178 METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
in general remain dark for either axis of rotation, and only do so for sec- 
tions in the principal zones of the optical ellipsoid. Biaxial minerals show, 
moreover, two directions of apparent isotropism, those of the optic axes 
(optic binomials). To trace out the relations obtaining for orthorhombic, 
monoclinic, and triclinic minerals and their distinguishing features is not 
a difficult matter, but one for which space is not here available. They are, 
in effect, those used for the same purpose with ordinary methods. 
THE ACCURATE DETERMINATION OF THE POSITION OF AN OPTIC AXIS IN THE 
FIELD OF VISION. 
Although the underlying principles of determination by means of the 
universal stage are the same for all sections of a mineral, it has been found 
by experience that, for certain sections, definite courses of procedure for 
measuring the optic axial angles are best adapted to produce the best results. 
Fedorow has divided the possible sections of any biaxial mineral into four 
convenient groups, each of which has its special characteristics and to each 
of which certain methods are best suited. The relative positions of the 
optic axes to and in the field of vision have been made the criteria for 
distinguishing these different groups ; thus, in group ( i ) both optic axes are 
within the field of vision; (2) one optic axis is within the field of vision and 
makes an angle of less than 20 with the normal to the section ; the second 
optic axis can not be brought within the field of vision by any rotation of 
the stage; (3) one optic axis only appears in the field and makes an angle 
of over 20 with the normal to the section ; the second optic axis lies entirely 
outside the field; (4) both optic axes lie outside of the microscopic field, 
the section in question being cut more or less nearly perpendicular to the 
optic normal, or about parallel to the plane of the optic axes, or approxi- 
mately normal to the obtuse bisectrix of a mineral with small optic axial 
angle. 
In case one or both optic binormals of a biaxial mineral section can be 
brought by rotation to coincidence with the axis of the microscope, it is 
necessary to determine these angles of rotation with the greatest possible 
accuracy. In all cases an approximate determination is first effected by 
rotating the section about V"i and // 2 until it is dark and remains dark during 
a complete rotation of the microscope stage H\. In weakly convergent 
polarized light the optic axis can be seen in the center of the field. In 
ordinary microscopes, where absolutely plane parallel polarized light can 
not be obtained, the section in such a position will not be perfectly dark, 
owing to internal conical refraction and to the fact that the incident light 
is not strictly parallel,* but will preserve the same degree of slight uniform 
illumination for all positions of the microscope stage. 
More accurate determinations of the position of an optic axis can then 
be made by means of extinction angles along definite directions, which, 
when plotted in projection, give rise to curves, all of which pass through the 
optic axis. The average point of intersection of a set of such curves is then 
the correct position of the optic axis in projection (Fig. 109). 
SeeB 
345 .'90S 
. Kalkowsky. ZeiUchr. Kryst. (). 486-497. 1884; W. Voigt. Verb, dcutsch. phys. Gcael.. 7, 340- 
;; Ann. d. Phys. (4). 19, 14-21. M, 108-126 1906; Physik. Zeitsch., *, 672-673, 1903. 
