Biaxial interference figures are obtained the same way as uniaxial interference figures.

To obtain and observe an interference figure using the microscope.

  1. With high power, focus on a mineral grain free of cracks and inclusions
  2. Flip in the auxiliary condensor and refocus open aperture diaphragm up to its maximum.
  3. Cross the polars
  4. Insert the Bertrand lens or remove the ocular and look down the microscope tube.

Will not see the grain, but the interference figure, which appears on the top surface of the objective lense.

The appearence of the interference figure is dependant on the orientation of the mineral grain and its indicatrix.

We will examine 5 cases:

  1. Centred Acute Bisectrix
  2. Centred Biaxial Optic Axis
  3. Centred Obtuse Bisectrix
  4. Centred Optic Normal or Biaxial Flash Figure
  5. Random Orientations