A mineral grain is oriented with it's optic axis horizontal. This orientation exhibits the maximum birefringence, for this mineral in the thin section, and produces a flash figure.
The flash figure results because the vibration directions, of the indicatrix, within the field of view are nearly parallel to polarisation directions of the microscope.
With the grain at extinction the optic axis is oriented either EW or NS in the resulting interference figure. The interference figure produced occupies most if not all of the field of view and consists of a very broad, fuzzy isogyres cross.
Upon rotating the stage, < 5° rotation, the isogyres will split and move out of the field of view in opposite quadrants.
The quadrants into which the isogyres move correspond to the quadrants into which the optic axis is moving, as the stage is being rotated.
With the optic axis in the 45° position, no isogyres will be present, and the field of view may exhibit some interference colours. Isochromes, if present, will be concave outward.
The colour in the centre of the field of view is the normal interference colour for that mineral under crossed polars.
In quadrants which contain the optic axis, the interference colours decrease away from the centre.
In remaining two quadrants the interference colours increase away from centre.
The number of isochromes observed is dependant on the thickness of the thin section and the birefringence of the specific mineral.
If the central portion of the figure in the 45° position is white, the optic axis quadrants will be first order grey, other quadrants will be pale first order yellow.
Optic sign can be determined using flash figure, but it is not definitive.
Biaxial minerals will also produce a flash figure. It is better to look for a centred or off centred figure, either uniaxial or biaxial to determine the optic sign of the unknown mineral.