  The standard formula for the rotation measure, RM, which determines the
position angle, $\psi={\rm RM}\lambda^2$, due to Faraday rotation, includes
contributions only from the portions of the ray path where the natural modes of
the plasma are circularly polarized. In small regions of the ray path where the
projection of the magnetic field on the ray path reverses sign (called QT
regions) the modes are nearly linearly polarized. The neglect of QT regions in
estimating RM is not well justified at frequencies below a transition frequency
where mode coupling changes from strong to weak. By integrating the
polarization transfer equation across a QT region in the latter limit, I
estimate the additional contribution $\Delta\psi$ needed to correct this
omission. In contrast with a result proposed by \cite{BB10}, $\Delta\psi$ is
small and probably unobservable. I identify a new source of circular
polarization, due to mode coupling in an asymmetric QT region. I also identify
a new circular-polarization-dependent correction to the dispersion measure at
low frequencies.
