Wilhelm Conrad Roentgen lived in the years 1845-1923. Among his other researches he studied the occurrence of cathode radiation. That occurred while the electric current flowed through the tube filled with rare gas. The tube started to illuminate to the negative electrode. Invisible rays coming out of the cathode were supposed to be responsible for that. He observed their interaction with metals. He noticed that fluorescent crystals, placed near the set of instruments used in the experiment, started to illuminate. That happened even when he conducted the experiment on the absolute darkness. That occurrence he tried to explain by the subsistence of an unknown mysterious, new kind of radiation - X-radiation. Roentgen discovered that the radiation was absorbed by different materials in a different degree. That let him make the first photo of alive person's hand skeleton.