8 BULLETIN 679, TJ. S. DEPARTMENT OF AGRICULTURE. 
MEASURING THE OPTICAL PROPERTIES OF AN ALKALOID. 
Before measuring the optical properties of any substance, its 
purity must be established, for admixtures may modify some or all 
of its constants to a marked degree. The medium from which it is 
crystallized must also be taken into consideration, as some sub- 
stances take up the liquids, in the form of " solvent-of-crystalliza- 
tion," and the crystallographic and optical characters may differ 
widely from one solvent to another. For instance, the alkaloid 
quinidine is recorded in the literature as crystallizing from acetone 
in the tetragonal system, being optically uniaxial, from methyl and 
other alcohols in the rhombic system, with axial ratios near 0.8:1:0.7 
and with an optic axial angle (2 E) about 75°, and from benzene, 
also rhombic, but with the axial ratios nearer 0.7:1:1.1 and an optic 
axial angle of 85°. These differences are such that the crystals from 
these several solvents would be identified crystallographically or 
optically as belonging to different chemical substances by an observer 
not previously informed as to their character. Anyone desiring to 
gain familiarity with the manipulation of the microscope, and with 
the sort of effects upon which this method of identification of sub- 
stances is based, may follow the directions outlined for the typical 
case described in the next paragraph. 
Dissolve a 5-gram sample of cinchonine alkaloid in 100 cc of 
boiling 95 per cent alcohol. Filter the solution and allow it to cool, 
when the alkaloid is deposited on the walls of the vessel in small, 
brilliant crystals. Take out a few crystals, dry, crush, and mount 
them on a slide in potassium-mercuric iodid solution with a refrac- 
tive index of about 1.61. Examined first with a low-power objective, 
in ordinary light, cut down by the substage diaphragm, the material 
shows plates and rods, many of them with hexagonal outlines. When 
the lower nicol is inserted, with its plane of vibration horizontal 
(right and left), all of the grains with their long dimensions horizontal 
stand out boldly, and light appears to pass into them as the tube is 
raised, indicating their refractive index lengthwise to be higher than 
that of the liquid. Many grains with their long dimensions vertical 
show faint orange and blue borders, indicating that the refractive 
index crosswise approximates that of the liquid, although, as this 
index is the mean between the two lower ones of the substance, it 
varies with the position in which each grain happens to lie. When 
both nicols are inserted and crossed, the diaphragm being now opened 
to obtain as bright an illumination as possible, the majority of the 
grains are white, or have hues of the fourth order, because of the 
extremely strong double refraction, with several marginal color bands, 
due to the wedge-shaped edges. A few grains show brilliant hues of 
lcwer orders, because of extreme thinness or because they are so 
