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. 
When 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 nlumination, 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 
