On rotating the stage, the shape and appearance of the isogyres change because the vibration directions for the light are also being rotated.
Rotation of the Isogyres, Bxa Figure, 2V < 40°.
When the grain is at extinction, and the acute bisectrix is vertical the interference figure will be centred in the field of view and will look like the left image above. In this orientation the optic plane (OAP) will be oriented either NS or EW, forming the thin arm of the isogyre cross. The arm of the cross which contains the optic plane also contains the melatopes (M). Near the melatopes the isogyres are at their narrowest.
The second arm of the isogyre cross, containing the optic normal (ON), is wider and fuzzier than the arm containing the optic plane.
Rotating the stage (center and right images above) causes the cross to split into two separate isogyres, with each isogyre pivoting about one melatope.
With the optic axis in the 45° position (right image above), the isogyres form two curved hyperbola with the melatopes acting as the vertices.
The isogyres are narrowest at the melatopes and thicken in both directions away from the melatope.
The isochrome pattern remains fixed relative to the position of the melatopes as the stage is rotated. The position and shape of the isochromes can be used to infer where the melatopes lie.
For minerals with a 2V angle of < 40°, the melatopes and isogyres will remain in the field of view as the stage is rotated.
For minerals with a 2V > 40° the position of both melatopes will lie outside the field of view when the interference figure is examined. The result is that at extinction the isogyre cross is visible, however on rotation the isogyres leave the field of view and are not visible in the 45° position, as the 2V angle is too large (> 40°). In this case the shape of the isochromes can be used to infer the position of the melatopes, the optic plane and the optic normal.
Rotation of the Isogyres, Bxa Figure, 2V > 40°
The 2V angle for a biaxial interference figure can be estimated by comparison of the image viewed under the microscope with Figure 7.32 (p. 103 in Nesse) or with the image below.