158 ELEMENTS OF APPLIED M.:CROSCOPY. 
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’, 
