488 FRED. EUGENE WRIGHT 
reference to a triaxial ellipsoid, the principal axes of which are 
equal to the three principal refractive indices. Having given the 
lengths and positions of the three principal axes of this optic 
ellipsoid within the crystal, it is possible to predict definitely the 
optical behavior of any section cut from the crystal. The validity 
of the optic ellipsoid has been proved so frequently and its use- 
fulness in practical work is so great that its importance, independent 
of all theory, in optical work cannot be too strongly emphasized. 
That this has not been adequately done in the past, is evident 
from the current terms used to designate the optical properties of 
a mineral and of a mineral section. Thus we determine whether 
a mineral is isotropic, uniaxial, or biaxial and classify it accord- 
ingly, but there is no collective term which states that by this 
determination we actually ascertain the particular type of optic 
ellipsoid by which the optical behavior can be expressed; whether 
by a triaxial ellipsoid in which the three principal axes are 
unequal, or by an ellipsoid of revolution in which one axis is 
unique and different in length from the other two equal axes, or by 
a sphere in which all three axes are of the same length. For this 
characteristic the term optic ellipsoidity’ is here suggested as a 
suitable group expression; thus the optic ellipsoidity of a biaxial 
mineral may be considered biaxial (two axial ratios being required 
to define the shape of its ellipsoid); the optic ellipsoidity of a uniaxial 
mineral, uniaxial (one axial ratio being sufficient to define the 
shape of its optic ellipsoid); and the optic ellipsoidity of an isotropic 
mineral, zsoaxial (the three axes of its optic ellipsoid being equal). 
The optic ellipsoidity of a mineral is one of its most important 
diagnostic features; 1t is employed as a primary group-characteristic 
in nearly all the determinative tables for use with the petrographic 
microscope which have been published. 
The lengths of the principal axes of the optic ellipsoid are 
ascertained by measuring the principal refractive indices of the 
crystal. From these in turn the principal birefringences, the 
optic axial angle, and the optical character of the mineral can be 
derived; these last properties are, therefore, subordinate, in a 
1 The writer is indebted to Mr. C. E. van Orstrand of the U.S. Geological Survey 
for aid in devising both this term and the expression oPfic ellipsity noted below. 
