4 BULLETIN 97, U. S. DEPARTMENT OF AGRICULTURE. 
different velocity in different directions. Amorphous substances and 
substances crystallizing in the isometric system are isotropic. All 
others are anisotropic. Whether a given substance is isotropic or 
anisotropic can be readily determined by crossing the nicols; that is, 
by putting the analyzer in at right angles to the polarizer, and rotating 
the stage. Isotropic substances remain dark during a complete 
rotation, whereas anisotropic substances alternately light up and 
become dark four times during a complete rotation. It should be 
noted here that sections of anisotropic uniaxial substances cut at right 
angles to the principal axis remain dark during rotation; but this 
case can be easily determined by means of interference figures, as will 
be seen later. Should the substance be isotropic, there remains only 
the refractive index to be determined. 
REFRACTIVE INDEX. 
By focusing sharply on an edge of the grain, analyzer out, and then 
raising the tube of the microscope slightly by means of the fine adjust- 
ment, a line of light will be seen to move into or out of the grain. If the 
refractive index of the grain is higher than that of the oil, the line will 
move in; if the index of the oil is the higher, the line will move out. 
In other words, the line of light moves into the medium of higher index 
on raising the tube. The reverse phenomena takes place on lowering 
the tube. Another method is to use an objective of medium power 
with the condenser in and raised. A shadow is thrown across half the 
field by placing the finger between the reflecting mirror and the 
polarizer. The grains with an index higher than that of the oil appear 
dark with a bright band on the side toward the shadow. Should the 
index of the oil be higher, the bright band is on the side from the 
shadow. It being known whether the index of the grain or of the oil 
is the higher, by trial an oil can be found in which the substance com- 
pletely disappears. The indices of the grain and of the oil are then 
the same; and since the index of the oil is known, the index of the 
substance is also known. As a supplementary test, a shadow can be 
again thrown across the field. Then, the indices of the substance and 
of the oil being the same, one side of the grain is colored blue and the 
other red. The isotropic nature of the substance in conjunction with 
the index of refraction is sufficient to identify it. 
ANISOTROPIC SUBSTANCES. 
Should the substance be anisotropic, the extinction angles are first 
measured. This is done by noting the position of the grain when it 
is completely dark and then rotating it until a crystallographic or a 
cleavage face is parallel to one of the cross hairs. The angle of rota- 
tion read off from the stage is the angle of extinction. A more exact 
method is that of the Bertrand ocular. This ocular consists of two 
