MICEOSCOPIC IDEIs^TIFlCATION OF i:t«rOKGANIC 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 3. 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 hyperbolae 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 il p<v and tJie reverse if p > v. 



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 any real value. Especially was this so in the tables 

 made by BoUand.^ 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 fiir Krystallographie und Mineralogie. 

 Except for a very few common s.ubstances, 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. 



» Monatsh. fur Chemie, 31:387-419 (1910). 

 1531— 22— Bull. 1108 2 



