Various opinions have been expressed regarding the importance of such factors as a 'diffusing effect' due to small angle scattering in determining the visual range in a cloud. If a reduction of apparent contrast and an influence on brightness level are the only important factors, conditions at the obscuration point should be described by,whereBtdenotes the brightness of an object which has a contrastCwith its background,Bcand μDrepresent respectively the cloud brightness and optical density in the line of sight, andPdenotes the contrast limen value. Experiments to test this relation have been performed with an ammonium chloride smoke in a chamber 1.8 m. long. The results indicate that the equation is adequate and that factors not taken into consideration in it play a negligible role in total obscuration under laboratory conditions. The experiments were performed at various brightness levels within the 1 to 100 millilambert range.