hite,
and the white tidy very dark.

[Illustration: FIG. 83.--A negative.]

The lighter the object, such as tidy or collar, the more salt is
changed, or, in other words, the greater the portion of the silver
salt that is affected, and hence the darker the stain on the plate at
that particular spot. The plate shows all gradations of intensity--the
tidy is dark, the black tie is light. The photograph is true as far as
position, form, and expression are concerned, but the actual
intensities are just reversed. How this plate can be transformed into
a photograph true in every detail will be seen in the following
Section.

124. The Perfect Photograph. Bright objects, such as the sky or a
white waist, change much of the silver chloride, and hence appear
dark on the negative. Dark objects, such as furniture or a black coat,
change little of the chloride, and hence appear light on the negative.
To obtain a true photograph, the negative is placed on a piece of
sensitive photographic paper, or paper coated with a silver salt in
the same manner as the plate and films. The combination is exposed to
the light. The dark portions of the negative will act as obstructions
to the passage of light, and but little light will pass through that
part of the negative to the photographic paper, and consequently but
little of the silver salt on the paper will be changed. On the other
hand, the light portion of the negative will allow free and easy
passage of the light rays, which will fall upon the photographic paper
and will change much more of the silver. Thus it is that dark places
in the negative produce light places in the positive or real
photograph (Fig. 84), and that light places in the negative produce
dark places in the positive; all intermediate grades are likewise
represented with their proper gradations of intensity.

[Illustration: FIG. 84.--A positive or true photograph.]

If properly treated, a negative remains good for years, and will serve
for an indefinite number of positives or true photographs.

125. Light and Disease. The far-reaching effect which light has upon
some inanimate objects, such as photographic films and clothes, leads
us to inquire into the relation which exists between light and living
things. We know from daily observation that plants must have light in
order to thrive and grow. A healthy plant brought into a dark room
soon loses its vigor and freshness, and becomes yellow and drooping.
Plants do not a