The image on the plate is then developed and fixed, and in order to secure a printed image exactly like the copy as to right and left position it is necessary to reverse the negative. This is done by cutting the film square, as seen in Fig. 210, and then peeling it off the glass, as seen at Fig. 211, and transferring it to another glass plate in reversed relation. The copper printing plate is produced as follows: The plate is first polished, as seen at the top of Fig. 213, and is then sensitized with a solution of organic matter and an alkaline bichromate. The face of the reversed negative is laid flat against and in direct contact with the face of the sensitized copper plate, and tightly held thereto by the screw clamps of the half tone printing frame. The printing on the sensitized copper face through the stippled or half-tone negative is then effected either by daylight or by the electric light. The application of the electric light for this purpose is shown in Fig. 212. The copper plate is then taken out and subjected to the three lower operations seen in Fig. 213. It is first developed under a stream of water from a faucet, seen on the left, and is then taken in a pair of pliers and held over a gas stove, as seen at the bottom, to “burn-in” the image, and then placed in a tray containing an etching bath of chloride of iron seen on the right, by which the copper is eaten away around the little stipples, and the latter, representing the half tones of the original picture, are left raised, or in relief, to form the inkable surfaces of the printing plate. So fine are these stipples, however, that the picture is to the eye perfectly reproduced. The several views illustrating this process are made in this way, the lines of the reticulated screen being 175 to the inch. The plate is next subjected to the mechanical operation of “routing out” or cutting away the undesirable portions by a routing machine, seen in Fig. 214. It then receives further mechanical treatment to correct imperfections and finish its edges, and is finally mounted upon a block ready for the printer. [Illustration: FIG. 213.--TREATMENT OF COPPER PLATE.] [Illustration: FIG. 214.--ROUTER AT WORK ON HALF-TONE PLATE.] The most striking application made of photography in recent years is in the production of so-called moving pictures, in which a series of photographic figures thrown upon the screen have all the motion of animated scenes which have been caught and imprisoned by the swiftly acting and never failing memory of the camera, to be again turned loose in active play through the Kinetoscope or Biograph. Perhaps the most valuable contribution to science at the end of the century made by this art is in surgery, for photographing through opaque bodies by the aid of the Roentgen rays, but for the latter subjects treatment in separate chapters must be reserved. CHAPTER XXV. THE ROENTGEN OR X-RAYS.