346 F. F. Wright — Measurement of the Optic Axial Angle 



metric mineral is isotropic for all directions of light wave 

 propagation. Uniaxial minerals (hexagonal "and tetragonal) 

 appear isotropic for light waves passing along the principal 

 crystallographic axis. For all other directions, they are aniso- 

 tropic, but even then can generally be distinguished from 

 biaxial minerals at once by the fact that every section of a 

 uniaxial mineral contains the w ellipsoidal axis, parallel with 

 and normal to which it extinguishes. If the section be placed, 

 therefore, in the position of darkness between crossed nicols 

 and be revolved about a horizontal axis, V 1? it will continue to 

 remain dark, if the ellipsoidal axis co coincides with the axis 

 of revolution, while if the ellipsoidal axis co be normal to the 

 latter, the crystal will exhibit interference colors of polariza- 

 tion on revolution except for sections of the prism zone. Biax- 

 ial minerals, on the other hand, do not in general remain dark 

 for either axis revolution, and only do so for sections in the 

 principal zones of the optical ellipsoid. Biaxial minerals 

 show, moreover, two directions of apparent isotropism, those 

 of the optic axes or optic binormals. To trace out the rela- 

 tions obtaining for orthorhombic, monoclinic and triclinic min- 

 erals and their distinguishing features, is not a difficult matter, 

 but one for which space is not here available. They are in 

 effect those which are used for the same purpose with ordinary 

 methods. 



The Accurate Determination of the Position of an Optic 

 Axis when 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, certain courses of pro- 

 cedure 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 (1) 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 cannot be brought within the field of vision by any 

 revolution 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 sec- 

 tion in question being cut more or less nearly perpendicular to 

 the optic normal, or about parallel to the plane of the optic 

 axes ; or about normal to the obtuse bisectrix of a mineral 

 with small optic axial angle. 



