6 BULLETIN 679, L T . S. DEPARTMENT OF AGRICULTURE. 
violet between the first and second orders. Even though the order, 
can not be determined on the crystal alone, the order of the combi- 
nation of crystal and selenite can usually be told in one direction at 
least. As the selenite adds or subtracts just one order to or from 
the color produced by the crystal alone, the order of the crystal can 
thus be calculated. The thickness of the grain is best measured by 
focusing first on the top and then on the bottom, and noting how 
much the fine adjustment head has been turned. 
Sign of elongation x (positive or negative) . — The elongation is posi- 
tive when the greater of any two indices lies in the direction of the 
longest axis or dimension of a given grain, and negative when the 
lesser of two lies in that direction. This may be ascertained by 
observing elongated grains immersed in a medium possessing the 
same mean refractive index, in light passing through but one nicol 
prism, of which the direction of vibration is known. This last prop- 
erty is most readily determined by observing a few needlelike crystals 
of a dark-colored variety of the mineral tourmaline. The darkest 
color is always shown when these lie with their long axis perpendicular 
to the vibration direction of the nicol prism. If the grain under 
observation shows the lowest refractive index when its long dimen- 
sion is perpendicular to the vibration direction of the nicol, and the 
highest when parallel to this, its elongation is positive. In the 
reverse case it is negative. The sign may also be ascertained by the 
use of the selenite plate. Crystals showing as low double refraction 
as possible are turned so that their longer dimensions are parallel to 
direction c of the plate, which is placed so as to make an angle of 45° 
with the vibration planes of the nicol. If the polarization color is 
intensified, so that the crystal appears blue, or other color of a higher 
order than the red- violet due to the selenite plate, the elongation is 
positive; if the crystal appears yellow, white, or gray, that is, of 
lower order color than the red- violet field, the elongation is negative. 
Turning the crystal through 90° so that its long direction is perpen- 
dicular to axis c of the plate gives the opposite result, the color decreas- 
ing in order if the elongation is positive and increasing if it is negative. 
III. IN CONVERGENT POLARIZED LIGHT. 
Convergent polarized light is produced by raising the substage 
condenser as close to the slide as possible and using a high-power 
objective, preferably one of -J- inch or 4 millimeters. After a grain is 
centered, the eyepiece is removed, or a Bertrand lens inserted, if 
the microscope is provided with one, and the stage, or nicol prisms, 
slowly revolved. 
JStrictly, "character of principal zone." 
