OPTICAL IDENTIFICATION" OF ALKALOIDS. 3 
hairs in corresponding positions must then be used. It is a great 
convenience if one of these prisms can be rapidly inserted and with- 
drawn without changing the position of its vibration plane. A revolv- 
ing stage, an Abbe condenser, and a substage iris diaphragm are 
necessary. One 4-millimeter (J-inch) and one 8-millimeter Cl- 
inch) or 16-millimeter (§-inch) objective are adequate for every pur- 
pose. A slot should be cut in the microscope tube between the top 
of the objective and the bottom of the upper nicol prism for the 
insertion of a selenite plate yielding the " sensitive tint," red- violet of 
the first order. This plate, which may be purchased from any dealer 
in petrographic microscopes, is inserted so that its direction c lies at 
45° from the vibration planes of the nicols. Finally, liquids of known 
refractive index are needed, directions for preparing which are given 
on page 4. 
OBSERVATIONS. 
Samples are prepared by gently crushing the dry substance. A 
minute amount is immersed in a drop of a liquid in which it is insolu- 
ble, on a glass slide, and protected by a small cover glass. The obser- 
vations are made as follows : 
I. IN ORDINARY LIGHT. 
A 16-millimeter (f-inch) objective is usually most satisfactory for 
the preliminary examination of a substance. 
Color or hue. — If the substance is pleochroic, showing different 
colors in different crystallographic directions, the several colors and 
their directions are recorded. 
Crystal habit. — This observation includes the outline of crystals, etc. 
Angles. — Interfacial angles are measured by revolving the stage of 
the microscope until the trace of each face in turn lies parallel to one 
of the cross hairs, and reading the angular position of each. This 
process is repeated several times, and the results averaged. 
Cleavage — Cleavage refers to the directions in which splitting 
along definite planes occurs. 
Refractive indices. — These are inversely proportional to the rates 
at which light is transmitted in different crystallographic directions. 
They are determined by immersion in liquids of known indices, 
trying one after another until the boundaries in each direction 
successively disappear. The following special instructions are im- 
portant: In crystals of the rhombic, monoclinic, and triclinic sys- 
tems there are three indices, the smallest being called a, the inter- 
mediate |8, and the largest 7. The directions in which these values 
hold are respectively X, Y, and Z. In substances of the tetragona . 
trigonal, and hexagonal systems there are two indices, one called co 
and the other e. Tn the cubic system there is but one index, called n. 
Determination of refractive indices by the immersion method. — A 
series of liquids of known indices should first be prepared, the com- 
