c_ sence 7#* -_ 
MICROSCOPIC IDENTIFICATION OF INORGANIC SALTS. 9 
If the refractive indices have been determined with sufficient 
accuracy, the double refraction is simply their arithmetical difference. 
But it is sometimes desirable to determine the value experimentally. 
This can be done with the quartz wedge and an appropriate map of 
interference colors, such as is given in several of the textbooks 
referred to on page 38. For further details the references must be 
consulted. 
Dispersion means simply that the axial angle is greater or less for 
red than for violet light. This may be determined by a measurement 
of the axial angles with lights of these two colors; but usually it is 
quite sufficient simply to observe the position of the colors in the 
interference figures. If both the hyperbole have the same dispersion, 
then a reddish color should be noted on the concave and a bluish 
color on the convex side of the hyperbola if p<» and the reverse if p>». 
The orientation of a crystal is simply a statement of the relation of 
the optic directions and planes to the crystallographic axes and faces, 
and necessarily presupposes familiarity with and the possibility of 
identifying the crystallographic forms. This identification is not 
always possible with rounded or broken grains showing no crystalline 
outlines. In certain cases, however, the position of the crystallo- 
graphic axes can be established by the optical data, and vice versa. 
In compiling the tables an effort has been made to gather together 
all substances for which the optical constants are known with any 
degree of completeness and accuracy. It has been found necessary 
to eliminate a large amount of material for which the data are too 
meager to be of anyreal value. Especially was this so in the tables 
made by Bolland.* In many cases he failed to specify the particular 
hydrate of the salt he described or even note whether it was hydrous 
or anhydrous. It is hoped later on to take up these incomplete data 
in other publications treating of the differentiation of salts of known 
chemical groups. The main mass of data has been taken from Groth’s 
“Chemische Krystallographie,” (2 vols., Leipzig, 1906 and 1908), and 
from the pages of the Zeitschrift fir Krystallographie und Mineralogie. 
Except for a very few common substances, all naturally occurring 
salts have been eliminated; for them reference should be made to the 
admirable tables of Larsen. Except where otherwise noted, all 
optical constants refer to measurements with the D line of the spec- 
trum. 
3 Monatsh. fiir Chemie, 31:387-419 (1910). 
1531—22—Bull. 1108-2 
