In for a calcite crystal, ${\mu }_0$ and ${\mu }_e$ are the refractive indices of the crystal for O-ray and E-ray respectively, then along the optic axis of the crystal

$$\begin{gathered} Calcite is a negative crystal . \\ Negative crystals are crystals in which refractive index corresponding to E-Ray \left({\mu }_e\right) \\ is less than the refractive index corresponding to O-Ray ( {\mu }_0) in all directions except for Optic axis. \\ So , along the optic axis {\mu }_0={\mu }_e \end{gathered}$$