402 WHERRY AND ADAMS: PINAVERDOL 



that is, very close to rhombic in character, so marked deviation 

 of the optical directions from axes a and c is not to be expected. 

 For practical purposes, therefore, the optical phenomena may be 

 considered in the general direction of the three crystallographic 

 axes. 



The faces of the clinopinacoid h (loo) reflect light of brilliant 

 brass-yellow color. Those of the orthopinacoid a (loo) have not 

 been observed broad enough to determine the color accurately 

 but it does not appear to be materially different from that of h; 

 on theoretical grounds, of course, it must be at least slightly 

 different. The prism faces m (no), which lie about midway 

 between these two pinacoids, are correspondingly also brass- 

 yellow. 



Faces lying perpendicular to axis c, or nearly so, show a deep 

 bronze-violet reflection color. Intermediate colors are of course 

 shown by all forms lying between the base and the prism, namely, 

 by the domes and pyramids, and the actual color is a brilliant 

 metallic green, which may perhaps best be described as beetle- 

 green. The nearer one of these forms lies to the base, the darker 

 the green, and the nearer the prism, the more yellow appears 

 in it. In fact, the colors of a form may be correlated directly 

 with the value of its coordinate angle p, which represents the 

 angle between the zone axis of the prism and the perpendicular 

 to the face in question. For forms of sufficient size to permit the 

 certain recognition of their color, the relations are as follows : 



TABLE 2 

 Colors of Crystals 



