ark gray or black.

HOW PHOTOGRAPHS ARE MADE. All photography depends on this action of
light. The plates or films are coated with a silver salt,--usually a
more sensitive salt than silver chlorid. This is exposed to the light
that shines through the lens of the camera. As you have learned, the
lens brings the light from the object to a focus and makes an image
on the film or plate. The light parts of this image will change the
silver salt to silver; the dark parts will not change it. So wherever
there is a white place on the object you are photographing, there will
be a dark patch of silver on the film or plate, and wherever there
is a dark spot on the object, there will be no change on the film or
plate.

[Illustration: FIG. 173. The silver salt on the paper remains white
where it was shaded by the key.]

As a matter of fact, the film or plate is exposed such a short
time that there is not time for the change to be completed. So the
photographer develops the negative; he washes it in some chemicals
that finish the process which the light started.

If he exposed the whole plate to the light now, however, all the
_unchanged_ parts of the silver salt would also be changed by the
light, and there would be no picture left. So before he lets any light
shine on it, except red light which has no effect on the silver salt,
he dissolves off all the white unchanged part of the silver salt,
in another kind of chemical called the _fixing bath_. This is called
"fixing" the negative.

The only trouble with the picture now is that wherever there should
be a patch of white, there is a patch of dark silver particles; and
wherever there should be a dark place, there is just the clear glass
or celluloid, with all the silver salt dissolved off. This kind of
picture is called a _negative_; everything is just the opposite shade
from what it should be. A white man dressed in a black suit looks like
a negro dressed in a white suit.

HOW A PHOTOGRAPHIC PRINT IS MADE. The negative not only has the lights
and shadows reversed, but it is on celluloid or glass, and except
for moving pictures and stereopticons, we usually want the picture on
paper. So a print is made of the negative. The next experiment will
show you how this is done.

EXPERIMENT 101. In a dark room or closet, take a sheet of
blueprint paper from the package, afterwards closing the
package carefully so that no light can get to the papers
inside. Hold the p