hrough 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 w