Introduction

Professor W. L. Bragg has just discussed a whole series of radiation phenomena in which the electromagnetic theory is confirmed. He has even dwelt on some of the limiting cases, such as the reflection of X-rays by crystals, in which the electromagnetic theory of radiation gives us, at least approximately, a correct interpretation of the facts, although there are reasons to doubt that its predictions are truly exact. I have been left the task of pleading the opposing cause to that of the electromagnetic theory of radiation, seen from the experimental viewpoint.

I have to declare from the outset that in playing this role of the accuser I have no intention of diminishing the importance of the electromagnetic theory as applied to a great variety of problems. It is, however, only by acquainting ourselves with the real or apparent failures of this powerful theory that we can hope to develop a more complete theory of radiation which will describe the facts as we know them.