121. The Magic Wand in Photography. Suppose we coat one side of a glass plate with silver chloride, just as we might put a coat of varnish on a chair. We must be very careful to coat the plate in the dark room,[B] otherwise the sunlight will separate the silver chloride and spoil our plan. Then lay a horseshoe on the plate for good luck, and carry the plate out into the light for a second. The light will separate the silver chloride into chlorine and silver, the latter of which will remain on the plate as a thin film. All of the plate was affected by the sun except the portion protected by the horseshoe which, because it is opaque, would not allow light to pass through and reach the plate. If now the plate is carried back to the dark room and the horseshoe is removed, one would expect to see on the plate an impression of the horseshoe, because the portion protected by the horseshoe would be covered by silver chloride and the exposed unprotected portion would be covered by metallic silver. But we are much disappointed because the plate, when examined ever so carefully, shows not the slightest change in appearance. The change is there, but the unaided eye cannot detect the change. Some chemical, the so-called "developer," must be used to bring out the hidden change and to reveal the image to our unseeing eyes. There are many different developers in use, any one of which will effect the necessary transformation. When the plate has been in the developer for a few seconds, the silver coating gradually darkens, and slowly but surely the image printed by the sun's rays appears. But we must not take this picture into the light, because the silver chloride which was protected by the horseshoe is still present, and would be strongly affected by the first glimmer of light, and, as a result, our entire plate would become similar in character and there would be no contrast to give an image of the horseshoe on the plate.
[Footnote B: ] That is, a room from which ordinary daylight is excluded.]
But a photograph on glass, which must be carefully shielded from the light and admired only in the dark room, would be neither pleasurable nor practical. If there were some way by which the hitherto unaffected silver chloride could be totally removed, it would be possible to take the plate into any light without fear. To accomplish this, the unchanged silver chloride is got rid of by the process technically called "fixing"; that is, by washing off the unreduced silver chloride with a solution such as sodium thiosulphite, commonly known as hypo. After a bath in the hypo the plate is cleansed in clear running water and left to dry. Such a process gives a clear and permanent picture on the plate.
FIG. 82.—A camera.
122. The Camera. A camera (Fig. 82) is a light-tight box containing a movable convex lens at one end and a screen at the opposite end. Light from the object to be photographed passes through the lens, falls upon the screen, and forms an image there. If we substitute for the ordinary screen a plate or film coated with silver chloride or any other silver salt, the light which falls upon the sensitive plate and forms an image there will change the silver chloride and produce a hidden image. If the plate is then removed from the camera in the dark, and is treated as described in the preceding Section, the image becomes visible and permanent. In practice some gelatin is mixed with the silver salt, and the mixture is then poured over the plate or film in such a way that a thin, even coating is made. It is the presence of the gelatin that gives plates a yellowish hue. The sensitive plates are left to dry in dark rooms, and when the coating has become absolutely firm and dry, the plates are packed in boxes and sent forth for sale.
Glass plates are heavy and inconvenient to carry, so that celluloid films have almost entirely taken their place, at least for outdoor work.
123. Light and Shade. Let us apply the above process to a real photograph. Suppose we wish to take the photograph of a man sitting in a chair in his library. If the man wore a gray coat, a black tie, and a white collar, these details must be faithfully represented in the photograph. How can the almost innumerable lights and shades be produced on the plate?
The white collar would send through the lens the most light to the sensitive plate; hence the silver chloride on the plate would be most changed at the place where the lens formed an image of the collar. The gray coat would not send to the lens so much light as the white collar, hence the silver chloride would be less affected by the light from the coat than by that from the collar, and at the place where the lens produced an image of the coat the silver chloride would not be changed so much as where the collar image is. The light from the face would produce a still different effect, since the light from the face is stronger than the light from the gray coat, but less than that from a white collar. The face in the image would show less changed silver chloride than the collar, but more than the coat, because the face is lighter than the coat, but not so light as the collar. Finally, the silver chloride would be least affected by the dark tie. The wall paper in the background would affect the plate according to the brightness of the light which fell directly upon it and which reflected to the camera. When such a plate has been developed and fixed, as described in Section 121, we have the so-called negative (Fig. 83). The collar is very dark, the black tie and gray coat white, and the white tidy very dark.
