OPTICAL IDENTIFICATION OF ALKALOIDS. 5 
II. IN PARALLEL POLARIZED LIGHT. 
Parallel polarized light is produced by inserting the crossed nicol 
prisms. 
Extinction {the directions in which the crystal becomes dark). — 
Extinction is said to be parallel when it occurs on bringing a long 
axis or side of a grain into parallelism with one of the cross hairs, and 
inclined when it occurs in some other position. When inclined, the 
angle between the axis or side and the cross hair is measured and 
recorded as the extinction angle. Several measurements are made, 
and the maximum value used. Crystals of the tetragonal, trigonal, 
hexagonal, and rhombic systems show normally parallel extinction, 
as do also monoclinic crystals elongated parallel to axis b. Crystals 
of the cubic system are dark in all positions, and are said to be 
isotropic; they show none of the phenomena hereafter listed. 
Double refraction (birefringence). — This is the difference between 
the highest and lowest indices of refraction. It is described as weak, 
moderate, strong, etc., and is estimated by the order of the polari- 
zation colors shown when the grain is midway between two extinction 
positions. 
Determination of the double refraction by order of polarization colors. — 
The polarization color obtained by placing any doubly-refracting 
crystal grain between crossed nicols is dependent on its thickness 
and the strength of its double refraction. Table 1 gives the colors 
with different thicknesses and double refractions. 
Table 1. — Colors observed with various thicknesses and double refractions. 
Color 
Double 
refraction 
Xthickness. 
Color. 
Double 
refraction 
Xthickness. 
First order: 
Black 
Mm. 
0.00000 
.00005 
.00020 
.00035 
.00045 
.00052 
.00055 
.00057 
.00065 
.00080 
.00090 
.00095 
.00105 
.00110 
Third order: 
Mm. 
0.00115 
Gray 
Light blue 
. 00120 
White 
.00135 
Yellow 
Light yellow 
.00140 
Light orange 
.00145 
Red 
.00155 
Red- violet • 
Light red- violet 
.00160 
Second order: 
Violet 
Fourth order: 
.00170 
Blue 
Pale blue 
.00180 
.00190 
Yellow 
. 00200 
Pale orange 
.00210 
Red 
.00230 
Pale red- violet 2 
.00240 
» Sensitive tint, shown by the selenite plate. 
2 Fifth order colors are extremely faint, and higher orders are white. 
The selenite plate may be used to determine the order of the color, 
provided it is not too high. The plate is inserted, and the crystal is 
observed when parallel and again when perpendicular to its direc- 
tion c. In one direction the color of the grain will be of lower order, 
in the other of higher order, than the selenite, which itself yields red- 
