light by turning black in color, they could capture
a transient image. Their process involved the sen-
sitization of a copper plate. By exposing the plate
to iodine vapor, a coating of the light-sensitive
silver iodide would form on the plate. They then
used this plate in place of the canvas or paper in
the camera obscura, and would expose it for a
period of time to the chosen scene. The difficult
part, Daguerre found, was making that image
permanent so that it could be viewed under the
same light that created it. He discovered that he
could develop the image by using heated mercury.
The mercury vapors would cling to the areas that
had been previously exposed. The greater the
amount of exposure in a given area, the greater
the concentration of mercury would be. The
image was then made permanent or fixed in a
solution of sodium thiosulfate, known historically
as hyposulfite of soda, the derivation of the dark-
room term ‘‘hypo.’’
The resulting product, called a daguerreotype,
was an image of stunning quality and detail. The
silvered image was at once positive and negative,
depending on how it caught the light. While more
sensitive than Nie ́pce’s original emulsion of bitu-
men of Judea and lavender oil, daguerreotype still
required that the subject remain motionless for
several minutes. In addition, the process was
incredibly difficult and inherently hazardous.
Despite these factors, the popularity of daguerreo-
types grew to staggering proportions, particularly
for portraiture.
The Calotype
At the same time, an Englishman by the name of
William Henry Fox Talbot was working on his own
technique. Instead of a metal plate, Fox Talbot sen-
sitized paper by saturating it alternately with com-
mon salt (sodium chloride) and silver nitrate,
producing silver chloride. This compound gained
density in direct proportion with the amount of
light striking it. The resulting negative images were
also fixed in a hypo solution. Fox Talbot then took
these paper negatives and placed them on top of
another piece of sensitized paper, and exposed the
negative sandwich to bright sunlight. The second
paper was then developed and fixed, resulting in a
positive image called a calotype. Unlike the daguer-
reotype, multiple identical images could be created
from the same negative. However, the nature of the
paper was such that the images lacked the clarity and
detail of daguerreotypes.
Wet Collodion
The next advance in photographic materials was in
1851 with the wet collodion process. Fredrick Scott
Archer, also of England, made use of collodion, a
substance that suspended the light-sensitive particles
within a sticky emulsion that could be poured over a
glass plate. When dry, the emulsion became a tough
transparent suspension for the latent exposed image,
whichwasthendevelopedinpyrogallicacidoriron
sulfate. The plate was then fixed, washed, and dried.
The resulting negative on a transparent base pro-
vided the ability to make multiple positives of the
images, like Fox Talbot’s calotype. However, the
image also had the clarity of the daguerreotype.
Soon, the wet collodion process would replace both
the calotype and the daguerreotype as the dominant
photographic process.
The disadvantage of the wet collodion process
was that the plate was only sensitive while the emul-
sion was still wet. That meant the photographer
would need a veritable darkroom wherever he was
going to take the picture. Despite this drawback,
the process was used widely in some of the first
examples of photojournalism during the American
Civil War. War photographer Mathew Brady do-
cumented never-before-seen images of war with a
traveling darkroom inside a horse-drawn wagon.Color Films
Meanwhile, avenues for reproducing the colors of
reality on film were being pursued. James Clerk
Maxwell, a Scottish physicist, demonstrated that
all colors could be represented through combina-
tions of the three additive primaries (red, green,
and blue). It was not until 1907, however, that
practical commercial color photography was
achieved by brothers Auguste and Louis Lumie`re.
The duo were inventors of the autochrome process,
which involved the integration of several layers of
dyed starch grains (red, green, and blue) with a
coating of photographic emulsion over a glass
plate. The results were impressionistic, with poin-
tillism-like dots of color.
Kodachrome was the product of two research
scientists associated with the Eastman Kodak Com-
pany. Using a subtractive process, Leopold Mannes
and Leopold Godowsky came up with a way to
manufacture an emulsion containing layers sensitive
to each primary color (red, green, and blue). The
result was the first example of realistic color in photo-
graphy when Kodachrome was introduced in 1935.FILM