With a Société Genevoise double crystal spectrometer, the mass absorption coefficients of eight different elements extending from hydrogen (from paraffin and carbon) to lead were determined for a wave-length of 208.6 x-units. An attempt to make a similar series of measurements on the $K{\beta }_1$ line of tungsten failed because of the low intensity of the x-rays. With the same instrument but with the first crystal removed as a single crystal spectrometer, measurements were extended down to 138 x-units. The atomic absorption coefficients determined from these measurements were found to be given accurately by the equation $\left(\frac{{\mu }_a}{Z}\right)=C_k{Z}^{3.18}{\lambda }^q+\left(\frac{\sigma }{{\sigma }_0}\right){\sigma }_0$, where $\frac{\sigma }{{\sigma }_0}$ is given by the Klein-Nishina formula rather than the Breit and Dirac or Compton formulas for scattering. In the case of hydrogen however the atomic absorption coefficient (calculated from the difference between measurements on paraffin and carbon) was within experimental error equal to the Thomson scattering for the range of wave-lengths used. The constant $q$ appears to have the value 2.50 for this region, and $C_k$ the value 5.81×${10}^{-7\mathrm{}}$, if $\lambda$ is measured in centimeters. In general the absorption coefficients agree fairly well with those of other observers except for a slight discrepancy in the case of copper; a peculiarity is pointed out in the case of silver.

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