492 FRED, EUGENE WRIGHT 
doubtedly be included in the second essentially quantitative 
group. In the following paragraphs a concise description of the 
essentials of a few of the best available methods for the determin- 
ation of these properties will be given. 
DETERMINATION OF THE OPTICAL PROPERTIES OF THE FIRST CLASS 
For this group few new methods of determination have been 
developed in recent years; color, pleochroism, absorption, crystal 
habit, dispersion of the optic axes and of the bisectrices being ascer- 
tained by practically the same methods which have been in use 
since the introduction of the petrographic microscope. 
The optic ellipsoidity—In the determination of this feature, 
two methods, in particular, have proved useful in recent years: 
(a) Uniaxial minerals are readily distinguished from biaxial miner- 
als, by noting that the achromatic brushes (zero isogyres) in inter- 
ference figures from uniaxial plates are parallel with the principal 
nicol planes and pass through the center of the field on rotating 
the stage, while in biaxial minerals the dark bars (zero isogyres) of 
the interference figure rotate on rotation of the stage and may 
include any angle with the principal plane of the polarizer. If 
the dark axial bar in an interference figure does not remain straight 
and in the same azimuth on rotating the stage, the optic ellip- 
soidity is biaxial; otherwise the birefracting mineral is in general 
uniaxial. (0) To ascertain whether a very weakly birefracting 
plate is isotropic or birefracting and at the same time to determine 
the relative value of the axes of its optic ellipse (its optic ellipsity), 
the sensitive tint plate should be inserted, not in the diagonal 
position as is usually the case, but in such a position that the 
axes of its optic ellipse include only a small angle with one of the 
principal nicol planes; under these conditions the field illumina- 
tion due to the sensitive tint plate itself is very slight while its 
path difference is still effective. The changes in the faint color 
hues from the mineral grain are clearly visible against the darker 
background and extremely minute traces of birefringence can thus 
«See F. Becke, Denkschr. Wiener Akad. Wissen. Math.-Natur. Kl., LXXV, 1904; 
Tscherm. Min. Pet. Mitteil., XXIV, 30, 1905; XXVII, 177-78, 1908. 
