  It is known that the concept of limiting polarization introduced seventy
years ago by K G Budden has the capacity to explain the magnitude of circular
polarization seen in normal pulsars with rotation periods of the order of one
second under the assumption of a high-multiplicity electron-positron plasma.
However, a review of limiting polarization under the same assumption in
millisecond pulsars indicates that it is inapplicable there because the region
of limiting polarization lies far outside the light cylinder. The present
paper, using the ion-proton model, evaluates circular polarization both
generally and specifically for J2144-3933, and gives a fairly detailed
understanding of the observations in normal pulsars including the change of
sign as a function of frequency seen in J0908-4913. But it also fails to
explain circular polarization in millisecond pulsars owing to the smaller
particle number densities and birefringence of the magnetosphere in these
objects. However, the review of limiting polarization finds that, within the
ion-proton model, this distinct process can describe their circular
polarization. It is argued that certain features of millisecond pulsar Stokes
profiles are clearly consistent with limiting polarization.
