1. |l'q t,vo recent papers t in the Philosophical 1V[agazine the authors have published the results of measurements made on the intensity of reflexion of X-rays by rock-salt. The mathematical formula for the intensity of reflexion, as calculated by Darwin :~, involves as one o~ its factors the amount of radiant e'nergy scattered in various directions by a single atom when X-rays of given amplitude fall upon it. The other factors in'the formula can b~,~, evaluated. By measuring the intensity of

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... eflexion experimentally we can therefore obtain an absolute measurement of the amplitude of the wave, scattered bv a single atom, in terms of the amplitude of the incident ra~liation. This measurement is of considerable interest, because it may throw some light on the distribution of the electrons around the nucleus of the atom. We regard the wave scattered by the atom~ as a whole, as the resultant of a number of waves, each scattered independently by the electrons in the atom. A formula first evaluated by J. J. Thomson is used in order to calculate the amplitude of the wave scattered by a single electron. If an incident beam of plane polarized X-rays consists of waves of amplitude A, then the amplitude A r at a distance R fr()m the electron in a plane containing the direction of the incident radiation, and at right angles to the electric displacement, is given by A / 1 e 2