The variance of the X‐ray spectral distribution in the CuKα multiplet has been redetermined with improved accuracy and over a wider range than previously. This investigation is parallel to that of Edwards&Toman on iron radiation, with which comparison is made on the following points: (i) the effect of using a β‐filter and (ii) the effect of interpolating the line profile under theKα satellite group. Both definitions of the variance are considered and three linear regions can generally be distinguished in the variance–range function. The choice of definition and the usefulness of the different linear regions in any practical application are also d