857
photographic background until he joined in the Société
française de photographie, 1873 in the hope of improv-
ing his technique.
He brought back from the United States the use of
the melainotype process and was one of the fi rst who
used the carbon printing: he published many treatises
on this subject.
Liébert also patented (1873) his own enlarger and
invented a kind of color tint to put on pictures (1894);
he also operated a sensitized paper factory between 1895
and 1900. Finally, he proposed the fi rst studio equipped
with the electric light (1879) and realized photographs
of the Expositions universelles balloon (1889).
Moreover, Alphonse Liébert is known as one of those
who recorded and photographed the events of the Paris
Commune in 1870.
After his death, in February 1913, his son Georges
Auguste Liébert continued his work in the studio.
Marion Perceval
LIGHT-SENSITIVE CHEMICALS
Light, as a form of energy, has the power to promote
chemical change. Of the many photochemical reac-
tions, the few that are suited to making permanent
photographic images employ light-sensitive salts of the
metals silver, iron, uranium, and chromium, and some
purely organic substances.
Silver halides were essential to the fi rst 150 years
of camera photography owing to their unique ability to
capture an image “instantaneously.” No other substance
matches their unparalleled sensitivity, which depends
on the formation of an invisible latent image, and its
subsequent development, whereby the action of light is
amplifi ed enormously—a few hundredfold in the earliest
development processes discovered by Henry Talbot and
Louis Daguerre; but about ten million times in modern
emulsions. All other photosensitive substances provide
little or no amplifi cation, and have no practical use in the
camera, only for making positive prints or photograms,
where intense illumination and lengthy exposures are
no disadvantage.
Light of shorter wavelengths has greater intrinsic
energy; the portion of the prismatic spectrum most
effective photochemically is therefore the blue and
ultraviolet—the “actinic” radiation discovered pho-
tographically by Johann Ritter in 1801. Photography
usually entails the promotion by light of a chemical
reduction of metal cations (positively-charged metal
atoms) which take up, and are neutralized by the nega-
tively-charged electrons supplied by an oxidisable
substance, to form the elemental metal. Description
of these photochemical reduction-oxidation reactions
by traditional, balanced chemical equations tends to
obscure the essence of the process, so this account
will use ionic “half-reactions” explicitly involving the
transfer of electrons, represented as e–. The half-reac-
tions are then combined proportionally to balance out
the electrons in the overall equation.Silver
Silver chloride was discovered in 1565 by Georg
Fabricius in the mines of Bohemia, as the mineral
“horn-silver” or luna cornua. By the 17th century,
it was known to darken in sunlight; in 1614, Angelo
Sala observed the same behaviour in silver nitrate
(the lapis lunearis of the alchemists) when in contact
with organic matter. Johann Heinrich Schulze was the
fi rst to demonstrate a primitive photographic effect in
silver salts in 1725, and Carl Wilhelm Scheele showed
in 1777 that the violet rays of the spectrum were most
effective in decomposing silver chloride, and that the
dark product was fi nely-divided silver. Knowledge of
the light-sensitivity of the other silver halides had to
await the discoveries of the parent halogens, bromine
(by Antoine Balard, 1826), and iodine (by Bernard
Courtois, 1811). In the following equations, X repre-
sents any of these, i.e., Cl, Br, or I:Light + X– → X + e–
Halide anion → halogen atom + electron
Ag+ + e– → Ag
Silver cation + electron → silver metal
X + X → X 2
Halogen atoms → halogen molecule
The overall net reaction is:
Light + AgX → Ag +^1 / 2 X 2
silver halide → silver metal + halogen moleculeTo prevent the reversal of this reaction and destruc-
tion of the image silver, a halogen absorber should be
present: sodium citrate is used in many Printing-out
papers. This process is discussed further under the
entries for Photogenic drawing negative and Salted
paper print. The chemistry of Development is described
under Calotype or Talbotype. Pure silver halides react
chiefl y to blue and ultraviolet light. To render a balance
of tones, negative emulsions must respond to the entire
visible spectrum, which was achieved by Hermann Wil-
helm Vogel’s introduction of sensitizing dyes in 1873,
extending the response to green light (orthochromatic
plates), and eventually in 1904 to red wavelengths (pan-
chromatic plates).Iron
Many salts of iron(III) with organic acids are photo-
sensitive; Johann Döbereiner fi rst observed in 1831
that a green solution of iron(III) oxalate in sunlight