With his preceptor, Louis Ducos du Hauron, when he was a teenager, became initiated into pictorial art. He already knew that mixing blue, yellow and red in equal proportions produced neutral grey. But by dosing these colours in unequal proportions quite different colours were produced in an infinite number of nuances. Hence the idea at first confused in his mind of physicist, to separate the three colours and to reunite them afterwards. By layering them, he thought, the initial colour of a landscape would be restored.
He was not really a photographer and until then had never handled a photographic chamber. But he was interested in the discovery of the Nièpce brothers who had already found a technique to reproduce a landscape not on a canvas with a colour palette and a brush.
But the reproduction was only in black and white and even rather sepia.
It was the starting point of thorough research that lasted seven years before being successful. He developed beforehand his research on animated images that heralded the cinématograph.
He immersed himself in his physics books to familiarize himself with light and its interactions with colours, and optics, as well as his chemistry books to discover the philosophical emulsion capable in addition to black and white of also fixing colours. His objective is clear: capture in the darkroom the ephemeral rainbows, that Newton knew how to create, that sometimes draw their arches in the rain clouds with the complicity of a facetious sun.
In fact, Louis Ducos du Hauron moved towards not an additive system already unsuccessfully and unsatisfactorily experimented by R. Hunt, Thomas Yung or still the Englishman James Clerk Maxwell (1800-1874) who, as for them, resorted to three filters, red, green and blue. Although they did obtain a coloured image by projection, it was nevertheless imperfectly coloured since reds were lacking.
As for Ducos du Hauron, he decided to resort to the substractive method.
He hoped to obtain in that manner a perfect trichrome, in other words a polychrome photograph.
In fact Louis Ducos du Hauron started from Chevreul’s basic principle according to which all the colours with their varied nuances proceed from varied dosages of three basic layers, red, yellow and blue. The colours of his initial painter’s palette By mixing such colours close to 1,500 nuances can be obtained. A combination he applied with three coloured glasses he had had cut at a mirror cutter’s. This was the idea of what we call filter today.
The blue or cyan glass stops yellow rays of light, as for the yellow glass, it stops blue rays and the red or magenta glass stops blue-green rays.
Louis Ducos du Hauron behind his photographic chamber
Starting from this basic observation, he acquired a chamber whose focal length he modified at the beginning by adding a simple spectacle glass. Although his first trials confirmed the efficiency of his filter system, they were not satisfactory to his liking. Since by exposing in turn three negative plates, you cannot obtain three perfectly identical pictures of the same photographic subject, and then superimpose them sandwichlike.
Never mind! Louis Ducos du Hauron, far from losing heart and with a dogged will to succeed, kept at his task tirelessly, "Penelope-like".
His first investigations did enable him to note that the process was the right one. It needed to be improved.
In particular, the filtering of the three coloured glasses had to be refined because the glasses did not present an ideally plane face. This brought about a parallax effect. Neither did their colour present the perfect nuance.
He then resorted to tanks with perfectly parallel sides full of the coloured solution close enough to the three initial colours. Although it was difficult to use, his new trichrome process, he himself named heliochromic turned out to be more judicious to achieve his three successive negatives, each of which was exposed by a single colour in the different natural nuances of the photographed subject.
More or less red, yellow and blue in accordance with lighting provided by natural light.
Three transparent negatives the ideal sensitive coating for which had to be found.
There again, Louis Ducos du Hauron multiplied investigations to find in the chemical product range the ideal product exposed by rays of light filtered by his darkroom.
He used in turn bituminous coal, collodion, a cotton powder solution initially meant for surgery because of its agglutinative properties.
He was a perfectionist. He did not stop until he found the ideal gelatin to cover his initially neutral plates.
He obtained silver bromide mixed with eosin, another medical mixture that proved most adapted to retain colours and their various nuances on each of these three negatives that were exposed in part by real light beams.
Once developed into positive, his three plates were in fact the ancestors of the slide then named Diaphanie (literally: transparency).
Superimposed and projected simultaneously they then produced an immaterial coloured picture on a screen.
But Louis Ducos du Hauron did not stop his research after achieving this first result, however satisfactory it was, because it enabled him to obtain three monochrome positives from three prints. As proved by research recently carried out by the Synchrotron of Grenoble. A multidisciplinary team made up of European specialists of l'ESRF, du CNRS, of C2RMF, of musée d’Orsay, of l’École nationale supérieure (ENS) Louis-Lumière, of the Faculty of Science and engineering of Sorbonne-Université, of chemistry Paris-Tech, and of a specialised photographic conservator, with innovative means, hardly possible until then. They studied various photographs of Louis Ducos du Hauron and thus discovered chemical products and pigments, trichrome coal, potassium dichromate, Prussian blue, chrome yellow, carmine red and alizarine madder varnish, he had in turn tried and retained to improve the fixing of a coloured image on the glass plates of his darkroom, thus bringing a compelling proof, if need be, of exclusive authorship of Heliochromy he was entitled to.
Louis Ducos du Hauron also continued to try and improve his shooting. He thus replaced his initial darkroom by the Melanochromoscope then a more efficient Chromographoscope whose model he made a precise drawing of and that he had had built by a Parisian specialist after filing a patent.
To make it simple, without falling into the trap of an obscure optical and physical technique, this camera makes it possible, thanks to an initial prism and a system of mirrors, to leave simultaneously an imprint and not one after the other, on the three negative plates of the same exterior image, but each time separately and exclusively filtered.
Respectively, each negative has its red, yellow and blue filter.
Louis Ducos du Hauron was thus assured to obtain three formally identical and super-imposable images without risking a lag of one of the three photographs.
Coteaux de l'Ermitage (hillside) in Agen
The other advantage of the process is to reduce exposure that initially took Niépce a whole day.
« On approche du but, on l'a atteint! », s'exclame enthousiaste son frère Alcide.
Faubourg de l'Ermitage à Agen
Le premier concerne « Les sensations lumineuses ». Le second « Les couleurs en photographie, solution du problème » ne sera rédigé qu'une fois qu’à la place de la photo projetée et immatérielle, il réalisera une épreuve sur papier polychrome à l'aide de ses trois filtres monochromes, reproduisant ainsi les couleurs de l'original.
Ces deux héliochromies qu'il présentera publiquement ne sont autres que ses deux vues d'Agen.
La ville photographiée en surplomb depuis les hauteurs du coteau de l'Ermitage et le coteau de l'Ermitage pris en contrebas.
Alors seulement Louis Ducos du Hauron peut confier satisfait:
« Le soleil n'a pas trompé mon attente. Il fait un usage judicieux de la palette réduite à trois couleurs ».
Il a bel et bien inventé, mais aussi réalisé, en exclusivité, la première photo en couleurs du monde et à Agen et mérité ce surnom que lui donne la Société française de photographie : « Le jeune savant du Midi ».