4 BULLETIN 1108, U. S. DEPARTMENT OF AGRICULTURE. 
All of the optical properties named may usually be observed and 
measured where measurement is possible in about 15 minutes, with 
no more material than can be held upon the point of a knife blade, 
assuming of course that one has ready all the essential equipment for 
the work and is not under the time-consuming necessity of making a 
microscopical search for the material to be identified in a mass of other 
material. In certain cases, however, the time of search can be mate- 
rially reduced or eveneliminated. For instance, if one has a mixture 
of NaCl and KCl, the presence of either one but not both being 
known, the mass can be mounted in an oil of the same refractive index 
as that of the known substance, when the latter will be invisible or 
practically so and the unknown will stand out with a considerable 
degree of prominence, thus permitting ready “‘spotting”’ and a sub- 
sequent working down or up to its index with different oils. The 
value of this point in the rapid detection of impurities in supposedly 
pure substances is obvious. 
EXPLANATION OF SYMBOLS. 
For convenience in identification the substances have been tabu- 
lated in five groups as follows: Isotropic, uniaxial positive, uniaxial 
negative, biaxial positive, and biaxial negative substances. In the 
isotropic group the refractive index is the only property to be ob- 
served, and the arrangement of the substances is according to increas- 
ing values of the index. In the uniaxial positive and uniaxial nega- 
tive groups there are two refractive indices to be measured, designated 
w and e, denoting respectively the ordinary and extraordinary rays of 
light as observed in polarized light. The difference between these 
two indices constitutes the birefringence of the substance. Other 
properties, such as pleochroism, are occasionally observable, and the 
crystal system should always be noted where possible. The arrange- 
ment of the substances is according to ascending values of the w 
index. 
In the biaxial positive and biaxial negative groups there are three 
refractive indices to be measured—that is, the a, 8, and y indices— 
indicating respectively the least, the intermediate, and the greatest 
index of the substances. The differences between any two of these 
are significant and are tabulated. The true optical angle is indicated 
by 2V. The symbols 2K and 2H indicate the value of this axial 
angle when measured in air and oil, respectively. Dispersion is tabu- 
lated as greater or less for red, p, than for violet, v, light. In the 
columns headed “‘orientation,’’ X, Y, and Z signify the directions of 
vibration of the a, 8, and y rays. Ax.pl. is an abbreviation for plane 
of the optic axes of the crystal. All the other symbols in this column 
are either geometrical or simple crystallographic designations of crys- 
