ray B' at right angles to A and A' is most refracted, 

 passes through a greater distance in the mica than does 

 the extraordinary ray, and is retarded by an amount 

 depending on the thickness of the plate and the strength 

 of double refraction of the substance. In the analyzer 

 each of the two rays A' and B' is split up again into two 

 components at right angles to each other. A' a and A'h, 



