side of the screen next to the observer in the viewing apparatus. [Illustration: FIG. 217.--EDISON’S SURGEON’S X-RAY APPARATUS.] It is not to be understood that such viewing apparatus is necessary in taking a surgical photograph. In such case only the X-Ray tube, means for exciting it, the patient’s body, and the sensitive photographic plate, are essential factors, the patient’s limb or body being interposed between the light tube and photographic plate, so as to cause the X-Rays emanating from the tube to cast the shadow of the patient’s bones, the bullet in his body, or other foreign object, directly upon the photographic plate, the sensitive and conscious plate obeying the will of these subtle rays, and receiving the impress of their actinic effect under conditions which it denies to ordinary light. [Illustration: FIG. 218.--COMPLETE X-RAY APPARATUS IN USE.] For exciting the vacuum tube any electrical machine capable of throwing a series of sparks across a gap of about five inches is sufficient. Various electrical machines may be used for this purpose, the Holtz, or the Wimshurst glass plate machine, the Ruhmkorff, or induction coil, or even the high frequency transformer. A good example of a complete X-Ray apparatus is that in use at the Army Medical Museum at Washington, made by Otis Clapp & Son, and shown in Fig. 218. The electrical generator is of the Wimshurst type, and is shown in a large glass-enclosed cabinet on the right. The glass disks within are rotated either by a small electric motor shown on the floor, or by a hand crank above. The X-Ray tube, of globular or bulb shape, is shown just above the patient’s hip, and its opposite poles are connected by wires to the opposite electrodes of the generator. When the current is switched on by the operator, the bulb is illuminated with the cathode rays, and the X-Rays, proceeding therefrom through the clothing and flesh of the patient, cast a shadow of the patient’s hip joint upon the photographic plate placed on the cot beneath the patient. [Illustration: FIG. 219.--X-RAY FOCUS TUBE.] In the effort to secure greater sharpness in the image cast by the X-Rays, various forms of vacuum tubes have been devised. That shown in Fig. 219 represents one of the most important improvements. K is the cathode plate, formed of a concave disk of aluminum, which focuses the rays at a point near the center of the bulb. At this point a plate of platinum A, which metal allows practically none of the X-Rays to pass through it, is mounted on the anode in such an angular position that it gathers the focused rays and reflects them through the side of the tube. They thus make a sharper shadow than when radiating from the more extended surface of the glass. [Illustration: FIG. 220.--LOCATING A FOREIGN BODY IN THE BRAIN.]