Greatly struck by this observation he tried various other arrangements. He put a deal board between the tube and the screen, but found that the board made no difference whatever, and that whatever rays from the tube were lighting up the screen, these rays could pass through the deal board. He then took away the board and put in its place a thick book, but again this made no impression whatever upon the rays. Thinking that perhaps the rays were coming round to the screen in some unexplained way, he held his hand between the tube and the screen. Imagine his surprise on seeing, not the shadow of his hand, but only the shadow of the bones in his hand on the phosphorescent screen. It must have given him something of a shock, and we may suppose that he quickly turned on the light to see if his hand had really shrivelled up into a skeleton. But no, the hand looked just as usual, and yet on holding it in the path of these wonderful rays, the shadow of the bones was again distinctly visible, the flesh showing only in faint outline. It was a most wonderful discovery. The great news was not long in travelling all over the world. The experiment was repeated thousands of times, and it was soon found out that a great new fact had been added to the range of human knowledge. It was found that metallic bodies such as coins, bullets, and needles could be located in the flesh by means of the rays playing through it on to the screen. Instead of the screen, a photographic plate could be used, and pictures of these bodies could be obtained in a few minutes. [Illustration: Fig. 74.--A Foot inside a Boot as seen by means of the Röntgen Rays.] Surgeons were not slow in using the great discovery in the hospitals, and every large hospital has had a special staff of men trained in the use of the Röntgen Rays. But it often happens that the blessings of a new discovery are endangered by evil effects which they bring in their train, and this was very much the case with the Röntgen Rays. The men who spent all their days taking photographs by means of the new rays found to their dismay that wherever the rays played upon them for a long time the skin became dry and hot. The hair fell off, and even the nails dropped out. Worse than that, very painful sores began to appear after a time, which it was found almost impossible to heal. In some cases death was the result of prolonged exposure to the Röntgen Rays; but, as usual, science eventually succeeded in separating the good from the bad effects, and protected the skin by means of screens of lead which cannot be pierced by X-rays. Now this new great aid to surgery and medicine has been established properly and regulated satisfactorily.