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== Characteristic figures of uniaxial and biaxial minerals ==
[[File:Uniaxial interference figures.jpg|thumb|center|upright=2.5|Sketches of uniaxial interference figures, viewed along the optic axis of each mineral. The colours approximate [[birefringence]] colours which might be seen if this were a mineral with second order maximum birefringence. The dark "maltese cross" pattern is characteristic of uniaxial minerals. Also shown are schematics of the shape of a cross section through the mineral's optical [[index ellipsoid|indicatrix]] (recording its refractive index in 3D) that would be seen at each position. The elongated direction could be distinguished by adding a [[sensitive tint plate]] to the microscope, letting the user discriminate between "uniaxial positive" (left) and "uniaxial negative" (right) minerals.]]
An interference figure produced looking straight down or close to the optic axis of a uniaxial mineral will show a characteristic [[Maltese cross|"Maltese" cross]] shape to its isogyres. If you are looking perfectly down the optic axis, the pattern will remain completely unchanging as the stage is rotated. However, if the viewing angle is slightly away from the optic axis, the centre of the cross will revolve/orbit around the central point as the stage is rotated. The form of the cross will stay constant as it moves.
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