SYNOPSIS OF MR. HUNT'S TREATISE ON "THE INFLUENCE OF THE SOLAR RAYS ON
COMPOUND BODIES, WITH ESPECIAL REFERENCE TO THEIR PHOTOGRAPHIC
APPLICATION."
OXIDE OF SILVER exposed for a few hours to good sunshine, passes into a
more decided olive color, than characterises it when first prepared by
precipitation from nitrate of silver. Longer exposure renders this
color very much lighter, and the covered parts, are found much darker,
than those on which the light has acted directly. In some instances
where the oxide of silver has been spread on the paper a decided
whitening process in some parts, after a few days exposure, is noticed.
Oxide of silver dissolved in ammonia is a valuable photographic fluid;
one application of a strong solution forming an exceedingly sensitive
surface. The pictures on this paper are easily fixed by salt or weak
ammonia.
NITRATE OF SILVER.--This salt in a state of purity, does not appear to
be sensibly affected by light, but the presence of the smallest portion
of organic matter renders it exceedingly liable to change under
luminous influence.
If a piece of nitrated paper is placed upon hot iron, or held near the
fire, it will be found that at a heat just below that at which the
paper chars, the salt is decomposed. Where the heat is greatest, the
silver is revived, and immediately around it, the paper becomes a deep
blue; beyond this a pretty decided green color results, and beyond the
green, a yellow or yellow brown stain is made. This exhibits a
remarkable analogy between heat and light,--before spoken of in chap.
II--and is of some practical importance in the preparation of the paper.
PRISMATIC ANALYSIS.--The method of accomplishing the prismatic
decomposition of rays of light by the spectrum has already been
described on pages 22 and 23. The color of the impressed spectrum, on
paper washed with nitrate of silver, is at first, a pale brown, which
passes slowly into a deeper shade; that portion corresponding with the
blue rays becoming a blue brown; and under the violet of a peculiar
pinkey shade, a very decided green tint, on the point which corresponds
with the least refrangible blue rays, may be observed, its limits of
action being near the centre of the yellow ray, and its maximum about
the centre of the blue, although the action up to the edge of the
violet ray is continued with very little diminution of effect; beyond
this point the action is very feeble.
When the spectrum is made to act on paper which has been previously
darkened, by exposure to sunshine under cupro-sulphate of ammonia, the
phenomena are materially different. The photographic spectrum is
lengthened out on the red or negative side by a faint but very visible
red portion, which extends fully up to the end of the red rays, as seen
by the naked eye. The tint of the general spectrum, too, instead of
brown is dark grey, passing, however, at its most refracted or positive
end into a ruddy brown.
In its Photographic application, the nitrate of silver is the most
valuable of the salts of that metal, as from it most of the other
argentine compounds can be prepared, although it is not of itself
sufficiently sensible to light to render it of much use.
CHLORIDE OF SILVER.--This salt of silver, whether in its precipitated
state, or when fused, changes its color to a fine bluish grey by a very
short exposure to the sun's rays. If combined with a small quantity of
nitrate, the change is more rapid, it attains a deep brown, then slowly
passes into a fine olive, and eventually, after a few weeks, the
metalic silver is seen to be revived on the surface of the salt. Great
differences of color are produced on chlorides of silver precipitated
by different muriates. Nearly every variety in combination with the
nitrate, becomes at last of the same olive color, the following
examples, therefore, have reference to a few minutes exposure, only, to
good sunshine; it must also be recollected that the chloride of silver
in these cases is contaminated with the precipitant.
Muriate of ammonia precipitates chloride to darken to a fine chocolate
brown, whilst muriate of lime produces a brick-red color. Muriates of
potash and soda afford a precipitate, which darkens speedily to a pure
dark brown, and muriatic acid, or aqueous chlorine, do not appear to
increase the darkening power beyond the lilac to which the pure
chloride of silver changes by exposure. This difference of color
appears to be owing to the admixture of the earth or alkali used with
the silver salt.
The prismatic impression on paper spread with the chloride of silver is
often very beautifully tinted, the intensity of color varying with the
kind of muriate used. Spread paper with muriate of ammonia or baryta
and you obtain a range of colors nearly corresponding with the natural
hues of the prismatic spectrum. Under favorable circumstances the mean
red ray, leaves a red impression, which passes into a green over the
space occupied by the yellow rays. Above this a leaden hue is
observed, and about the mean blue ray, where the action is greatest, it
rapidly passes through brown into black, and through the most
refrangible rays it gradually declines into a bluish brown, which tint
is continued throughout the invisible rays. At the least refrangible
end of the spectrum, the very remarkable phenomenon has been observed,
of the extreme red rays exerting a protecting influence, and preserving
the paper from that change, which it would otherwise undergo, under the
influence of the dispersed light which always surrounds the spectrum.
Not only the extreme red ray exerts this very peculiar property, but
the ordinary red ray through nearly its whole length.
In photographic drawing this salt is of the utmost importance. Mr.
Talbot's application of it will be given hereafter in another portion
of this work.
IODIDE OF SILVER--Perfectly pure, undergoes very little change under
the influence of light or heat; but if a very slight excess of the
nitrate of silver be added it becomes infinitely more sensitive than
the chloride.
The spectrum impressed upon paper prepared with a weak solution of the
hydriodate of potash presents some very remarkable peculiarities. The
maximum of intensity is found at the edge of the most refrangible
violet rays, or a little beyond it, varying slightly according to the
kind of paper used, and the quantity of free nitrate of silver present.
The action commences at a point nearly coincident with the mean red of
the luminous spectrum, where it gives a dull ash or lead color, while
the most refrangible rays impress a ruddy snuff-brown, the change of
tint coming on rather suddenly about the end of the blue or beginning
of the violet rays of the luminous spectrum. Beyond the extreme violet
rays, the action rapidly diminishes, but the darkening produced by
these invisible rays, extends a very small space beyond the point at
which they cease to act on the chloride of silver.
In its photographic application, it is, alone, of very little use; but
in combination with other reagents it becomes exquisitely sensitive.
With gallic acid and the ferrocyanate of potash it forms two of the
most sensitive photographic solutions with which we are acquainted.
These are used in the calotype process.
IODURET OF SILVER.--If upon a plate of polished silver we place a small
piece of iodine, and apply the heat of a lamp beneath the plate for a
moment, a system of rings is speedily formed. The first ring, which
spreading constantly forms the exterior of the circle, is of a bright
yellow color; within this, there arises, successively, rings of green,
red and blue colors, and then again a fine yellow circle, centred by a
greyish spot on the place occupied by the iodine. On exposing these to
the light, the outer yellow circle almost instantly changes color, the
others slowly, in the order of their position, the interior yellow
circle resisting for a long time the solar influence. These rings must
be regarded as films of the ioduret of silver, varying, not only in
thickness, but in the more or less perfect states of combination in
which the iodine and metal are. The exterior circle is an ioduret in a
very loose state of chemical agregation; the attractive forces increase
as we proceed towards the centre, where a well formed ioduret, or
probably a true iodide of silver, is formed, which is acted upon by
sunlight with difficulty. The exterior and most sensitive film
constitutes the surface of Daguerreotype plates. The changes which
these colored rings undergo are remarkable; by a few minutes exposure
to sunlight, an inversion of nearly all the colors takes place, the two
first rings becoming a deep olive green; and a deep blue inclining to
black.
The nature of the change which the ioduret of silver undergoes on
Daguerreotype plates, through the action of light, Mr. Hunt considers
to be a decided case of decomposition, and cites several circumstances
in proof of his position. These with other facts given by Mr. Hunt in
his great work on the Photographic art, but to voluminous to include in
a volume of the size to which I am obliged to confine myself, should be
thoroughly studied by all Daguerreotypists.
PRISMATIC ANALYSIS.--The most refrangible portion of the spectrum, (on
a Daguerreotype plate) appears, after the plate has been exposed to the
vapor of mercury, to have impressed its colors; the light and delicate
film of mercury, which covers that portion, assuming a fine blue tint
about the central parts, which are gradually shaded off into a pale
grey; and this is again surrounded by a very delicate rose hue, which
is lost in a band of pure white. Beyond this a protecting influence is
powerfully exerted; and notwithstanding the action of the dispersed
light, which is very evident over the plate, a line is left, perfectly
free from mercurial vapor, and which, consequently, when viewed by a
side light, appears quite dark. The green rays are represented by a
line of a corresponding tint, considerably less in size than the
luminous green rays. The yellow rays appear to be without action, or
to act negatively, the space upon which they fall being protected from
the mercurial vapor; and it consequently is seen as a dark band. A
white line of vapor marks the place of the orange rays. The red rays
effect the sensitive surface in a peculiar manner; and we have the
mercurial vapor, assuming a molecular arrangement which gives to it a
fine rose hue; this tint is surrounded by a line of white vapor, shaded
at the lowest extremity with a very soft green. Over the space
occupied by the extreme red rays, a protecting influence is again
exerted; the space is retained free from mercurial vapor and the band
is found to surround the whole of the least refrangible rays, and to
unite itself with the band which surrounds the rays of greatest
refrangibility. This band is not equally well defined throughout its
whole extent. It is most evident from the extreme red to the green; it
fades in passing through the blue, and increases again, as it leaves
the indigo, until beyond the invisible chemical rays it is nearly as
strong as it is at the calorific end of the spectrum.
Images on Daguerreotype plates which have been completely obliterated
by rubbing may be restored, by placing it in a tolerably strong
solution of iodine in water.
BROMIDE OF SILVER.--This salt, like the iodide, does not appear to be
readily changed by the action of light; but when combined with the
nitrate of silver it forms a very sensitive photographic preparation.
Paper prepared with this salt, blackens over its whole extent with
nearly equal intensity, when submitted to the prismatic spectrum. The
most characteristic peculiarity of the spectrum is its extravagant
length. Instead of terminating at the mean yellow ray, the darkened
portion extends down to the very extremity of the visible red rays. In
tint it is pretty uniformly of a grey-black over its whole extent,
except that a slight fringe of redness is perceptible at the least
refracted end. Beyond the red ray, an extended space is protected from
the agency of the dispersed light, and its whiteness maintained; thus
confirming the evidence of some chemical power in action, over a space
beyond the luminous spectrum, which corresponds with the rays of the
least refrangibility.
This salt is extensively used in photographic drawing.
PREPARATIONS OF GOLD.--Chloride of Gold, freed from an excess of acid
is slowly changed under the action of light; a regularly increasing
darkness taking place until it becomes purple, the first action of the
light being to whiten the paper, which, if removed from the light at
this stage, will gradually darken and eventually develope the picture.
This process may be quickened by placing the paper in cold water.
Chloride of gold with nitrate of silver gives a precipitate of a yellow
brown color. Paper impregnated with the acetate of lead, when washed
with perfectly neutral chloride of gold, acquires a brownish-yellow
hue. The first impression of light seems rather to whiten than darken
the paper, by discharging the original color, and substituting for it a
pale greyish tint, which by slow degrees increases to a dark slate
color; but if arrested, while yet, not more than a moderate ash grey,
and held in a current of steam, the color of the parts acted upon by
light--and of that only--darkens immediately to a deep purple.
Here I must leave the subject of the action of light upon metalic
compounds--referring to Mr. Hunts work for any further information the
student may desire on the other metals--as I find myself going beyond
my limits. I cannot, however, entirely dismiss the subject without
giving a few examples of the action of light on the juices of plants,
some of which produce very good photographic effect.
CORCHORUS JAPONICA--The juice of the flowers of this plant impart a
fine yellow color to paper, and, so far as ascertained, is the most
sensitive of any vegetable preparation; but owing to its continuing to
change color even in the dark, photographic images taken on paper
prepared with it soon fade out.
WALL FLOWER.--This flower yields a juice, when expressed with alcohol,
from which subsides, on standing, a bright yellow finely divided
faecula, leaving a greenish-yellow transparent liquid, only slightly
colored supernatant. The faecula spreads well on paper, and is very
sensitive to light, but appears at the same time to undergo a sort of
chromatic analysis, and to comport itself as if composed of two very
distinct coloring principles, very differently affected. The one on
which the intensity and sub-orange tint of the color depends, is
speedily destroyed, but the paper is not thereby fully whitened. A
paler yellow remains as a residual tint, and this on continued exposure
to the light, slowly darkens to brown. Exposed to the spectrum, the
paper is first reduced nearly to whiteness in the region of the blue
and violet rays. More slowly, an insulated solar image is whitened in
the less refrangible portion of the red. Continue the exposure, and a
brown impression begins to be percieved in the midst of the white
streak, which darkens slowly over the region between the lower blue and
extreme violet rays.
THE RED POPPY yields a very beautiful red color, which is entirely
destroyed by light. When perfectly dried on paper the color becomes
blue. This blue color is speedily discharged by exposure to the sun's
rays, and papers prepared with it afford very interesting
photographs.-- Future experiments will undoubtedly more fully develope
the photogenic properties of flowers, and practically apply them.
Certain precautions are necessary in extracting the coloring matter of
flowers. The petals of fresh flowers, carefully selected, are crushed
to a pulp in a mortar, either alone or with the addition of a little
alcohol, and the juice expressed by squeezing the pulp in a clean linen
or cotton cloth. It is then to be spread upon paper with a flat brush,
and dried in the air. If alcohol be not added, it must be applied
immediately, as the air changes or destroys the color instantly.
Most flowers give out their coloring matter to alcohol or water--but
the former is found to weaken, and in some cases to discharge
altogether these colors; but they are in most cases restored in drying.
Paper tinged with vegetable colors must be kept perfectly dry and in
darkness.
To secure an eveness of tint on paper it should be first moistened on
the back by sponging, and blotting off with bibulous paper. It should
then be pinned on a board, the moist side downwards, so that two of its
edges--the right and lower ones--project a little over those of the
board. Incline the board twenty or thirty degrees to the horizon, and
apply the tincture with a brush in strokes from right to left, taking
care not to go over the edges which rests on the board, but to pass
clearly over those that project; and also observing to carry the tint
from below upwards by quick sweeping strokes, leaving no dry spaces
between them. Cross these with other strokes from above downwards,
leaving no floating liquid on the paper. Dry as quickly as possible,
avoiding, however, such heat as may injure the tint.