The covered tube still lit the screen
In November 1895 Wilhelm Rontgen wrapped a discharge tube in black cardboard. A fluorescent screen several steps away still glowed. Something unknown had crossed the covering.
Books, wood, metal, and flesh blocked the rays by different amounts. Lead stopped them strongly; bone stood out inside a hand. Unsure what they were, Rontgen borrowed mathematics’ symbol for an unknown and called them X-rays.
Diagram description
Static physics diagram: rontgens_hand_image.
A hand became an interior image
Rontgen exposed his wife Anna Bertha’s hand on a photographic plate. Her bones and wedding ring appeared. Within months, doctors were using the rays to locate fractures and foreign objects.
Use raced ahead of safety. Early operators did not understand how ionizing radiation damages tissue, and long exposures injured many of them.
Rontgen declined to patent the discovery and received the first Nobel Prize in Physics in 1901. The temporary letter remained. It still carries the mood of that first night: an application can become obvious while the thing producing it remains unknown.