482 REPORT OF NATIONAL MUSEUM, 1900. 



Among- minerals the many varieties of color are classed, first, as 

 metallic and nonmetallic, and all shades are referred to eij^ht funda- 

 mental colors — white, gray, black, blue, green, yellow, red, and brown; 

 second, according to peculiarities in the arrangement of color, as play 

 of color, opalescence, iridescence, and asterism; third, as to difference 

 in color shown for light transmitted in ditierent directions through the 

 stone. This case of color absorption is called pleochroism and is 

 peculiar to certain minerals. 



Diaplianelty. — The capacity of transmitting light materially affects 

 the beaut}' and value of g-ems. There is a wide difference in the degree 

 of this property possessed by most gems and the amount of light 

 transmitted, or the degree of transparency is classed as transparent 

 when the outline of an object seen through the stone is perfectly dis- 

 tinct; subtransparent when an object may be seen but its outline is 

 indistinct; translucent when light is transmitted but oljjects are not 

 seen; subtranslucent when merely the edges are translucent; opaque 

 when no light is transmitted. , 



Luster. — This is that character depending upon the power and man- 

 ner of reflecting light and is dependent upon the nature of the reflect- 

 ing surface and the quantity'' or the intensity of the light reflected. 

 The kinds of luster are described as metallic, the lirilliant appearance 

 seen upon the surface of polished metal; adamantine, the luster of the 

 diamond; vitreous, having the luster of flint glass; resinous, having 

 the appearance seen upon the surface of pine resin; waxy, the luster 

 of beeswax; greas}', when resembling that of a freshl}' oiled surface; 

 pearly, the luster of mother-of-pearl; silky, when having a sheen like 

 that of silk. The degrees of intensity are splendent, shining, glisten- 

 ing, glimmering, and dull. There being no standard of description in 

 regard to luster, these terms are loosely used and intermediate ones 

 may be substituted in describing a particular kind of luster according 

 to the judgment of the observer. 



Refraction of light. — The familiar case of the apparent breaking of 

 an oar where it enters the water is an illustration of the bending back 

 or refraction of light. This phenomenon occurs in the majority of 

 cases where a ray of light passes obliquely from one transparent 

 medium to another. Part of the incident ray enters the medium and 

 changes its direction, or is refracted. 



For example, if in fig. 1 ao is a ra}^ of light passing from air into 

 water, its path will be changed after ])assing the surface at o and it will 

 continue in the direction oh. Conversely, if a ray of light, ho., pass 

 from the denser medium, water, at o, it will take the direction oa. 

 Now, if yox is a perpendicular to the surface, ww^ at o, it will be seen 

 that the angle aoy., called the angle of incidence (/) of the ray, «o, is 

 greater than the angle hox., called the angle of refraction (/■), and what 

 is observed in this case is found to ])e universall}^ true. Again, how- 



