Of the various possible methods of approaching present day problems of electromagnetic radiation—particularly the problem of reconciling with the older electromagnetic theory those experimental facts which have led to the formulation of the Quantum Theory—the most profitable is probably to be found in the study of the phenomena of Röntgen radiation. Primarily this is due not to the accuracy which may be attained in measurements upon radiation of this type, though this is an important factor, but to the fundamental nature of the phenomena themselves. The result of this is shown both in the simplicity and in the generality of the laws observed.