1. Optic axis Figure

   The thin section is perpendicular to the c axis = optic axis.
   The mineral appears isotropic, or nearly isotropic under crossed polars, exhibiting a very low first order grey to black interference colour.
   

2. Off centred Optic Axis Figure

   The c axis (optic axis) is not vertical, but inclined from the vertical axis of the microscope.
   Will only see isogyre in the field of view at a time, which will sweep out of the field of view parallel to one crosshairs to be replaced by a new isogyre which sweeps into the field of view parallel to the other crosshair.
   This orientation will exhibit an intermediate colour, between the lowest and highest colour exhibited by this mineral in the thin section being examined.
   

3. Flash Figure

   The c axis is parallel to stage.
   The isogyres split and leave field of view rapidly with only a slight rotation, <10°.
   The maximum interference colour will be observed under crossed polars.
   

Sign determination

• + ve n_omega<n_epsilon
  slow ray = epsilon, fast ray = omega
• - ve n_omega>n_episilon
  slow ray = omega, fast ray = epsilon.

Interpretation of Uniaxial Interference Figures