HF The History of Photography 600pág

522 The evolution of colour photography
Sir David Brewster's theory of red, blue and yellow as primary colours, which is valid
for pigments only (subtractive colour processes) .
. . .
Ducos du Hauron proposed the taking of three separation negatives behmd red,
blue and yellow filters, and visually superimposing three diapositives behind identical
filters in a viewing instrument or chromoscope.
Collen suggested2 taking three separation negatives, one coated with a substance
sensitive to red only, one to blue and one to yellow. From these he recommended
making colour diapositives, each combination of two superimposed negatives to
produce the colour of the third: and finally to superimpose the three diapositives
upon each other. But, as Collen realized, there were no substances sensitive to one
colour only.
In November 1868 du Hauron applied for a patent for colour processes by the
additive and subtractive methods, which with certain modifications formed the basis
of all later methods. In the former, instead of re-forming the picture by projecting
a positive transparency from each separation in register with the others upon the
same surface, each illuminated by light of a single colour-as Clerk-Maxwell had
demonstrated-du Hauron suggested making the triple operation on a plate through
a glass screen covered with minute red, blue and yellow dots or lines. His book Les
Couleurs en Photographie : Solution du Probleme (I 869) explained this technique in
greater detail, but a full description of this first idea of a screen-plate process had to
wait for twenty-eight years3 -after it had already been put to practical use by other
inventors-with the important difference of using the correct primary colours on the
screen.
In du Hauron's subtractive method three separation negatives were taken behind
green, orange-red and blue-violet filters; from these, positives were printed on three
thin sheets of bichromated gelatine incorporating carbon pigments of red, blue and
yellow colour respectively, i.e. the complementary colours to those by which the
negatives were taken. On treatment with hot water, the parts of the gelatine unaffected
by light were washed away, leaving red, blue and yellow carbon prints
which, when mounted superimposed, formed a colour photograph. Either colour
transparencies or colour prints could be made by this method, depending on whether
the carbon prints were mounted on glass or on paper.
CHARLES CROS, a doctor and writer, had like du Hauron been working independently
on the theory of three-colour photography for several years. By an extraordinary
coincidence he published in Le Monde the correct principle of the subtractive
colour method only two days after the granting of Ducos du Hauron's patent on 23
February 1869. A more detailed account appeared in a pamphlet the same year.4
Like all experi ? 1enter working on the problem of colour photograph, Ducos du
Hauron was. seriously impeded by the comparative insensitivity of plfotographic
negative materials to colours other than blue and violet. Even the most correct theory
was bound to lead to unbalanced colour pictures until good panchromatic material
was available. Ducos du Hauron's earliest surviving colour photograph, a view of
Angouleme, dates from 1877, and though taken after the introduction of certain dyestuffs
which had the effect of increasing the colour range, the picture is far from
perfect. The unequal reaction of the sensitive collodion coating to the three primary
colours was stated by du Hauron5 to be 25-30 minutes behind the red filter, 2-3
minutes behind the green filter, and only 1-2 seconds with the blue-violet filter.
HERMANN WILHELM VOGEL's pioneer work in orthochromatism (see page 332) led
a number of other investigators to experiment on similar lines, with the result that
through new and better colour sensitizers the photographic plate was by degrees