  An expression for the mutual coherence function (MCF) of an electromagnetic
beam wave propagating through atmospheric turbulence is derived within the
confines of the Rytov approximation. It is shown that both the first and second
Rytov approximations are required. The Rytov MCF is then compared to that which
issues from the parabolic equation method of strong fluctuation theory. The
agreement is found to be quite good in the weak fluctuation case. However, an
instability is observed for the special case of beam wave intensities. The
source of the instabilities is identified to be the characteristic way beam
wave amplitudes are treated within the Rytov method.
