1. The nature of X-rays X-rays were discovered in 1895 by Rontgen in Wurtzburg. For the next 17 years the nature of these rays was one of the dominant questions in physics: were they particles or were they waves? W.H. Bragg, of Adelaide, found indisputable evidence that they were particles; C.G. Barkla, of Liverpool and Edinburgh, found even more indisputable evidence that they were waves. The decisive experiment was carried out by Friedrich and Knipping (1) in 1912 in Munich, under the guidance of von Laue; they passed a fine beam of X-rays through a crystal of copper sulphate, hoping that it would behave as a diffraction grating. It did! The background was emotionally described bv von Laue in 1948 (2). * W.H. Bragg had a son, W.L. Bragg, who was a research student under J.J. Thomson at Cambridge. He was, as he himself said later, rather upset that his father seemed to have been proved wrong, and he tried to think up an alternative explanation of particles travelling through tunnels in the crystal. But he soon realized that the wave explanation had to be accepted.
