THE tremendous stimulus which the discovery of X-rays 1 and radio-activity 2 has given to the progress of the physical and other sciences, during the last three decades, is a matter of common information. Notwithstanding this, the situation which preceded the period is not so widely known. As late as in the seventies of the past century the marked successes of the older concepts and developments of dynamics, with the marvellous background of their earlier applications throughout astronomical time and space, in dealing with the phenomena of heat, sound, optics, electricity and magnetism, led eminent men to venture the opinion that the time for large advances in physics was past; to a great degree the house of physics was in order and the progress of the future would be concerned, principally, with more and more accurate evaluations of the constants characteristic of these great divisions of the subject. Clouds were not long forming upon such an horizon. During the last two decades of the nineteenth century four of the most interesting, but perplexing, discoveries in the history of science were made known, Two of these, mentioned above, were quite strikingly novel. A third grew out of the past in a more logical fashion, having to do with more general aspects of radiation--namely, the radiation of a " black body" in a condition of equilibrium, 3 whose interpretation upon a " quantum " hypothesis * is now a well-known but somewhat controversial matter. The earliest of the four, and perhaps the most surprising to members of the older school of physicists, was the result of the famous experiment of Michelson and Morley, in which it was found impossible to detect a relative motion * Communicated by Prof.