IDENTIFICATION OF FERTILIZER MATERIALS. 
right and two left handed quartzes of the same thickness cut perpen- 
dicular to the axis. The lines of contact between the four parts are 
parallel to the principal sections of the nicols. This ocular is inserted 
in the tube of the microscope in place of the ordinary ocular and a cap 
nicol placed over it in such a position as to cross the polarizer. The 
analyzer is out. When a double refracting substance is placed under 
the microscope with this arrangement, the adjacent quadrants appear 
dissimilarly colored and the diagonal quadrants similarly colored. 
Upon rotating the stage a position is reached at which the four quad- 
rants are uniformly colored. Then the principal sections of the grain 
are parallel to those of the nicols. The angles can be read off as 
above. In tetragonal, hexagonal, and orthorhombic substances the 
extinction is always parallel to a crystallographic direction, and con- 
sequently the angle of extinction is zero. In monoclinic substances 
the extinction is parallel or symmetrical only in the zone parallel 
to the b-axis. The extinction is oblique in all other sections. In 
triclinic substances the extinction directions are all inclined. 
PLEOCHROISM. 
Some anisotropic substances possess the property of changing 
their color or the intensity of their color when rotated under the 
microscope with the analyzer out. This phenomenon is due to differ- 
ent degrees of absorption in different directions in the crystal. For 
example, in one direction one color constituent of white light may be 
absorbed more than another color. The degree of absorption in one 
direction is expressed as greater, less, or equal to the degree of absorp- 
tion in another direction; and the absorption axes are assumed to 
coincide with the axes of vibration of the light. 
REFRACTIVE INDICES. 
Anisotropic substances belonging to the tetragonal and hexagonal 
systems have two indices of refraction; and those belonging to the 
orthorhombic, monoclinic, and triclinic systems have three. These 
indices may be determined substantially as given for the indices of 
isotropic substances. One index may be observed in one position of 
the crystal. The stage is then rotated and another index determined 
Except in cases of strongly double refracting substances, the deter- 
mination of the mean index is sufficient for practical purposes. 
BIREFRINGENCE. 
When the refractive indices are known accurately, the maximum 
birefringence is obtained directly from them by subtracting the 
lowest index from the highest. Since, however, it is not always 
practicable to determine the indices with the required degree of 
accuracy, other methods are used. The table of Michel-Levy is here 
very useful. A copy of it will be found in most textbooks of optical 
