The obtuse bisectrix figure can be interpreted in the same way as the acute bisectrix, except that the trace of the optic plane has an index of nBxa rather than nBxo.
In the obtuse bisectrix figure if this ray, parallel to optic plane, is the fast ray than the acute bisectrix is the X axis.
In the obtuse bisectrix figure if this ray, parallel to optic plane, is the slow ray, then Bxa = Z axis.
The optic plane will lie in those quadrants of the field of view into which the isogyres move as the stage is rotated. These quadrants will also be those from which the isogyres leave the field of view.
If the 2V is large, e.g. 80 to 90°, it is difficult to distinguish the Bxa figure from the Bxo figure. The sign determination is impractical and a new grain, which produces an optic axis figure, i.e. one with the lowest interference colour, should be selected.