If the optic axis of the mineral is vertical, the grain will exhibit 0 birefringence and remain black or nearly black upon rotating the stage.

The interference figure produced by such a grain is a centred optic axis figure which consists of a centred black cross superimposed on circular bands of interference colours.

The cross is formed of black bars - isogyres, point where the two isogyres cross is the melatope and marks the point where the optic axis emerges.

Interference colours increase in order outward from the melatope, near melatope colours are low first order Each colour band is called an isochrome.

If the optic axis is vertical the interference figure for the mineral does not move as the stage is rotated.

Isochromes form and are exhibited by the interference figure due to varying retardation of convergent light rays on the sample.