  We use the transfer equation in relativistic form to develop an expansion of
the one-photon distribution for a medium with constant photon mean free path,
$\epsilon$. The resulting radiative viscosity tensor may be expressed either as
a simple integral operator acting on the Thomas stress tensor or as the
solution of an inhomogenous, linear partial differential equation. The
expression obtained for the radiative viscosity tensor applies for media with
long, as well as short, photon mean free paths. We also develop results
applicable for relatively smooth flows by using the form of the Thomas stress
tensor with generalized transport coefficients derived by the application of a
suitable operator to the bare Thomas coefficients.
