If both melatopes lie in the field of view, an optic axis figure can be treated as an off centered acute bisectrix figure and the optic sign can be determined using the method outlined previously.
If only a single melatope is visible, when the interference figure is obtained, then it can be considered to be half of a Bxa figure. With a single melatope the convex side of the isogyre will point towards the location of the acute bisectrix, when the trace of the optic plane is oriented in 45° position.
The isogyre will be curved in this position and the degree of curvature is a direct reflection of the amount of the corresponding 2V angle.
If 2V = 90°, the degree of curvature is difficult to determine. The isogyre appears as a straight line parallel to the crosshair when the grain is at extinction. It may be confused with an off-centred uniaxial optic axis figure, however when the stage is rotated the biaxial interference figure isogyre will also rotate while the unixial figure isogyre will move parallel to the crosshairs.
In this case the mineral is optically neutral and the sign is neither positive nor negative, and with 2V for the mineral = 90°.