Theories of light extinction and contrast reduction by atmospheric aerosols are reviewed. Measurements by a telephotometer of the relatively turbid atmosphere of the eastern U.S. are described. Because of current levels of extinction, it is feasible to apply a dual target technique first described by Ruhle, which corrects for the targets' imperfect blackness and stray light imperfections in the telephotometer optics. Measurements have been made during an intensive visibility study in the Ohio River Basin. These results compare favorably to independent measures of scattering and absorption coefficients. Complications introduced by uneven illumination due to clouds are found to be surprisingly small; only the most obvious cases of poor illumination need be excluded.