2 BULLETIN 679, U. S. DEPARTMENT OF AGRICULTURE. 
tals that are formed." 1 When it is considered how greatly crystal 
habit may be modified by the conditions of formation, the presence 
of impurities in the solutions, etc., the desirability of such quantita- 
tive data is apparent. 2 
A number of crystalline alkaloids, when obtained in a pure or 
nearly pure state, can be fairly well recognized by certain micro- 
chemical and color reactions. When mixed, however, and particu- 
larly when small amounts of one acccompany large amounts of another, 
it is difficult to establish the presence and identity of each. As there 
seemed every reason to hope that optical-crystallographic measure- 
ments would furnish a ready and certain means for the recognition 
of these substances, even when mixed in widely divergent propor- 
tions, a series of observations was made by the methods outlined in 
this bulletin. The results were satisfactory. 3 
METHODS FOR DETERMINATION OF OPTICAL CONSTANTS. 
APPARATUS. 
The instrument known as the petrographic microscope, because it 
was developed for use in the science of petrography, the study of 
rocks, is really an apparatus for the accurate measurement of crystal- 
lographic and optical constants of crystals in general. For instance, 
the system to which a crystal belongs, the axial ratios of the sub- 
stance, the refractive indices, double refraction, and optical axial 
relations can be ascertained by its use. The determination of several 
such features on the same crystal will naturally be of enormously 
greater diagnostic value than the mere recording of more or less acci- 
dental shapes assumed by a crystalline substance under ill-defined 
conditions, which is all that is usually done. Moreover, it is not 
necessary that large or well-developed crystals be available. Most 
of the data can be readily measured on crystalline grains 0.025 mil- 
limeter (yJ-q- inch) in diameter, and in favorable cases on even 
smaller ones, down to 0.01 millimeter ( 25 1 00 inch), or, in other 
words, on what would ordinarily be classed as finely powdered 
materials. 
While for the complete study and characterization of a crystal- 
lized substance a somewhat elaborate microscope outfit is necessary, 
comparatively few unusual accessories are needed to apply the 
methods herein described to the identification of alkaloids. The 
microscope must be equipped with nicol prisms which can be adjusted 
so that their vibration planes are exactly horizontal (right and left) 
and vertical (forward and backward) ; and an eyepiece with cross 
i Kraemer, Henry. Applied and Economic Botany (Philadelphia, 1914), p. 768. 
2 Fry, W. H. Identification of Commercial Fertilizer Materials, 1914, U. S. Dept. Agri. Bui. 97. Wright, 
F. E. The Petrographic Microscope in Analysis, Jour. Amer. Chem. Soc, 1916, 38: 1647-1658. Chamot, 
E. M. Chemical Microscopy, Jour. Ind. Eng. Chem., 1918, 10: 60-66. 
> To be published in the Jour. Amer. Chem. Soc. 
