In the summer of 1914 I began, with the assistance of Mr. W. F. Rawlinson, an investigation of the velocities of the secondary cathode particles ejected from metallic surfaces by X-rays of known frequency. The source of X-rays then available was not very powerful, and little progress had been made when the work was interrupted by the war. A preliminary account of the experimental method, and of the few results which had been obtained, was communicated to the ‘ Philosophical Magazine,' and published in August, 1914. Briefly, the method consisted in exposing a narrow strip of metal to the beam of X-rays, and using a uniform magnetic field to bend the stream of ejected electrons onto a photographic plate, the experiment being performed in a high vacuum. The product rH, where H is the strength of the magnetic field and r the radius of curvature of the path of an electron moving normally to the field, is characteristic of that electron : from it the kinetic energy of the electron may readily be calculated. A special focussing device was used, and described in the paper referred to, by means of which quite a wide beam of electrons could be employed without impairing the definition of the resulting image on the photographic plate: this device not only appreciably shortens the exposures required, but also enhances the effect of the rays under investigation, relatively to that of the general scattered radiations. These latter are always troublesome, and cannot be entirely eliminated.