The Röntgen rays are certain invisible rays resembling, in many respects, rays of light, which are set free when a high pressure electric current is discharged through a vacuum tube. A vacuum tube is a glass tube from which all the air, down to one-millionth of an atmosphere, has been exhausted after the insertion of a platinum wire in either end of the tube for connection with the two poles of a battery or induction coil. When the discharge is sent through the tube, there proceeds from the anode--that is, the wire which is connected with the positive pole of the battery--certain bands of light, varying in color with the color of the glass. But these are insignificant in comparison with the brilliant glow which shoots from the cathode, or negative wire. This glow excites brilliant phosphorescence in glass and many substances, and these "cathode rays," as they are called, were observed and studied by Hertz; and more deeply by his assistant, Professor Lenard, Lenard having, in 1894, reported that the cathode rays would penetrate thin films of aluminium, wood, and other substances and produce photographic results beyond. It was left, however, for Professor Röntgen to discover that during the discharge another kind of rays are set free, which differ greatly from those described by Lenard as cathode rays The most marked difference between the two is the fact that Röntgen rays are not deflected by a magnet, indicating a very essential difference, while their range and penetrative power are incomparably greater. In fact, all those qualities which have lent a sensational character to the discovery of Röntgen's rays were mainly absent from these of Lenard, to the end that, although Röntgen has not been working in an entirely new field, he has by common accord been freely granted all the honors of a great discovery. [Illustration: SKELETON OF A FROG, PHOTOGRAPHED THROUGH THE FLESH. THE SHADINGS INDICATE, IN ADDITION TO THE BONES, ALSO THE LUNGS AND THE CEREBRAL LOBES. From a photograph by Professors Imbert and Bertin-Sans; reproduced by the courtesy of the "Presse Medicale," Paris. In taking this photograph the experiment was tried of using a diaphragm interposed between the Crookes tube and the plate; and the superior clearness obtained is thought to result from this.] [Illustration: RAZOR-BLADE PHOTOGRAPHED THROUGH A LEATHER CASE AND THE RAZOR-HANDLE. From a photograph taken by Dr. W.L. Robb of Trinity College. The shading in the picture indicates, what was the actual fact, that the blade, which was hollow ground, was thinner in the middle than near the edge.] [Illustration: SKELETON OF A FISH PHOTOGRAPHED THROUGH THE FLESH. From a photograph by A.A.C. Swinton, Victoria Street, London. Exposure, four minutes.]