484 REPORT OF NATIONAL MUSEUM, 1900. 



and all minerals belonging- to sj^stems of ciystallization other than the 

 isometric are doubly refracting. 



Dk2jersion of light.— ^Nh^n a ra}^ of light passing from one mcdinm 

 to another is refracted, it maj^ be decomposed into several kinds of 

 light, separated more or less widely from each other and difi'erently 

 colored. A familiar example of this is seen when a beam of sunlight 

 passes through a Hint-glass prism and produces a band of colors in all 

 the hues of the rainbow. This decomposition of light by refraction 

 is called dispersion. Upon it depends that peculiar quality of "fire" 

 or play of prismatic hues in gems, a marked characteristic in the 

 diamond and zircon. 



Polarization of light. — The light which passes through a doubly 

 refracting cr3^stal undergoes a peculiar change. A ra}^ of light which 

 has been once split by passing through a doubly refracting substance 

 will not again be divided on passing through another doubly refract- 

 ing substance, nor can it again be reflected at a certain angle, nor 

 again traverse in a certain direction the substance in which it has suf- 

 fered this change. Light which has acquired these properties is said 

 to be polarized. 



Pleochroism. — This is the variation in color observed in doubly 

 refracting minerals when viewed in difierent directions. This prop- 

 erty is conspicuous in the tourmaline, iolite, chrysoberyl, and epidote. 

 The pleochroism of gems can best be observed with the aid of an 

 instrument called the dichroscope. 



This consists of an oblong rhombohedron of Iceland spar, at the 

 extremities of which is cemented a glass prism of 18°. This is con- 

 tained in a cylindrical metal case having a convex lens at one end and 

 a square opening at the other. On looking through the lens the 

 square opening appears double. When a pleochroic gem is examined 

 in transmitted light with this instrument, the two squares on a revolu- 

 tion will be found to have different colors at certain intervals. 



Phosj^horescence. — The emission of light or phosphorescence may be 

 produced in different ways; by rise of temperature, by mechanical 

 effect, and by insolation — that is, by exposure to the direct action of 

 sunlight. Phosphorescence is a property possessed by some gems, 

 notabl}^ the diamond. 



Hardness. — ^This is the degree of resistance the mineral offers to 

 abrasion. It is usually referred to an arbitrary scale of ten minerals 

 showing a regular gradation in hardness from 1, talc, the softest; 2, 

 gypsum; 3, calcite; 4, fluorite; 5, apatite; 6, orthoclase; 7, quartz; 8, 

 topaz; 9, corundum, to 10, diamond, the hardest. The scale is used 

 as follows: Fragments of the minerals comprising the scale are applied 

 in succession to the stone under examination. Should the test stone 

 neither scratch nor be scratched by a particular unit in the scale, the 

 hardness of the two are the same. Should the test piece scratch one 



