OBTUSE BISECTRIX FIGURE

The Obtuse Bisectrix Interference Figure is produced when the obtuse bisectrix (Bxo) of the indicatrix is perpendicular to microscope stage.

The angle between the Bxo and the optic axes is > 45°. The result is that the melatopes will always lie outside the field of view.

The pattern of the isochromes and vibration directions are similar to those of Bxa figure, however the isogyre cross is generally fuzzier than Bxa figure. The optic plane will still parallel the EW or NS crosshair.

On rotating the stage the isogyre cross splits and leaves the field of view in the quadrants into which the optic plane (OP) is being rotated, as with Bxa figure.

The isogyre cross splits and leaves the field of view usually with a rotation of 5° to 15°.

For a Bxo figure the isogyres, when they split, will not be in the field of view.

If 2V = 90°, the Bxa and Bxo are identical, making the mineral optically neutral.

If 2V is small (~5° or less), the Bxo figure resembles an optic normal figure.

Obtuse Bisectrix Interference Figure - Overhead