  The transmission Jones matrix of an arbitrary stack of reciprocal plane
parallel plates which has been turned through 180 degrees about an axis in the
plane of the stack is, in an appropriate basis, the transpose of the
transmission matrix of the unturned slab with a change in the sign of the
off-diagonal elements. We prove this convention-free result for the case where
reflection at the interfaces can be ignored and use it to devise an
experimental scheme to separate isotropic and topological phase changes in a
reciprocal optical medium.
