  We first derive the round-trip Jones matrix for double passage through a
reciprocal optical medium by means of reflection off a plane mirror that could
be optically anisotropic. We then show that if a medium with only linear
birefringence and linear dichroism is placed between a pair of orthogonal
quarterwave plates with principal axes at 45 deg to that of the medium and the
sandwich is placed in front of an isotropic mirror it behaves, under double
passage, like an isotropic medium. We describe a simple liquid crystal device
that behaves, in reflection, as an isotropic medium whose refractive index can
be varied by application of an electric field, thus acting as a phase only
modulator for light in any polarization state.
