IT has been shown by Herr Laue and his colleagues that the diffraction patterns which they obtain with X-rays and crystals are naturally explained by assuming the existence of very short electromagnetic waves in the radiations from an X-ray bulb, the wave length of which is of the order 10-9 cm. The spots of the pattern represent interference maxima of waves diffracted by the regularly arranged atoms of the crystal. Now, if this is so, these waves ought to be regularly reflected by a surface which has a sufficiently good polish, the irregularities being small compared with the length 10-9 cm. Such surfaces are provided by the cleavage planes of a crystal, which represent an arrangement of the atoms of the crystal in parallel planes, and the amount by which the centres of atoms are displaced from their proper planes is presumably small compared with atomic dimensions.