Dcterminatio)i of tJic Feldspars. — Winchell. 35 
present the orientation in which it gives the figure of the axis 
sought. The operation is facihtated by the previous knowl- 
edge of its cleavages, and of the probable nature of the species 
in hand. 
The axis n^ is preferable in the examination of the acid 
plagioclases, and «g in that of the basic. M. Fouque ascer- 
tained by the examination of numerous inclined sections that, 
in case of slight obliquity, the decentring of the image and 
the consequent error in the result, is less in sections perpen- 
dicular to «p than in those perpendicular to //j, . He also 
ascertained that the error is greater (except in the basic feld- 
spars) when the inclination is in the direction of the trace of 
the plane of the optic axes than in a direction perpendicular 
to it. An inclination of 5° removes the figure one-third of 
the radius of the field of the microscope away from the central 
position. An inclination of 10° removes it two-thirds of the 
same radius. 
If a bisectrix is exposed favorably in the field of the micro- 
scope, as frequently happens in a rock section cut at random, 
it is important to know whether it is n-^ or ;/g . Resort may 
be had to the quartz of sensitive tint, which with the feldspars 
in sections not over 0.03mm in thickness, is the most ready 
and reliable test. It is also possible to know whether the axis 
so examined is in the acute or the obtuse angle. After some 
experience the observer becomes able to judge by the appear- 
ance of the i'nterference figure, and its changes on rotation of 
the stage, in nearly all cases, whether the axial angle is acute 
or obtuse. Thus, in the first place, the sections perpendicular 
to Wp are more dark in parallel polarized light than those .per- 
pendicular to «g . This observation is frequently sufficient, 
at least if the angle 2V does not exceed 80°. The delicacy 
of the observation is increased by interposing a quartz plate 
which gives the rose color of the first order of the color scale. 
Again it is frequently possible to judge whether the axial angle 
under examination is acute by noting the comparative amount 
of rotation of the stage necessary to produce a marked separa- 
tion of the hyperbolas. The promptly separating hyperbolas 
belong to the obtuse angle. When the hyperbolas are tardy 
in moving from the black cross the angle is acute. In case 
such rough observation be not sufficient, the hyperbolas may 
