An in-situ Optical Transfer Function measurement system has been developed and was successfully tested in the Sargasso Sea to 1000+ m depths in water 5000+ m deep. Concurrently obtained water samples were analyzed with a 16-channel Coulter counter for particulate content. Mie theory was applied to obtain the Volume Scattering Function σ (or β) for 8 samples at various depths, and the Fourier transform relation first developed by Wells was then used to obtain Modulation Transfer Functions H for comparison with those directly measured. Finally the transfer functions were inverse-transformed to obtain Volume Scattering Functions for comparison with those obtained by Mie calculations. Agreement was excellent in almost all cases, indicating that the equipment worked correctly, that Mie theory is applicable and the Wells' transform relation is valid. The experimental data display particle scattering plateaus which directly yield the beam attenuation coefficient a (or c). Integration of the Volume Scattering Function yields the total scattering coefficient s (or b) whereby the absorption coefficient a is obtained by simple subtraction from α. The range of spatial frequencies employed in the single successful experiment precluded specification of the diffuse attenuation coefficient γ (or k∞ the coefficient of attenuation of the asymptotic distribution at great depths). But the theoretical calculations indicate that H plateaus (again) near zero spatial frequency and that this plateau extends sufficiently beyond ψ = 0 to permit a reasonable determination of γ with obtainable optics. But the range of spatial frequencies employed did indicate the existence of an inverse-square roll-off of H with Li. Despite the lack of ancillary observations, this behavior is attributed to so-called "turbulence" or patches of "coherent" refractive index. The Coulter analyses themselves permit selection from among reported values of the slope of the so-called hyperbolic distribution of particle sizes. And the MIE scattering σ to H calculations suggest a selection from among similarly reported values of an average particle refractive index. Finally, evidence indicates, surprisingly, that although the larger particles have a large effect on σ near θ = 0 (greatly affecting the forward scattering peak) their effect on both the particle scattering (and absorption) coefficient and H is nil.