German physics professor Wilhelm Röntgen stumbled on X-rays November 8, 1895, while experimenting, and about seven weeks later wrote the first scientific paper on them, calling them "X rays" because he did not know what type of ray they were. Their medical use was discovered when he made a picture of his wife's hand on a photographic plate formed due to X-rays, which was the first X-ray of a human body part. When she saw the picture, she said "I have seen my death." wikiThe X in X-Ray is of the Algebraic formula. It was the inventor admitting he didn't know how the X-ray actually worked. I don't know how this works-ray doesn't sound as good I guess.