Classical and quantum reflections of X-rays in crystals

It appears desirable that we reply, though very briefly, to the comments that have appeared in Nature on our first note on this subject and which were evidently written before our second and supplementary note had been published. Our purpose in these communications was very definite, namely, to indicate it as a necessary consequence of classical optics and of quantum mechanics that there should be two types of X-ray reflection in crystals, due respectively to static and to dynamic stratifications of electron density; in the language of quantum mechanics, these correspond respectively to an elastic collision of the photon with the crystal lattice and to an inelastic one in which part of the energy of the photon is transferred to the crystal as an optical vibration of its lattice structure