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.

[Illustration:  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.