IDENTIFICATION" OF FERTILIZER MATERIALS. 5 



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 



