4 BULLETIN 97, U. S. DEPARTMENT OP 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 01 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 Bcrtrand ocular. This ocular consists of two 



