The range bisecting the angle involving the optic axes of a biaxial crystal
- The line bisecting the perspective between your optic axes of a biaxial crystal.
(letter.) The range bisecting the direction between your optic axes of a biaxial crystal.
The acute bisectrix of the optic axes never deviates from the normal to the basal plane by more than a degree or two, hence a cleavage flake of mica will always show an optic figure in convergent light when placed on the stage of a polarizing microscope.