66 PHYSICAL PROPERTIES OF MINERALS. 
4. REFRACTION anp POLARIZATION. 
Minerals differ widely in their refracting and polarizing 
properties, and hence these properties are a convenient means 
of distinguishing species. The explanations of the subject, and 
the methods of careful experimenting, will be found in treatises 
on optics, and also at considerable length, and with minute 
directions as to the use of instruments, in the Text-Book of 
Mineralogy. Only a few of the simpler facts required for the 
ordinary purposes of the mineralogist are here mentioned. 
The character of the refraction varies according to the sys- 
tem of crystallization. 
A. In isometric crystals there is simple refraction alike in 
all directions, and no polarization. 
B. In dimetric and hexagonal crystals the vertical axis, or 
axis of symmetry, is the direction of the optic axis; in all 
directions except this a transmitted ray of light is doubly re- 
fracted. Such crystals are optically weiaxiad. 
C. In trimetric, monoclinic, and triclinic crystals, which 
have the three axes unequal, there are ¢wo directions of no 
double-refraction. Such crystals are optically biaxial. 
1. Lsometric System.—In the isometric system there is no 
reference whatever in the refraction to crystalline structure, 
and in this respect substances thus crystallizing are like water. 
There is only simple refraction. The index of refraction is ob- 
tained by dividing the sine of the angle of incidence of a ray of 
light by the sine of its angle of refraction. ‘hus if a ray of 
light strike the surface of a transparent plate of the mineral at 
an angle of 40° from the perpendicular, and then passes through 
the plate at an angle of 30° from the perpendicular, owing to 
the refraction, the sine of 40° divided by the sine of 30° will 
be the index of refraction. Now the index of refraction of air 
being made the unit, that of water is 1°335; of fluorite, 1434; 
of rock salt, 1:557; of spinel, 1-764; of garnet, 1:8155. of 
blende, 2°260; of diamond, 2°439. 
2. Crystals Umaxial in Polarization.—A transparent Cleav- 
ace plate from a crystal of calvite shows what is called double 
refraction. Placed over a line drawn on any surface, two 
parallel lines are seen, one produced by the ordinary ray, and 
the other by the extraordinary vay. Both rays are polarized, 
and in planes at right angles to each other. Prisms, called 
Nicol prisms, made from transparent calcite (Iceland Spar), 
are employed for obtaining polarized light. Transparent 
