6 BULLETIN 1108, U. S. DEPARTMENT OF AGRICULTURE, 
erty, aside from isotropy, which is to be determined; that is, the 
refractive index. 
Since light, in passing through anisotropic media, is always doubly 
refracted, this group presents complications of refractive index and 
other determinable phenomena, all of which have diagnostic value. 
Uniaxial crystals have two refractive indices, one of the rays 
vibrating parallel to the optic axis and another at right angles to this 
direction. Biaxial crystals have three indices, two lying in one plane 
and the third in another plane of the particle. Not only is each 
index of diagnostic value; but also the differences between the in- 
dices of a given substance, since these differences are a measure of 
the double refraction and therefore of the order of the interference 
colors shown between crossed nicols. Furthermore, the plus or minus 
value of the difference of the indices of rays vibrating in given direc- 
tions determines the optically positive or optically negative character 
of the substance. If e>w in a uniaxial substance, the substance is 
optically positive; if the reverse is true, negative. In biaxial sub- 
stances, if B—a is decidedly greater than y— the substance is op- 
tically negative, and if the reverse is true, positive. 
Biaxial substances, suitably oriented, show the emergence of two 
optic axes, and the angle between these two axes is called the optic 
angle. Uniaxial materials may be considered as the limiting case of 
the biaxial group where the optic angle is zero. 
Frequently red light is dispersed more or less than violet light, 
and this fact is of diagnostic value. 
In any given substance other than amorphous or isometric there is 
always a specific relation between the directions of the optic axes or 
planes and the crystallographic axes, and therefore between the optic 
axes or planes and any given external face or line on the crystal. 
In other words, a given substance lying in a particular crystallo- 
graphic position will always show the same value and orientation 
of its optical properties, whereas a change in the crystallographic 
position would usually change the orientation and might change the 
value of some given property—apparent refractive index, for example. 
The absorption of certain colors of light in one optical direction 
in a crystal to a greater or less extent than in another direction 
gives rise to a change in color of the material depending upon the 
direction in which it is viewed. This is called pleochroism, and is 
characteristic of certain compounds. 
The measurement of the refractive index is based on the principle 
that the contact between two substances of the same color and 
refractive index is invisible and the greater the difference between 
the indices of the two substances the more prominently one stands 
out from the other. If a substance is mounted in an oil of known 
refractive index, either the method of central or the method of in- 
