  Analysis of data involving nuclei far from stability often requires optical
potential (OP) for neutron scattering. Since neutron data is seldom available,
while proton scattering data is more abundant, it is useful to have estimates
of the difference of the neutron and proton optical potentials. This
information is contained in the isospin dependence of the nucleon OP. Here we
attempt to provide it for the nucleon-208Pb system. The goal of this paper is
to obtain accurate n+208Pb scattering data, and use it, together with existing
p+208Pb and 208Pb(p,n)208$Bi*_{IAS} data, to obtain an accurate estimate of the
isospin dependence of the nucleon OP at energies in the 30-60 MeV range. Cross
sections for n+208Pb scattering were measured at 30.4 and 40.0 MeV, with a
typical relative (normalization) accuracy of 2-4% (3%). An angular range of 15
to 130 degrees was covered using the beam-swinger time of flight system at
Michigan State University. These data were analyzed by a consistent optical
model study of the neutron data and of elastic p+208Pb scattering at 45 MeV and
54 MeV. These results were combined with a coupled-channel analysis of the
208Pb(p,n) reaction at 45 MeV, exciting the 0^+ isobaric analog state in 208Bi.
The new data and analysis give an accurate estimate the isospin impurity of the
nucleon-208Pb OP at 30.4 MeV, caused by the Coulomb correction to the proton
OP. The corrections to the real proton OP given by the CH89 global systematics
was found to be only few percent, while for the imaginary potential it was over
20% at the nuclear surface. Based on the analysis of the measured elastic
p+208Pb data at 40 MeV, a Coulomb correction of similar strength and shape was
also predicted for the p+208Pb OP at energy around 54 MeV.
