The conditions for the thicknesses of the foils of a Ross differential x‐ray filter are examined to yield the best accuracy of monochromatic intensity measurement for a given heterogeneous beam of x‐rays. The results are applied to the problem of monochromatizing theK&agr; radiation from a copper target x‐ray tube using nickel and cobalt foils. The improvement in the differential filter as a monochromator when used with a proportional counter and single‐channel pulse‐height discriminator is pointed out.