THE amiable insistence of my friend Andre? George has induced me to collect in the present Volume a number of Studies on contemporary Physics written from both the general and the more metaphysical point of view. Each of these Studies forms an independent whole, and can be read by itself. A slight degree of repetition-which the reader is asked to overlook-has been the inevitable result: for on more than one occasion I have been compelled to duplicate a summary of the great fundamental stages of contemporary Physics, such as the classification of simple substances, the investigation of the photo-electric effect and the origin of the Theory of Light Quanta and of Wave Mechanics: the subjects are somewhat technical, and I cannot well assume that they are common knowledge. But though the same subject is outlined in several of these Studies, I have tried to take up a different point of view in each, and have endeavoured to throw light on different aspects of the essential problems of Quantum Physics in order to facilitate a grasp of their importance. On comparing the different chapters the reader will observe that, while overlapping, they also complement one another; and he will feel the fascination and greatness inherent in the vast structure of modern Physics. And while admiring the vast number and the extreme delicacy of experimental facts which laboratory physicists have succeeded in revealing, and the strange and brilliant concepts devised by theorists to explain them, he will appreciate to what a degree the methods and ideas of physicists have grown in subtlety during recent years, and how great has been the progress from the somewhat ingenuous Realism and the over-simplified Mechanics of earlier thinkers. The more deeply we descend into the minutest structures of Matter, the more clearly we see that the concepts evolved by the mind in the course of everyday experience-especially those of Time and Space-must fail us in an endeavour to describe the new worlds which we are entering. One feels tempted to say that the outlines of our concepts must undergo a progressive blurring, in order that they may retain some semblance of relevance to the realities of the subatomic scales. Time and Space, in other words, are too loose a dress for the elementary entities; individuality becomes attenuated in the mysterious processes of interaction, and even Determinism, the darling of an older generation of physicists, is forced to yield. But the great book of Science is never finished: other surprises await us: who knows what mysteries are hidden within the nucleus of an atom, which, although a million million times smaller than the smallest living thing, is yet a universe in itself?