The principle of this art depends upon the property which certain
chemical preparations (chiefly those of silver) possess of being
blackened by exposure to light while in contact with organic matter,
and that in so gradual a manner that every degree of shade may be
represented. Now, as the various appearances of everything we see
depend upon the effects of light and shadow (together with the
peculiar color of the objects themselves), it follows that if these
lights and shadows can be transcribed, we shall have a representation
of the objects, minus their own proper colors--and this is what
photography effects. For the purpose of bringing the objects to be
taken within a small compass, and for increasing their vividness, an
instrument is used called a camera, in which there is an achromatic
arrangement of lenses (shown in figs. 2 and 3), which produces a
picture on the paper or glass to be affected by the light, in the
same way that a common magnifying glass will, if held at a proper
distance from a piece of paper, but much more perfectly (fig. 1).
The effect of this camera-picture on the paper or glass when properly
prepared and subjected to its influence, is to darken the paper in
all those parts which in the camera-picture are the brightest, and
to leave unaffected those parts which are the darkest, thus producing
what is called a “negative picture,” having the lights represented by
shadows and the shadows by lights. This is used to form the true or
“positive” picture, which is done by placing the negative on a piece
of prepared paper, and exposing it to the light. The negative having
been made partly transparent by wax (as will be further explained),
and the parts of the picture which are to be dark being left
transparent, the light passes through them and blackens those parts
of the prepared paper behind, while those which are to be light,
being dark in the negative, exclude the light and thus preserve the
paper at the back from being darkened; the paper is thus affected in
the contrary way to the negative, and is therefore a true picture of
the lights and shadows of the object copied. This is the principle
of all photographic pictures, but with numberless variations in its
practice; it would therefore be impossible here to describe the
particulars of each process, beyond the most simple.