Röntgen was experimenting with vacuum tubes at the University of Würzburg when he discovered the new ‘invisible light’ on 8 November 1895. Although his lab notes were burned after his death in 1923, Röntgen’s biographers describe him noticing a faint glow from a screen covered in fluorescent material about a metre away from his apparatus. This was despite the vacuum tube itself being covered with black cardboard that stopped all visible light.
Having reasoned that the tube itself must be giving off these invisible rays, Röntgen conducted a series of experiments over the next few weeks in which he found that they could pass through certain objects but not others. Due to his uncertainty over the exact nature of the new rays, he adopted the mathematical designation ‘X’ to reflect their mysterious nature.
As his experiments continued, Röntgen began to notice that the rays were able to penetrate the soft tissues in his body but were stopped by bone. After subsequently replacing the fluorescent screen with a photographic plate he made the first ever X-ray image clearly showing the bones of his wife Bertha’s hand and her wedding ring. On seeing the image she is said to have remarked, ‘I have seen my death.’
Röntgen published the paper “On a New Kind Of Rays” on 28 December 1895, and news of his discovery spread quickly. Within a year X-rays were being used as far away as the United States as both a diagnostic tool and for treating cancers. Röntgen refused to take out a patent on X-rays in order to allow the entire world to benefit from them but was awarded the first ever Nobel Prize in Physics in 1901.