What did Wilhelm Röntgen discover in 1895, and whose hand was in the first medical image?

Answer

He found an unknown ray passing through objects and named it X. His wife's hand showed bones and a ring.

Context

On 8 November 1895, Wilhelm Röntgen, a physics professor at the University of Würzburg, was experimenting with cathode-ray tubes covered in black cardboard. He noticed that a screen coated with barium platinocyanide glowed across the room, which implied some invisible radiation was escaping the tube. He spent the following weeks studying it, and because he did not know its nature, he called it X, the traditional symbol for an unknown. In German-speaking countries the rays are still often called Röntgen rays.

The famous image was made in December 1895 when he placed his wife Anna Bertha's hand on a photographic plate and exposed it for around 15 minutes (accounts vary on the exact time). The result showed her bones and her wedding ring as shadows. He described the discovery in a paper in late December 1895 and sent copies of the images to colleagues, and news spread through newspapers within weeks.

Doctors began using X-rays for locating bullets and fractures almost immediately, within months of the announcement. Röntgen received the first Nobel Prize in Physics in 1901. He declined to patent the discovery and donated his prize money to his university. Early users did not understand the hazards of radiation, and burns and injuries among early radiographers eventually led to protective standards.