On the evening of Nov. 8, 1895, Wilhelm Röntgen noted an unusual flicker in his lab in Würzberg, Germany. Like many scientists of the day, he was fascinated by the cathode-ray tube, and during one experiment, he covered the tube with heavy black paper. A small screen on his workbench covered with a fluorescent substance began to glow. Röntgen determined that some form of radiation was passing through the dark paper. He soon discovered that it could pass through most solid objects, with two notable exceptions: metal and bone. This was made clear a few days before Christmas when he placed his wife’s hand against a photographic plate and exposed it to the tube. Her bones and wedding ring were captured in the iconic image above. Röntgen would call the mysterious form of radiation X-rays.