In 1895, German physicist Wilhelm Konrad Rontgen observed that when an electric current was passed through a glass container (tube or tube) filled with liquefied gases, the tube vibrated. He closes the Rontgen tube with a cardboard box and is about to turn off the light and go home when he sees a light on the table.. It was green and the cyanide of barium was precipitated. This substance had the property of radiating when light fell on it. But the room is completely dark, and he thinks that the cathode ray tube is the cause of the crystals' radiation. How did the faint light of the tube pass through the cardboard box?!.