As this is the first example of a _negative_ image that has been
introduced into this work, it may be necessary to explain, in a few words,
what is meant by that expression, and wherein the difference consists.
The ordinary effect of light upon white sensitive paper is to _blacken_
it. If therefore any object, as a leaf for instance, be laid upon the
paper, this, by intercepting the action of the light, preserves the
whiteness of the paper beneath it, and accordingly when it is removed
there appears the form or shadow of the leaf marked out in white upon the
blackened paper; and since shadows are usually dark, and this is the
reverse, it is called in the language of photography a _negative_ image.
This is exemplified by the lace depicted in this plate; each copy of it
being an original or negative image: that is to say, directly taken from
the lace itself. Now, if instead of copying the lace we were to copy one
of these negative images of it, the result would be a _positive_ image of
the lace: that is to say, the lace would be represented _black_ upon a
_white_ ground. But in this secondary or positive image the
representation of the small delicate threads which compose the lace would
not be quite so sharp and distinct, owing to its not being taken directly
from the original. In taking views of buildings, statues, portraits, &c.
it is necessary to obtain a _positive_ image, because the negative images
of such objects are hardly intelligible, substituting light for shade, and
_vice versa._
But in copying such things as lace or leaves of plants, a negative image
is perfectly allowable, black lace being as familiar to the eye as white
lace, and the object being only to exhibit the pattern with accuracy.
In the commencement of the photographic art, it was a matter of great
difficulty to obtain good _positive_ images, because the original or
negative pictures, when exposed to the sunshine, speedily grew opaque in
their interior, and consequently would not yield any positive copies, or
only a very few of them. But, happily, this difficulty has been long
since surmounted, and the negative or original pictures now always remain
transparent during the process of copying them.
[PLATE XXI. THE MARTYRS’ MONUMENT]