An Absolute Scale of X-Ray Wave-Lengths

Absolute Scale of X-Ray Wave-Lengths Based on Discontinuities Due to Analyzing Crystal.—In a previous study of the continuous spectrum from a tungsten target, an absorption line was observed at 0.7928 × ${10}^{-8\mathrm{}}$ cm. This result has been confirmed with a molybdenum target, light lines being obtained at 0.7926 and 0.4288 × ${10}^{-8\mathrm{}}$ cm. To get these lines with a rock salt crystal a thickness of less than ${10}^{-2\mathrm{}}$ cm. was necessary. In explanation of the discontinuities it was suggested by A. W. Hull that when any wave-length is reflected by two sets of crystal planes, the energy is divided so that the intensity of the part which reaches the photographic plate is less than for wave-lengths on either side. For each pair of sets of planes the critical wave-length is fixed by the geometry of the crystal alone. Taking the grating constant for rock salt as 2.814 × ${10}^{-8\mathrm{}}$ cm., the wave-lengths computed for the planes 100/210 and 100/310 are 0.7959 and 0.4316 × ${10}^{-8\mathrm{}}$ cm. respectively. Using these as reference points, the center of the scale of wave-lengths may be accurately determined and a scale of wave-lengths may be established independent of everything except the grating constant.