The conditions for coherent scattering of starlight in the forward direction, and it's consequences

I have been too conservative in `A review of the properties of the scattered starlight which contaminates the spectrum of reddened stars' (NewA, 2002, 7/4, 191) concerning the conditions implied by an additional component of scattered light in the spectrum of reddened stars. I will review and simplify these conditions. Implications for the column density of the scatterers will then be discussed. Estimates for coherent Rayleigh scattering by hydrogen are in excellent agreement with the observations.