Note 75 — Chapter III · Investigation of Microscopic and Optical Characters of Minerals.

The interference figure, perpendicular to the obtuse bisectrix, would be of the same type with a larger axial angle. Ordinarily this figure would not come within the limits of the field of view of the microscope. Confusion may arise, however, but in a section perpendicular to the acute bisectrix the cross dissolves more slowly into the hyperbolas than in the case of a section perpendicular to the obtuse bisectrix. At times it may be necessary to measure the axial angle to be sure. When, however, the mineral is known, the section perpendicular to the acute bisectrix can be recognized, because if the mineral is optically positive the trace of the axial plane is parallel to a and if negative parallel to c.