The Centred Bxa Figure, pictured in the images above, is obtained when the acute bisectrix is oriented perpendicular to microscope stage. The left image is a view of an acute bisectrix figure from the microscope, the right image is a drawing of an acute bisectrix figure with its component parts labelled.
If the 2V angle of the mineral is low, than the melatopes lie within the field of view as the stage is rotated. The isochromes form an oval or figure 8 pattern around the melatopes, while the pattern of the isogyres changes as the stage is rotated.
At extinction, left image above, the isogyres will form a cross with arms parallel with the crosshairs. On rotating the stage to the 45° position, right image above, the cross will split and the isogyres will form two hyperbole which will lie in opposite quadrants of the field of view. The melatopes and/or isogyres will always leave the field of view along the optic axial plane when the stage is rotated and the figure breaks up.
Acute Bisectrix Intererence Figure (Overhead)