The x-ray in the public mind is associated chiefly with hospitals, physicians' offices, and dental chairs. It is well known to have brought about revolutionary changes in the diagnosis and treatment of disease, and to have a place in the investigations and teaching of physics, but fewer know that the trail of research, which led down the centuries to the discovery of the x-ray, has a much wider interest and significance. As we follow it we find ourselves witnessing the early development of electricity, magnetism, and the vacuum. We are led as directly to the radio, and to other manifold applications of electricity in vacuo, as to the x-ray itself, and the continuation of this same trail after the discovery of the x-ray brings us to the astonishing study of radiations, to radium, to the demonstration of the electron and to those revelations of the structure of atoms which are altering so profoundly the older concepts of matter and energy. A younger generation which has never lived in a world without x-ray ...