  It is shown that a transparent birefringent element whose eigenstates are a
pair of elliptical polarizations with principalaxes along the x and y axes,
sandwiched between a pair of orthogonal halfwave plates with principal axes
along directions at 45 deg to the x and y axes, is equivalent to an element
with the same eigenstates and eigenvalues but with the fast and the slow
eigenstates interchanged. The device thus produces the inverse of the original
unitary transformation. A similar result holds for a puredichroic element. With
electrically switchable halfwave plates such a device can be used to switch the
sign of optical activity or to rotate through 90 deg, without any moving parts,
a linear retarder or a linear polarizer.
