[Footnote: A communication to the Sheffield Photographic Society in the
_British Journal of Photography_.]
It is quite possible that in the remarks I propose making this evening
in connection with the photographic art I may mention topics and some
details which are familiar to many present; but as chemistry and optical
and physical phenomena enter largely into the theory and practice
of photography, the field is so extensive there is always something
interesting and suggestive even in the rudiments, especially to those
who are commencing their studies. Although this paper may be considered
an introductory one, I do not wish to load it with any historical
account, or describe the early methods of producing a light picture, but
shall at once take for my subject, "The Photographic Image: What It
Is," and under this heading I must restrict myself to the collodion and
silver or wet process, leaving gelatine dry plates, collodio-chloride,
platinum, carbontype, and the numerous other types which are springing
up in all directions for future consideration.
Now, in an ordinary pencil, pen and ink, or sepia sketch we have a
deposit of a dark, non-reflecting substance, which gives the outline of
a figure on a lighter background. The different gradations of shade
are acquired by a more or less deposit of lead, ink, or sepia. In
photography--at least in the ordinary silver process--the image is
formed by a deposition of metallic silver or organic oxide in a minute
state of division, either on glass, paper, or other suitable material.
This is brought about by the action of light and certain reagents. Light
has long been recognized as a motive power comparable with heat or
electricity. Its action upon the skin, fading of colors, and effect
on the growth of vegetable and animal organisms are well known; and,
although the exact molecular change in many instances is not clearly
understood, yet certain salts of silver, iron, the alkaline bichromates,
and some organic materials--as bitumen and gelatine--have been pretty
well worked out.
It is a remarkable and well-known fact that the chloride, iodide, and
bromide of silver--called "sensitive salts" in photography--are not
susceptible (at least only slowly) to change when exposed to the yellow,
orange, and red rays. The longer wave lengths of the spectrum, as you
know, form, with violet, indigo, blue, and green, white light. The
diagram on the wall shows this dispersion and separation of the
primitive colors. These--the yellow, orange, and red-- are called
technically "non actinic" rays, and the others in their order become
more actinic until the ultra violet is reached. The action of white
light, or rays, excluding yellow, orange, and red, has the effect of
converting silver chloride into a sub-chloride; it drives off one
equivalent of chlorine. Thus, silver chloride, Ag_2Cl_2=Ag_2Cl+Cl.
When water is present the water is decomposed. Hydrochloric acid, HCl,
hypochlorous acid, HClO is formed.
The iodide of silver in like manner is changed into a sub-iodide; but
with water hydriodic acid is formed unless an iodine absorbent be
present--then into hypoiodic acid. The silver bromide undergoes
a similar change. When with light alone, a sub-bromide,
Ag_2Br_2=Ag_2Br+Br, and with water hypobromous acid. It is important
to bear this in mind, as one or other, and frequently both iodide and
bromide of silver, is the sensitive salt requisite or used in producing
the invisible image.
The theory regarding these sensitive salts of silver is that, being very
unstable, _i. e._, ready to undergo a molecular change, the undulations
produced in the ether, which pervades all space, and the potential
action or moving power of light is sufficient to disturb their normal
chemical composition; it liberates some of the chlorine, iodine, or
bromine, as the case may be. This action, of course, applies to light
from any source--the sun, electricity, or the brighter hydrocarbons,
also flame from gas or candle, whether it comes direct as rays of white
light or is reflected from an object and conducted through a lens as a
distinct image upon the screen of a camera.
I have no time to speak on the subject of lenses, only just to mention
that they are, or ought to be, achromatic, so as to transmit white light
and of perfect definition, and the amount of light passed through should
be as much as possible consistent with a sharp image--at least when
rapid exposure is attempted.
I shall touch very lightly on the manipulative part of photography, as
that would be unnecessary; but a brief account of the chemicals in use
is essential to a right appreciation of the theory of developing the
image. In the first place, our object is to get a film of some suitable
material coated with a thin layer of a sensitive salt of silver--say
a bromo-iodide. By mixing certain proportions of ammonium iodide
and cadmium bromide, or an iodide and bromide of cadmium with
collodion--which is pyroxyline, a kind of gun-cotton dissolved in ether
and alcohol--a plate of glass is coated, and before being perfectly dry
is immersed in the nitrate of silver bath. The silver nitrate solution,
adhering and entering to a slight extent the surface of the collodion,
becomes converted by an ordinary chemical action of affinity into silver
iodide and bromide.
The ammonium and cadmium play a secondary part in the process, and
are not absolutely necessary in forming the image. The plate is now
extremely sensitive to light. When we have entered it into the dark
slide and camera, and then exposed to light, the change I mentioned
has taken place. The film is transformed into different quantities of
sub-iodide and sub-bromide of silver, according to brilliancy of light.
In addition, there is on the plate an amount of unchanged silver nitrate
which becomes useful in the second stage, or development. The image is
not seen as yet, being latent, and requiring the well-known developing
solution of sulphate of iron, acetic acid, alcohol, and water.
Practically we all recognize the effect of a nicely-balanced wave of
developer worked round a plate. The high lights are first to appear as a
darker color, till the details of shadow come out; when this is reached
the developer is washed off. The chemical action is briefly thus, and
it can be shown by solutions without a photographic plate, as in a test
tube: Pour into this glass a solution of silver nitrate, AgNO, and add a
solution of ferrous sulphate, FeSO_4. The ferrous sulphate combines
with the nitric acid, forming two new salts--ferric nitrate and ferric
sulphate. The silver is deposited. Any other substance which will remove
oxygen from silver nitrate without combining with the silver would do
the same, and metallic silver would be thrown down. The formula, as
shown on the diagram, explains the interchange.
When the developer is poured over the plate it attacks first the free
silver nitrate, and causes it to deposit extremely fine particles of
metallic silver. The question arises: How is it these particles arrange
themselves to form an image? This is explained by the physical movement
known as molecular attraction or affinity. These particles are attracted
first to the portions of the plate where there is most sub-iodide and
sub-bromide. In the shady parts less silver is deposited. When the image
is once started it follows that particles of silver produced by the iron
developer will cause more to fall down on the face of those already
present, and the image is, of course, built up if the silver nitrate
be all consumed on the plate. The developer then becomes useless or
injurious. The presence of acetic acid checks the reduction of the
silver, and the alcohol facilitates the flow when the bath becomes
charged with ether and spirit.
The molecular attraction just mentioned is made plainer by reference to
the simple lead tree experiment. We have here in this bottle a piece
of zinc rod introduced into a solution of acetate of lead. A chemical
change has taken place. The zinc has abstracted the acetic acid and the
lead is deposited on the zinc, and will continue to be so until the
solution is exhausted. The irregularities of surface and arborescent
appearance are well shown. If the change were rapidly conducted the lead
particles would from their weight sink directly to the bottom instead
of aggregating together like ordinary crystals. I have constructed a
diagram of colored card, which will perhaps more clearly demonstrate
the relation of the different constituents. The lower portion (Fig. a)
represents a section of the glass plate or support, the collodion film
(Fig. b) having upon its surface a thin layer of bromo-iodine silver
(Fig. c), which, when exposed to a well-lighted image, as in a camera,
changes into different gradations of sub-bromide and sub-iodide, as
indicated by irregular, dark masses in the film. The dotted marks
immediately above these are intended for the silver deposit (Fig.
d)--clusters of granules, more abundant in the well lighted and less
in the shaded parts of the picture, corresponding to the amount of
sub-bromide and iodide beneath.
[Illustration: SECTION OF SENSITIVE PLATE AFTER EXPOSURE AND DURING
DEVELOPMENT.
d Silver deposit--Image, c Sub-bromide and sub-chloride (gradations of),
b Collodion film--Substratum, a Section of glass plate--Support.]
The next point to consider is that of intensification--a process seldom
required in positive pictures, and would not be needed so often in
negatives if there was enough free silver nitrate on the plate during
development. The object, as we all know, in a wet-plate negative is to
get good printing density without destruction of half-tone. It is a
rule, I believe, in an over-exposed picture to intensify after fixing
the image, and in an under-exposed picture to intensify before fixing.
Whichever is done the intention is similar, namely, to intercept in a
greater degree the light passing through a negative, so as to make a
whiter and cleaner print. The usual intensifier--and, I suppose, there
is no better--is pyrogallic acid, citric acid, water, and a few drops of
silver nitrate solution. Pyrogallic is the most active agent, and might
be used alone with water; but for special reasons it is not desirable.
As a chemical it has a great affinity for oxygen, and will precipitate
silver from a solution containing, for instance, nitrate of silver. It
also combines with the metal, forming a pyrogallate--a dark brown, very
non-actinic material. The use of a few drops of AgNO_3 solution is very
evident. A deposit is added to the image already formed. Citric acid is
the retarder in this case. Alcohol is unnecessary, as the film is well
washed with water before the intensifier is used, consequently it flows
readily over the plate.
As regards fixing, or, more properly, clearing the image: it is the
simple act of dissolving out or from the film all free nitrate,
chloride, iodide, or bromide. Cyanide of potassium does not attack the
metallic deposit unless very strong. It has then a tendency to reduce
the detail in the shadows.