IT has long been known that some of the radio-elements emit a penetrating type of X-rays known as the gamma rays. It is clear that these radiations arising from the nucleus of the radioactive atom represent in a sense some of the characteristic modes of vibration of the nuclear structure. The wave-length and quantum energy of many of the stronger lines in the complicated gamma ray spectrum have been determined by different methods, with concordant results. It has been difficult to determine with certainty the origin of this radiation. It was at first supposed that it must arise from the motions of electrons in the nucleus, but in recent years there has been a growing belief that the radiation is connected with the transition of an α-particle or proton which forms part of the nuclear structure.