On the first and third the Photographic principle depends. In
explaining this principle the accompanying wood cuts, (figs. 3 and 4)
will render it more intelligible.
If a pencil of the sun's rays fall upon a prism, it is bent in passing
through the transparent medium; and some rays being more refracted than
others, we procure an elongated image of the luminous beam, exhibiting
three distinct colors, red, yellow and blue, which are to be regarded
as primitives--and from their interblending, seven, as recorded by
Newton, and shown in the accompanying wood cut. These rays being
absorbed, or reflected differently by various bodies, give to nature
the charm of color. Thus to the eve is given the pleasure we derive in
looking upon the green fields and forests, the enumerable varieties of
flowers, the glowing ruby, jasper, topaz, amethist, and emerald, the
brilliant diamond, and all the rich and varied hues of nature, both
animate and inanimate.
[Illustration: Fig. 3 (hipho_3.gif)]
Now, if we allow this prismatic spectrum (b. Fig. 3.) to fall upon any
surface (as at c.) prepared with a sensitive photographic compound, we
shall find that the chemical effect produced bears no relation to the
intensity of the light of any particular colored ray, but that, on the
contrary, it is dispersed over the largest portion of the spectrum,
being most energetic in the least luminous rays, and ever active over
an extensive space, where no traces of light can be detected. Fig. 4,
will give the student a better idea of this principle. It is a copy of
the kind of impression which the spectrum, spoken of, would make on a
piece of paper covered with a very sensitive photographic preparation.
The white space a. corresponds with the most luminous, or yellow ray,
(5, Fig. 3) over limits of which all chemical change is prevented. A
similar action is also produced by the lower end of the red ray c; but
in the upper portion, however we find a decided change (as at d). The
most active chemical change, you will perceive, is produced by the rays
above the yellow a; viz. 4, 3, 2 and 1 (as at b) the green (4) being
the least active, and the blue (3) and violet (1) rays the most so, the
action still continuing far beyond the point b which is the end of the
luminous image.
[Illustration: Fig. 4 (hipho_4.gif)]
Suppose we wish to copy by the Daguerreotype, or Calotype process, any
objects highly colored--blue, red and yellow, for instance
predominating--the last of course reflects the most light, the blue the
least; but the rays from the blue surface will make the most intense
impression, whilst the red radiations are working very slowly, and the
yellow remains entirely inactive. This accounts for the difficulty
experienced in copying bright green foliage, or warmly colored
portraits; a large portion of the yellow and red rays entering into the
composition of both--and the imperfections of a Daguerreotype portrait
of a person with a freckled face depends upon the same cause.
A yellow, hazy atmosphere, even when the light is very bright, will
effectually prevent any good photographic result--and in the height of
summer, with the most sensative process, it not unfrequently happens
that the most annoying failures arise from this agency of a yellow
medium. A building painted of a yellow color, which may reflect the
sun's rays directly into the operator's room will have the same effect.
Daguerreotypists, being ignorant of these facts, are very apt to charge
their want of success to the plates, or chemicals, or any thing but the
real cause; and it would be well to bear these facts constantly in mind
and as far as possible avoid them. This, may be accomplished, in a
measure, by a choice of location or by having the glass of your windows
tinged with blue; or a screen of thin blue paper may be interposed
between the light and sitter. In selecting subjects, all striking
contrasts in color should be avoided, and sitters for portraits should
be cautioned not to wear anything that may produce the effect spoken
of--dark dresses always being the best.
The action of light both combines and decomposes bodies. For instance,
chlorine and hydrogen will remain in a glass vessel without alteration
if kept in the dark; but if exposed to the rays of the sun, they
immediately enter into combination, and produce hydrochloric acid. On
the other hand, if colorless nitric acid be exposed to the sun, it
becomes yellow, then changes to red, and oxygen is liberated by the
partial decomposition effected by the solar rays.
Of the organic substances none are more readily acted upon by light
than the various combinations of silver.
Of these some are more, and others less sensitive. If Chloride of
silver, which is a white precipitate formed by adding chloride of
sodium (common salt) to a solution of nitrate of silver, be exposed to
diffused light, it speedily assumes a violet tint, and ultimately
becomes nearly black. With iodide of silver, bromide of silver,
ammonio-nitrate of silver, and other salts of this metal, the result
will be much the same.
Some bodies, which under the influence of light, undergo chemical
changes, have the power of restoring themselves to their original
condition in the dark. This is more remarkably displayed in the iodide
of platinum, which readily recieves a photogenic image by darkening
over the exposed surfaces, but speedily loses it by bleaching in the
dark. The ioduret of Daguerre's plate, and some other iodides, exhibit
the same peculiarity--This leads us to the striking fact, that bodies
which have undergone a change of estate under the influence of
day-light have some latent power by which they can renovate themselves.
Possibly the hours of night are as necessary to inanimate nature as
they are to the animate. During the day, an excitement which we do not
heed, unless in a state of disease, is maintained by the influence of
light and the hours of repose, during which the equilibrium is
restored, are absolutely necessary to the continuance of health.
Instead of a few chemical compounds of gold and silver, which at first
were alone supposed to be photographic, we are now aware that copper,
platinum, lead, nickel, and indeed, probably all the elements, are
equally liably to change under the sun's influence. This fact may be
of benefit to engravers, for if steel can be made to take photographic
impressions, the more laborious process of etching may be dispensed
with. In fact, in the latter part of this work, a process is described
for etching and taking printed impressions from Daguerreotype plates.
As yet this process has produced no decided beneficial results--but
future experiments may accomplish some practical discovery of intrinsic
value to the art of engraving.
A very simple experiment will prove how essential light is to the
coloring of the various species comprising the vegetable and animal
kingdoms. If we transplant any shrub from the light of day into a dark
cellar, we will soon see it lose its bright green color, and become
perfectly white.
Another effect of light is that it appears to impart to bodies some
power by which they more readily enter into chemical combination with
others. We have already said that chlorine and hydrogen, if kept in
the dark, will remain unaltered; but if the chlorine alone be
previously exposed to the sun, the chlorine thus solarised will unite
with the hydrogen in the dark. Sulphate of iron will throw down gold
or silver from their solutions slowly in the dark; but if either
solution be first exposed to sunshine, and the mixture be then made, in
the dark, the precipitation takes place instantly. Here is again,
evidence of either an absorption of some material agent from the
sunbeam, or an alteration in the chemical constitution of the body. It
was from understanding these principles and applying them that
philosophers were enabled to produce the Calotype, Daguerreotype, &c.
For the effects and action of light on the camera, see Chapter V.
Some advances have been made towards producing Photographic impressions
in color--the impossibility of which some of our best and oldest
artists have most pertinaciously maintained. The colored image of the
spectrum has been most faithfully copied, ray for ray, on paper spread
with the juice of the Cochorus Japonica, (a species of plant) and the
fluoride of silver; and on silver plate covered with a thin film of
chloride. The day may be still remote when this much to be desired
desideratum shall be accomplished in portrait taking; but I am led to
hope that future experiments may master the secret which now causes it
to be looked upon, by many, as an impossibility.
That great advantages have resulted, and that greater still will result
from the discovery of the Photographic art, few will deny. The
faithful manner in which it copies nature, even to the most minute
details, renders it of much value to the painter; but a few minutes
sufficing to take a view that formerly would have occupied several
days. Its superiority in portraits, over miniature or oil painting has
been tacitly acknowledged by the thousands who employ it to secure
their own, or a friends likeness, and by the steady increase in the
number of artists who are weekly, aye daily springing up in every town
and village in the land.