The Centred Optic Axis Figure is produced when 1 optic axis is vertical, and correspondingly the second optic axis and the bxa are inclined to vertical. See the block diagram below for the orientation of the indicatrix, the optic axes, Bxa, Bxo and the Optic Normal. The melatope corresponding to the vertical optic axis will be positioned directly beneath the crosshairs. This orientation is produced in a grain which displays the lowest interference colour for that mineral in the thin section being examined. WHY - light which travels along the optic axis sees the same electronic configuration in all directions yielding a single RI, and behaves as if it were travelling through an isotropic mineral.
At extinction the isogyre cross will be visible in the field of view, as shown in the left image above, if the 2V < ~ 30°, otherwise the center of the cross will lie outside the field of view. The center of the cross represents the position of the Bxa in the interference figure.
If the 2V is very small e.g., 1 - 5°, the interference figure looks like an off centred acute bisectrix figure.
Biaxial Optic Axis Figure (2V < 30°) Overhead
With a 2V > ~ 50° only one melatope with it's isogyre is visible within the field of view. The center of the cross representing the position of the Bxa will lie outside the field of view. With the mineral at extinction only a single arm of the cross, parallel to the crosshair, is visible and it narrows at the melatope. The optic plane of the indicatrix will lie along the isogyre and contains the melatope, in the field of view, and the Bxa and the second melatope, both of which lie outside the field of view.
Rotating the stage clockwise causes the cross to break up, outside the field of view, and the isogyre pivots about the melatope in a counter-clockwise direction.
When the optic plane is NS or EW the isogyre is straight, forming one arm of the cross, and the mineral is at extinction.
With the optic plane in the 45° position the isogyre will show its maximum curvature and the position of the acute bisectrix lies on the convex side of the isogyre.
In this position the approximate 2V angle can be estimated using the figure below as a guide. A rule of thumb is that the straighter the isogyre, in the 45° position, the higher the 2V angle.
Biaxial Optic Axis Figure (2V > 50°) Overhead