6 BULLETIN 679^ TJ. 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 ^ (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 tourmahne. 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 yeUow, 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. 



in. IN CONVERGENT POLARIZED LIGHT. 



Convergent polarized fight is produced by raising the substage 

 condenser as close to the sfide as possible and using a high-power 

 objective, preferably one of ^ 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. 



'Strictly, "character of principal zone." 



