Discovering a new type of ray

On November 8, 1895, Wilhelm Röntgen was conducting experiments using a cathode ray tube. He noticed that when he used the cathode ray tube, a board on the other side of his lab that was covered in phosphorus began to glow. Even if he covered the tube's light in a thick black cardboard box, the phosphorous board continued to glow.

It became clear to Röntgen that he had discovered a new type of ray.