In order to determine whether the idex of refraction of a mineral is greater than or less than the mounting material the Becke Line Method is used


BECKE LINE - a band or rim of light visible along the grain boundary in plane light when the grain mount is slightly out of focus.

Becke line may lie inside or outside the mineral grain depending on how the microscope is focused.

To observe the Becke line:

  1. use medium or high power,
  2. close aperture diagram,
  3. for high power flip auxiliary condenser into place.

Increasing the focus by lowering the stage, i.e. increase the distance between the sample and the objective, the Becke line appears to move into the material with the higher index of refraction.

The Becke lines observed are interpreted to be produced as a result of the lens effect and/or internal reflection effect.