Röntgen Observes X-Rays

A Screen Glows Where It Should Stay Dark

On November 8, 1895, Wilhelm Conrad Röntgen was experimenting with an electrical discharge tube in Würzburg when he noticed fluorescence on a nearby screen. The tube had been covered to keep visible light from escaping. Something was nevertheless reaching the screen and making it glow. The observation raised a useful experimental question: what passed between the tube and the screen when ordinary light had been blocked? The answer could not be read directly from the glow. Röntgen had to vary what he put in the path and examine what changed. The first observation mattered because it gave him a repeatable way of noticing an otherwise invisible effect.

Putting Materials Between the Source and Screen

Röntgen found that the rays passed through some substances more readily than others. Paper and certain other materials that blocked ordinary light could be relatively transparent to the new radiation. Dense objects could cast stronger shadows. A photographic plate could record differences that the investigator would otherwise see only while the apparatus operated. The combination made an image of hidden structures possible. It was the different degrees of transmission that did the work. A bone and the tissue surrounding it do not cast identical shadows, even though neither is visible to an observer looking at an intact hand under ordinary illumination.

A Famous Image Belongs to a Later Experiment

The familiar radiograph of a hand with a ring was made after the initial discovery and is generally identified as the hand of Röntgen's wife, Anna Bertha. It did not appear on the screen at the first instant of observation. By making such pictures, Röntgen could show readers something more persuasive than a description of an unexpected glow. His first report circulated at the end of 1895. Other investigators could reproduce the phenomenon with equipment related to apparatus already in laboratories, which helped the discovery travel quickly. Images soon drew public interest because the principle could be grasped without mastering the details of electrical discharge.

A New View and a New Responsibility

Early users recognized that the rays might reveal fractures or foreign objects without surgical exploration. Experience also exposed dangers from excessive radiation. The first investigators had neither the protection nor the knowledge expected in a modern radiology department. The modern usefulness of X-rays therefore depends on controlling exposure as well as making an image. Röntgen's Nobel Prize in 1901 recognized the discovery, but the November laboratory event was smaller and more specific. A screen glowed, an investigator noticed that the apparatus did not account for it in the expected way, and a series of experiments turned that discrepancy into evidence about a new kind of radiation.