From Delancey Place:
Physics professor Wilhelm Röntgen made an unexpected discovery when he blasted electrons from one electrode to another:
“In the mid-fall of 1895, Wilhelm Röntgen, a physics professor at Wurzburg, Germany, was working in a darkened laboratory with an electron tube — a vacuum glass cylinder that blasted electrons from one electrode to another — when he observed a strange phenomenon. An invisible energy that radiated from the tube had penetrated layers of surrounding cardboard and produced a faint green glow on a nearby fluorescent screen. Röntgen experimented with other materials (e.g., paper, rubber, and wood) that he wrapped around the tube but found the X-rays (he termed his discovery ‘X-rays’ because their composition was unknown) passed through all substances except for lead. The emissions also darkened photographic plates and, as an experiment, Röntgen had his wife place her hand between the source of the X-rays and a plate. To their amazement, the bones in her hand were distinctly outlined. The findings were so startling that Röntgen’s report on ‘shadow pictures’ soon appeared in a scientific periodical and, by early 1896, was translated and published in the United States.