especially strong in producing it. The substance employed by Dr. Röntgen was barium platino-cyanide, which is expensive. Experiment soon showed that other substances were more efficient as well as cheaper, hundreds having been tested under the rays for this effect. The best are tungstate of calcium, tungstate of zinc, barium platino-cyanide, and potassium platino-cyanide, the first named being at present the most common, though the last named has been a favorite with English experimenters. A screen of cardboard covered with a layer of fine crystals of either of these substances and exposed to the rays in a dark room, immediately lights up under their action, and a body impervious to the rays, when placed before the screen, is seen upon it as a shadow. If this screen is the front end of a light-proof box, into the other end of which the eyes can look while all light is excluded, we then have the fluoroscope, by which examinations can be made in a lighted room. Probably few X rays pass through and beyond the fluoroscopic screen. The effect of radiant energy upon a body is determined not by the rays that pass through it, but by those that are absorbed by it. It is difficult, therefore, to understand how the light which the blind have been said to see on peering into a fluoroscope can be really due to X rays.
The invention and improvement of the fluoroscope constitute an important part of the progress that has been made. The effect of the rays on photographic plates is heightened by similar means. The sensitive plate to be exposed to the rays is itself carefully in-closed in a wrapper so as to shut out every trace of light. If, before thus wrapping up the plate, a fluoroscopic screen is placed with its surface of crystals directly in contact with the photographic film, then where the rays penetrate to this crystalline surface it becomes luminous, and the light immediately affects the sensitive plate except in those spots where the object intercepts the X rays, and where consequently they do not cause fluorescence of the screen. This device has greatly reduced the time needed to obtain a photographic impression. Fig. 1 is an illustration of such action. A photographic plate was partly covered by such a screen, and the hand was placed partly over the screen and partly over the plate not covered by the screen. An exposure of twelve seconds was more than sufficient to produce a strong picture of the interior of the hand, under the screen, the flesh almost disappearing from view, while the effect upon the other portion of the plate (the dark part) is much less pronounced. The line of demarcation is very sharp.
Also photographic plates or films have been adapted to this particular use by preparing them so as to absorb the energy of the rays. X-ray plates are now used of which the mode of preparation is of course the manufacturer's secret, but which are
