  The microscopic mechanism beyond the optical anisotropy of an ionic crystal
which occurs for short wavelengths is investigated. The electron-hole, two
particle propagator and its analytical behaviour close to the band edge of the
one particle continuum plays a major role for the mechanism of this optical
anisotropy. Especially for an ionic crystal the two particle bound state, the
exciton, is of special importance. In this way we argue that the so called
``intrinsic birefringence'' in CaF$_2$ is neither intrinsic to the material nor
it is birefringence. Instead it is spatial dispersion caused by the vicinity of
a dispersive optical absorption given by the excitonic bound state. We propose
a model which connects the bound state dispersion with the band structure and a
model potential for a screened coulomb interaction. Based on these
considerations we predict a wavelength dependence of the dielectric function
approaching close to the bound state level $\epsilon \sim (\lambda -
\lambda_0)^{-1}$, where $\lambda_0$ is the wavelength of the excitonic bound
state level.
