An accidental glow through black cardboard revealed the skeleton
In 1895, German physicist Wilhelm Röntgen was investigating electrical discharges inside an evacuated Crookes glass tube in a dark room. Though he had carefully wrapped the tube in heavy black cardboard to block all visible light, a fluorescent barium-coated paper screen several feet away began mysteriously glowing. Realizing an unknown invisible radiation was escaping the glass, Röntgen investigated for weeks and produced the world's first X-ray image: the skeletal bones of his wife's hand.
A Fortuitous Glow in a Darkened Laboratory
In the late autumn of 1895, Wilhelm Conrad Röntgen was working in his laboratory at the University of Würzburg, examining the behavior of electrical discharges passed through evacuated glass vessels known as Crookes tubes. These devices, which physicists across Europe had been using to study cathode rays, consisted of sealed glass bulbs with positive and negative electrodes, pumped down to an extremely low pressure. When high-voltage electric currents pulsed through the near-vacuum, the glass walls of the tubes often produced a faint, characteristic greenish fluorescence. Röntgen was particularly interested in determining the limits of cathode rays, specifically how far they could travel outside the tube and whether they could penetrate various materials.
Cathode rays themselves were known to have very limited range in open air, typically scattering and extinguishing within just a few centimeters of the glass. To ensure that visible light from the discharge tube would not interfere with his observations, Röntgen meticulously encased the entire tube in a snug covering of thick, black cardboard. On the evening of November 8, 1895, after darkening the room completely to test the opacity of the paper jacket, he discharged the electrical apparatus. To his surprise, a faint, shimmering light caught his eye across the workbench, far beyond the reach of any known cathode rays. A small paper screen coated with barium platinocyanide was glowing in the dark.
The reaction was baffling because visible light could not possibly escape the heavy black shield, and conventional cathode rays could never travel several feet through the air to excite the chemical coating on the screen. Röntgen turned the high-voltage current off, and the glow instantly died away. When he switched the current back on, the phosphorescent glimmer returned. Something invisible was leaving the tube, passing completely through the light-blocking cardboard, traversing the open room, and transferring enough energy to the barium compound to make it fluoresce.