6 BULLETIN 97, U. S. DEPARTMENT OF AGRICULTURE. 



mineralogy. In its use it is necessary to know the thickness of the 

 grain. To determine this a sharp focus is first made with the fine 

 adjustment of the microscope on the upper surface of the grain and 

 then on the lower surface. The fine adjustment of course must be 

 first calibrated. The value thus obtained is multiplied by the refrac- 

 tive index of the substance, which gives the real thickness. The 

 thickness line is then followed across the table to the color given by 

 the grain between crossed nicols. The nearest oblique line is then 

 followed to the edge of the table, where the birefringence will be 

 found. It should be remembered that this table gives only the 

 maximum birefringence, and consequently it is necessary to try 

 several orientations of the grain in order to be sure that the bire- 

 fringence obtained is the maximum. Other more elaborate methods 

 for determining birefringence will be found in the standard texts. 



OPTICAL CHARACTER OF ELONGATION. 



To determine this, the grain is first rotated to extinction between 

 crossed nicols. It is then rotated 45°, thus bringing the interference 

 color to a maximum. For thin plates or weakly refracting crystals a 

 mica plate, for thick sections or strongly refracting crystals a gypsum 

 plate, or quartz wedge, is inserted between the crossed nicols. The 

 vibration directions of these plates must, of course, be known. The 

 colors of the grain will rise to higher orders when similar axes in the 

 grain and plate have the same direction, and will fall when different 

 axes are superimposed. Knowing the directions of vibration in the 

 plate, the directions of vibration in the grain are also known. When 

 the direction of elongation of the crystal is practically the same with 

 the axis of least ease of vibration, the grain has positive elongation. 

 TYhen the direction of elongation of the crystal is practically the same 

 as the axis of greatest ease of vibration, the grain has negative 

 elongation. 



UNIAXIAL SUBSTANCES. 



All anisotropic substances may be divided, by means of interfer- 

 ence figures, into two great classes, uniaxial and biaxial substances. 

 Interference figures are obtained, with good illumination, by using a 

 high-power objective, putting in and raising the condenser lens, cross- 

 ing the nicols, and removing the ocular. The figure is rendered more 

 distinct by placing over the top of the microscope tube a small black- 

 ened disk with a very small aperture through its center. Thus prac- 

 tically all light except that coming from the grain under examination 

 is excluded from the range of vision. The figure may also be seen 

 enlarged by leaving the ocular in and placing a Bertrand lens between 

 it and the analyzer. In the case of uniaxial crystals lying perpen- 

 dicular to the optic axis, the figure has the form of a dark cross, gen- 

 erally with colored rings around the intersection of the arms of the 



