Isogyres form when the vibration directions in the interference figure parallel the vibration directions of the polars. These are areas of extinction.
In the uniaxial indicatrix the ordinary rays vibrate perpendicular to the optic axis and are analogous to lines of latitude on the surface of the indicatrix. They vibrate as tangents to the circular isochromes.
Extraordinary rays vibrate parallel to the optic axis and are analogous to lines of longitude on the indicatrix surface and vibrate along radial lines from melatope outwards.
Once the interference figure has been obtained and identified as to whether it is uniaxial or biaxial, the optic sign of the mineral can be determined using an accessory plate, either gypsum, quartz or mica.