490 FRED. EUGENE WRIGHT 
respect to some definite crystal direction in the plate (extinction 
angle) are given. In actual determinative work with thin sections, 
the lack of a satisfactory method for measuring refractive indices 
directly under the microscope is a serious difficulty and the observer 
is compelled to make use of other properties, as birefringence and 
the relative axial lengths or axiality of the optic ellipse, which 
can be determined under these conditions but which, as noted 
above, are less important than the refractive indices and do not 
express, even together, as much as the refractive indices do 
alone. 
For the expression axiality of the optic ellipse the collective 
term optic ellipsity may be used to express the relative lengths of 
the axes of the optic ellipse of the section. This term is preferable 
to the usual term optical character of the section or optical character 
of its elongation. The term optical character serves primarily as a 
group expression for the terms optically positive and optically 
negative. Its further use, in the above sense as a term implying 
the determination, in plane-polarized light, of the relative lengths 
of the axes of the optic ellipse of any given section is not justifiable, 
because, in that case, the same term serves two masters and conveys 
to the mind two totally different impressions; and such usage is 
not conducive to precise statement. The term optic ellipsity or 
axtality of the optic ellipse may well be substituted for optical 
character in its second usage. 
In practical mineral determination under the microscope the 
observer may ascertain, in monochromatic light: (a) the optic 
ellipsoidity of a mineral, whether bzaxial, uniaxial, or tsoaxial; 
(b) the absolute lengths of the principal axes of its optic ellipsoid 
(=a, 8, y, the principal refractive indices); (c) the difference in 
absolute lengths of any two of the principal axes of its optic ellip- 
soid (= principal birefringences); (d) the angle between the nor- 
mals to the two circular sections of the optic ellipsoid (=optic 
axial angle); (e) the principal axis which bisects the acute angle 
between the normals to the two circular sections of the optic 
ellipsoid (the acute bisectrix Y or a and with it the optical character 
of the mineral whether positive or negative); (f) the relative position 
of the optic ellipsoid within the crystal (usually ascertained by 
