In 1895 the German physicist Wilhelm Conrad Rontgen (1845-1923) discovered X-rays. He called them "X" because their nature was then unknown. The nature of the rays was not understood until 1912 when another German physicist, Max von Laue (1879-1960), managed to diffract them through a lattice of crystal.
X-rays are electromagnetic waves which pass through material that is normally opaque to light. These waves have a very short wave-length. The discovery of X-rays immediately created a considerable stir. Rontgen became a national hero and was awarded the Nobel Prize for Physics in 1901.
The X-ray tube shown was used during the Great War, along with the Arsenic Plate. Using them the surgeon was able to detect where a bullet had penetrated, (usually in the head), many operations were successfully performed thanks to Rontgen's discovery