**UNIAXIAL OPTIC SIGN**

In Calcite n_{omega} > n_{epsilon}, 1.658 versus 1.485.
In other minerals, e.g. quartz, n_{omega} < n_{epsilon} , 1.544 versus 1.553.

This difference in this refractive index relationship provides the basis for defining the optic sign of uniaxial minerals.

- Optically positive uniaxial minerals n
_{omega} < n_{epsilon}
- Optically negative uniaxial minerals n
_{omega} > n_{epsilon}

Alternatively,

- if extrordinary ray is the slow ray, then the mineral is optically positive.
- if extraordinary ray is the fast ray, then the mineral is optically negative.

n_{epsilon} refers to the maximum or minimum index of refraction for the extraordinary ray, the value recorded in the mineral descriptions in the text.

n_{epsilon}' refers to an index of refraction for the extraordinary ray which is between n_{omega} and n_{epsilon}.

For uniaxial minerals any orientation will provide n_{omega}, but only one orientation, cut parallel to the c-axis will yield n_{epsilon} maximum. This orientation is the one which exhibits the highest interference colour as delta (birefringence), is greatest, and therefore retardation (DELTA) is greatest.

(DELTA = d(n_{s}-n_{f}))