06 PHYSICAL PBOPEBTTES OF MINERALS. 



4. REFRACTION axd 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 uniaxial, 



C. In trimetric, monoclinic, and triclinic crystals, which 

 have the three axes unequal, there are two directions of no 

 double-refraction. Such crystals are optically biaxial. 



1. Isometric 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. Ihus 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, T434 ; 

 of rock salt, 1-557 ; of spinel, 1*764; of garnet, 1*815; of 

 blende, 2'260 ; of diamond, 2"439. 



2. Crystals Uniaxial in Polarization. — A transparent cleav- 

 age plate from a crystal of ealcite 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 ray. Both rays are polarized, 

 and in planes at right angles to each other. Prisms, called 

 Nieol prisms, made from transparent ealcite (Iceland Spar), 

 are employed for obtaining polarized light. Transparent 



