MICROSCOPIC IDENTIFICATION OF INORGANIC SALTS. AT 
clined illumination may be used to determine whether the substance 
has an index higher or lower than that of the oil. This determined, 
the substance is mounted in another oil of appropriately higher or 
lower index, as the case may be, and the operation repeated. By 
this method of trial and error, an oil is found in which the given 
substance disappears. The refractive index of the substance is then 
the same as that of the oil. <A little practice enables one to estimate 
quite closely the index of a substance observed in an oil of a definite 
but different index, and this estimation very materially reduces the 
number of trials that must be made before the correct oil is found. 
The preceding description applies directly to the measurement of 
the refractive index of an isotropic substance. In the measurement 
of the indices of anisotropic substances, the problem is complicated 
by the fact that the index of the substance varies as the direction of 
the yibration of the light varies. With anisotropic substances, it is 
therefore necessary to ascertain carefully the optical orientation of all 
particles upon which the measurements are carried out. On a given 
substance in the proper orientation one index may be determined 
directly. A second index is in the same plane and consequently may 
be determined by a change in the orientation of the substance repre- 
sented by simple rotation of the stage of the microscope. The third 
index must be measured upon the substance in another plane repre- 
sented by a tilting of the grain. ‘This tilting may be accomplished 
by a universal stage attachment on the microscope or by moving the 
cover glass, as, for example, with the point of a pencil. In the. 
measurement of indices of anisotropic substances, regard to optical 
orientation is more often than not a considerable aid in identification. 
Whether a given crystal is isotropic or anisotropic—that is, whether 
it is isometric or belongs to one of the other crystalline systems—is 
readily determined by crossing the nicol prisms of the microscope. 
Isotropic substances transmit no light under these conditions; whereas 
anisotropic substances, being doubly refractive, do transmit light and 
are readily visible. In making this test care must be taken that an 
anisotropic uniaxial substance is not so oriented that the line of 
vision is parallel to the optic axis of the substance and that no par- 
ticles are observed only at their point of extinction. Tilting the grain 
and revolving the stage of the microscope will eliminate both of these 
possibilities of error. If the substance is known to be biaxial, simple 
revolution of the stage is sufficient. 
Uniaxiality or biaxiality is established by converting the micro- 
scope into a conoscope by crossing the nicols and removing the ocular 
or by inserting the Bertrand lens and observing the interference figure 
of the particle in question. Uniaxial substances show across, if lying 
on a plane perpendicular to the optic axis. As the particle is tilted 
more and more out of this position the center of the cross finally is out 
