OPTICAL IDENTIFICATION OF ALKALOIDS. 7 
Glass 1 may be uniaxial, possessing but one optic axis, or biaxial, 
possessing two. If biaxial, the angle between the two may be 
measured, for which purpose an apertometer or a micrometer eye- 
piece is needed. For the determinative work on alkaloids, however, 
it is necessary only to class the angle as small or large. 
A uniaxial interference figure consists of a dark cross surrounded 
by rings which are colored if white light is used, or alternately dark 
and light in monochromatic light. It may appear in the center of 
the field if the crystal is lying on face c, the base, or it may lie more 
or less to the side. Even when the cross is not visible the dark arms 
will, on revolving the stage, move across the field parallel to each 
cross hair in turn, remaining perfectly straight. 
A biaxial interference figure consists, in the most favorable case, of 
two dark spots, where the two optic axes emerge, surrounded by 
concentric oval or lemniscate curves, colored as in the uniaxial inter- 
ference figure, and traversed by two dark bars which form a cross 
when the vibration directions of the crystal are parallel to those of 
the nicol prisms, and open out into hyperbolas when the stage is 
rotated 45°. The apparent angle, 2 E, is proportional to the dis- 
tance separating these dark spots; the acute bisectrix lies midway 
between them. Two or three sheets of mica should be used for 
forming an idea of the appearance of the interference figures with 
different values of 2 E. Muscovite mica has 2 E from 60 to 70°; 
while phlogopite usually has 2 E much smaller, about 15°. In some 
positions only portions of the figure may be visible, but, in general, 
on revolving the stage the dark bars move across the field, neither 
parallel to the cross hairs nor remaining straight. 
Optical sign 2 (positive or negative). — Uniaxial crystals are positive 
if co is less than e; negative if co is greater than e. Biaxial crystals 
are positive if Z is the acute bisectrix and negative if X occupies 
that position. This is determined in the same way as is the sign of 
elongation (page 6). That is, the selenite plate is inserted, and the 
change of color of the interference figure observed; when the figure 
shows, on opposite sides of the central cross, areas of blue, marking 
an increase in the order of the color in a direction parallel to c of the 
plate, the sign is positive; when the color is yellow in that direction, 
indicating a decrease, the sign is negative. 
Dispersion. — Dispersion is the relative refraction of the red end of 
the spectrum as compared with the violet end, abbreviated to r and 
v, respectively. This can be observed in various ways, but, as it has 
in general little or no significance in the identification of alkaloids, 
it will not be further discussed here. 
1 Strictly "ellipsoidity." 
J Strictly "character." 
