The details of an observed absorption spectrum are shown to depend upon the thickness of the absorber. This effect is present, at least in principle, regardless of the type of radiation involved. It is discussed here specifically for x-rays, and specifically for the neighborhood of an absorption edge. Measurements of widths and of relative intensities of the component structure are the most sensitively involved, but wavelengths are also slightly affected. The explanation lies in the rôle of the effective spectral window of the spectrometer. The effect may be serious when the "tails" of the window are extensive, as is inevitably the case with two-crystal x-ray spectrometers. Features of the extent and shape of the spectral window for the (1, +1) position of the instrument used in this work are roughly determined from the thickness effect.

• Received 28 September 1956

DOI:https://doi.org/10.1103/PhysRev.105.1228