Absorption of X-Rays.---This paper contains an account of an experimental investigation concerning the discontinuity in the X-ray absorption corresponding to the K-series for the chemical elements from magnesium to chromium inclusively. The method followed was the same as that devised and employed by de Broglie. A specially designed vacuum spectrograph was used.Fine Structure of Absorption.---The spectrograms show that the discontinuity has a rather complex structure, a result in advance of those obtained by earlier investigators. A photometric study of the plates was made in order to obtain a more accurate knowledge of the detailed structure of the absorption limits.Results. These are recorded in tables which give for each element the wave-lengths of the different remarkable points in the structure of the discontinuities. The theoretical bearing of the new observations is briefly discussed.