X Rays, or Rontgen Rays. In 1896, Professor Rontgen, of Wurzburg, discovered that some invisible kind of rays are generated in the neighborhood of a Crookes' tube, i.e. a small glass tube into each end of which is fitted a wire from some form of electric generating apparatus, when (the tube being exhausted by an air-pump) the electric circuit is broken by the vacuum sparks in the tube between the two ends of the wires. If, when an electric current is made to pass along the wires, a living human hand be interposed between the Crookes' tube and a photographic plate, a shadow photograph can be obtained which shows all the outlines and joints of the bones. These invisible rays possess the property of passing through all bodies interposed in their path, but some bodies, being less permeable than others, cast a shadow. The discovery has proved of the greatest value in medicine and surgery.