Wilhelm Conrad Roentgen was the director of the Physical institute in Wuerzburg in Germany. In the evening on 8^th November 1895 he was experimenting with electric discharges in the vacuum tubes. At once he noticed that a piece of paper coated with tetrakyanoplatnatan barium was shining when being near the tube. Seemingly without any cause.

Roentgen's evacuated tube is a glass bulb with positive and negative electrodes.

Roentgen tried to cover the gas tube with a pie e of cardboard, but the paper was shining again. He suspected, that he discovered an invisible ray, which has got an ability to penetrate the objects. Today we now, that it is a kind of electromagnetic radiance with a 0,001 mm wave-length. When doing further experiments he found out, that the intensity of the light on the paper (screen)is in due proportion to the thickness of the material inserted between the tube and the screen, because the rays are partly absorbed with the material (depends on the thickness and kind of the material). But one fact was very surprising. When he had inserted his hand into the rays, the bones could been seen on the screen …

"If one holds hands between the discharge apparattus, one sees the darker shadows of the bones within the much fainter shadow picture of the hand itself." (Roentgen)
(Figs above from page http://www.shadow.net/~ebaumel/X_ray.html)

Thomas Hunt, at that time a well-known British physician, called this invention the “perhaps greatest margin stop in the history of diagnostic”. This happened just a month after the invention was done. These rays were originally called X-rays. Later, on honour of the inventor, they were called the Roentgen rays.

"As the thickness increases all materials become transparent." (Roentgen)
(Fig. from page http://www.cc.emory.edu/X-RAYS/century_04.htm)

Roentgen was rewarded for his invention with Nobel prize for physics in 1901.