The search for a direct color-sensitive medium continued, but only once was success in sight: in 1891, Gabriel Lippmann, professor of physics at the Sorbonne, perfected his interference process, which relied upon the phenomenon that a thin film, such as oil upon water, will produce all the colors of the rainbow. The results were startling. Steichen in 1908 wrote Stieglitz:
Professor Lippmann has shown me slides of still-life subjects by projection, that were as perfect in color as in an ordinary glass-positive in the rendering of the image in monochrome. The rendering of white tones was astonishing, and a slide made by one of the Lumiere brothers, at a time when they were trying to make the process commercially possible, a slide of a girl in a plaid dress on a brilliant sunlit lawn, was simply dazzling, and one would have to go to a good Renoir to find its equal in color luminosity.
Unfortunately the Lippmann process was not a practical technique, and is now obsolete.
The practical solution of photography in color was found in an indirect approach.
The British physicist James Clerk Maxwell performed a dramatic experiment at the Royal Institution of London in 1861. To prove that any color can be recreated by mixing red, green and blue light in varying proportions, he projected three lantern slides of a tartan ribbon upon a screen. In front of each projector was a glass cell filled with colored solution: one was red, a second, blue and the third, green. Each slide had been made from a negative which Thomas Sutton had taken through the identical glass cells or filters; each was theoretically a record of the red, blue and green rays reflected by the ribbon. The result was a color photograph — crude, but prophetic of the future.
Because Clerk Maxwell added red, green and blue light together, this technique is called additive. An equal addition of the three colors forms white; red and green add to form yellow; red and blue, magenta; green and blue, the blue-green known by photographers as cyan. It is important to bear in mind that this theory holds true only for colored light; the mixture of pigments is another matter.
Maxwell’s results were imperfect because the iodised collodion emulsion which Sutton used was not sensitive to all colors. When panchromatic emulsion was invented, his theory was put to practice with success. It is inconvenient to set up three magic lanterns whenever a color photograph is to be looked at. A portable apparatus, the Kromskop, was devised in 1892 by Frederick E. Ives of Philadelphia, which optically reunited three transparencies so that they could be viewed in proper register by looking through an eyepiece.
Still, people could not look at these transparencies as easily as they could look at a black and white photograph. The first practical method of making a single picture which could be viewed without any apparatus was invented in 1893 by John Joly of Dublin. Instead of taking three separate pictures through three colored filters, he took one negative through a screen minutely checkered with microscopic areas of red. green and blue. The screen was the exact size of the photographic plate and was placed in contact with it in the camera. After the plate had been developed, a transparency was made from it, and this was bound permanently to the color screen. The black, gray and white areas of the picture allowed more or less light to shine through the filters; if viewed from a proper distance the primary colors so modulated blended to form combinations reproducing the colors of the original scene. In 1903 the same principle was used by the brothers Lumiere in their autochromes which were put on the market in 1907. The photographic plate itself was covered with minute grains of starch which had been dyed. One third were red, one third green and one third blue, and they were mixed together before application to the plate so that the three primary colors were evenly distributed over the surface. After development the negative was turned into a positive by the reversal process, and a transparency resulted which reproduced the original colors. Dufaycolor combined these two techniques; a film was ruled to form a multiple filter somewhat similar to the Joly screen, and the image was reversed as in the autochrome process.
These methods have given way to techniques based on the subtractive theory.
A black object absorbs, or subtracts, all of the light falling upon it: nothing is reflected to the eye, and hence it looks black. A white object reflects all of the light rays falling upon it. If white light shines upon it, white light is reflected; if red alone shines upon it, red is reflected. A colored object, however, absorbs, or subtracts, some of the rays and reflects others. When white light falls upon a red object the green and the blue rays are subtracted: when it falls upon a cyan object, the red rays are subtracted, while the blue and green are reflected. But when red light falls upon a cyan object there are no blue or green rays to be reflected and the red rays are entirely subtracted. No light reaches the eye, and the object appears black.
Transparencies printed from negatives taken through red, blue and green filters will, if tinted in the respective complementary colors (cyan, yellow and magenta), superimposed in register and held against the light, reproduce all the natural colors in the scene which was photographed.
As early as 1869 Louis Ducos du Hauron wrote a remarkable little book, Les Couleurs en photographic; solution du probleme, in which he envisaged accurately not only the additive methods but also most of the subtractive processes employed today, at a time when satisfactory materials were not available. His summary of the subtractive technique could serve as an introduction to any textbook written today:
To obtain, by photographic techniques already known, and by the interposition of three colored media, three monochrome prints, one red, one yellow, the third blue, and then to form, by the superimposition or the mingling of these three prints, one unique print in which will be found reproduced at once the color and the form of nature.
Many variations of this basic principle have been devised. Prints can be made from each of the three color separation negatives, as they are called, on paper sensitized with emulsions containing cyan, yellow and magenta pigments. The emulsion is stripped from each print and the three are superimposed in register on a fresh piece of paper. This technique, a modification of the carbon process, is called carbro. Another method of superimposing the three images is by dye transfer. A gelatin matrix is prepared which will absorb dye in proportion to the lights and shadows, and which will yield up this colored image when pressed in contact with paper.
These techniques require three separate negatives. Where the subject is stationary, the exposures can easily be made in succession, but in photographing moving objects they must be made simultaneously. “One-shot” cameras have been devised, fitted with half-silvered mirrors, which will allow this to be done. But this apparatus is cumbersome, expensive, and inefficient.
In 1935 color films were introduced which eliminated the necessity of making more than one exposure and which could be used in any camera. The first to be announced was Kodachrome film, invented by Leopold Mannes and Leopold Godowsky and manufactured by the Eastman Kodak Company.