Introduction

X-rays were discovered in 1895 by Röntgen¹ during the course of an examination of the fluorescence produced in the walls of a discharge tube when an electrical discharge occurred in the residual gas in the tube. It was known at that time that the fluorescence occurred when the pressure in the tube was low, so that no visible discharge took place and that it was under these conditions that cathode rays were most easily observed.² In order to observe the fluorescence more easily, Röntgen fitted the tube with a closely fitting sheath of thin black cardboard and the room was darkened. He also had a paper screen coated with barium platino-cyanide. He observed that when the potential difference produced by an induction coil was applied to the discharge tube the barium platino-cyanide fluoresced brilliantly whether the treated or untreated surfaces of the screen faced the tube. Furthermore, the screen lit up at appreciable distances from the tube—up to 2 m. Röntgen was able to convince himself that the agency which caused the fluorescence had its origin in that part of the discharge tube at which the walls were struck by the cathode rays. He also realised the importance of his discovery and began to study the properties of these new rays which he named ‘X-rays’. In the original paper in which the discovery was announced, he recorded his observations of some of the properties of X-rays, as follows: