So I have become a bit confused by the relationship between intensity of Rayleigh scattering and density of the gas. Multiple sources (ex. Salby, Atmospheric Physics) give the scattering cross section per molecule, σ, to be dependent on 1/λ4, the index of refraction of the material and 1/N2 where N is the number density. This would imply a scattering coefficient (fraction of a ray scattered per distance) of β=σN which should then be proportional to 1/N. This confused me (I thought more scatters should mean more scattering) so I consulted Intro to Optics by Hect. Here it stated multiple times that the intensity of lateral scattering did in fact decrease with increasing density, explaining that this was the result of interference effects when the molecules where close together. The problem is that all experimental data regarding Rayleigh scattering indicates that it increases with pressure/density. (ex. see figure 1, Experimental Verification of Rayleigh Scattering Cross Sections, Hans Naus and Wim Ubachs, Optical Letters, Vol. 25, No. 5). Does anyone have any idea what is going on here?