THE discovery of a process by which objects may be photographed in their natural colours has been the aim of innumerable researches, but as yet no direct method has been found. Methods based on the trichromatic process of colour analysis, which have had a considerable measure of success, are not colour photography, properly so-called. Frederick C. Ives, of Philadelphia, perfected the first successful three-colour process in 1888, utilizing a suggestion which James Clerk Maxwell had made in 1861. In the Ives process three negatives are made through red, yellow and blue screens. By placing these negatives in a special apparatus, lighting each of them by rays of the same colour as the screen by which it was made, and superimposing the resulting images by reflectors, a combined image is obtained in the natural colours of the object photographed. The Ives process is the foundation of three-colour printing; half-tone plates are made from the three negatives from which superimposed impressions are printed in red, yellow and blue inks.
M. Lumiere, in 1907, devised a process in which a film is made sensitive to colours by the use of a potato starch dyed in three colours. Exposure is made as usual, but with the plate reversed, so that the light reaches the sensitive emulsion, first passing through the covering of coloured starch grains. The plate is then developed, and, without fixing, treated with an acid permanganate reducer, rinsed, and redeveloped. The result is a positive photograph in natural colours. This process and others based upon it mark the highest point of achievement in photo-graphic reproduction in natural colours.
Nevertheless, something has been done in colour photography properenough possibly to satisfy those who have given careful attention to the subject that the problem will one day be solved. As early as 1810 observations were made by Dr, Seebeck, of Jena, which tend to encourage the view that direct colour photography is within the range of possibility. He was repeating the experiments made by Ritter in 1801 in respect to the ultra-violet rays of the solar-spectrum. To this end Seebeck passed a beam of white light through a prism, and received the spectrum upon a surface which had been rendered sensitive by means of chloride of silver, and upon which he subsequently perceived distinct traces of colour. Referring to the experiment in the ” Farbenlehre ” of Goethe, he says :
“When a spectrum produced by a properly constructed prism is thrown upon moist chloride of silver paper, if the printing be continued from fifteen to twenty minutes, whilst a constant position for the spectrum is maintained by any means, I observe the following: In the violet light the chloride becomes a reddish-brown (sometimes more violet, sometimes more blue), and this colouration extends well beyond the limit of the violet. In the blue part of the spectrum the chloride takes a clear blue tint, which fades away, becoming lighter in the green. In the yellow I usually found the chloride unaltered; sometimes, however, it had a light yellow tint In the red, and beyond the red, it took a rose of lilac tint. This image of the spectrum shows, beyond the red and the violet, a region more of less light and uncoloured.”
At a subsequent period, that is, after he has. heard of the discovery of photography in 1837 Sir John Herschel was led to experiment in the same direction, and discovered that ” the spectrum impressed upon a paper spread with the; chloride of silver is often beautifully tinted, giving, when the sunshine has been favourable, a range of colours very nearly corresponding with the natural hues of the prismatic spectrum. The mean red ray leaves a red impression, which passes into green over the space occupied by the yellow rays. Beyond this a leaden blue is discovered.” Both Daguerre and Fox-Talbot observed a reddish tinge on pictures from the effect of red objects in the landscape.
Another experimenter who tried to obtain the natural colours of objects by means of photography was Robert Hunt. These researches took place between 1840 and 1843, and the results were set forth in his work, ” Researches on Light,” published in 1844. They are interesting, and in some cases curious; but they led to nothing very definite beyond an expression of ” the hope that we may eventually arrive at a process by which external nature may be made to impress its images on prepared surfaces in all the beauty of their native colouration.”
The next man of any note to embark on the same enterprise was the French savant, Edmund Becquerel. His method was to take a silver plate, such as was used in daguerrotype, and to prepare its surface with a thin coating of chloride of silver. This was done either by soaking it in chlorine water, or by forming a surface of silver chloride by means of hydrochloric acid and chlorine chemically treated. On a plate thus prepared an exposure of a few minutes to the spectrum was sufficient to impress the different colours.
Other experiments of a similar description were tried with equal success. But then came the crux. Like Wedgwood and Davy with their photographic prints fifty years before, Becquerel could not fix the coloured images he secured. Some he appears to have rendered more or less permanent ; but the trouble was that they could only be examined in a faint light, and they would stand the air about as little as the light, oxygen speedily destroying the colours.
As we have seen, Niepce de St. Victor repeated Becquerel’s experiments. He found it preferable to form his chloride of silver by immersing the plate in a solution of chloride of lime. He also shortened the period of exposure. St. Victor sent specimens of his colour-photography to the Exhibition of 1862. They included vivid reproductions of some of the chief colours; but while several of the tints remained for a few hours, others faded almost directly they were exposed to diffused light.
Poitevin, St. Florent, Captain Abney, and others experimented on the lines so ably marked out by Herschel, Hunt, Becquerel, and St. Victor, though without adding much to the volume of our acquisitions in respect to colour photography.
Poitevin, it is true, states that the colours of pictures reproduced on paper rendered sensitive by silver chloride maybe fixed by means of sulphuric acid, although no one appears to have accomplished the feat.
High hopes were raised by the researches of Carey Lea in 1887; but in effect he carried us very little, if any, nearer to the goal of natural colours by photography than we were when, after patient research, Abney gave it as his opinion that the colours obtained upon silver salts by Becquerel and others were due to the oxidation of the silver compounds themselves.
In 1891, however, a new investigator came upon the scene, and in the results obtained by Professor Gabriel Lippmann, of Paris, it must be admitted that we have a distinct advance towards actual colour photography. His method, as de-scribed by him before the Royal Photographic Society, is as follows : The film in which the photograph is taken may be made of any substance, provided it is transparent and grainless. Exposure takes place in contact with a metallic mirror. The effect of the latter, which is formed by running a layer of mercury in behind the film, is to reflect back the incident coloured rays, and thus make the incident light waves stationary. The stationary vibrations, falling in the interior of the sensitive film, impress their own structure upon it, and by virtue of the structure thus imparted, the brown deposit of silver, when viewed by reflected white light, appears imbued with the same colours as are possessed by the image in the camera.
These colours are produced by ” interference ” in the same way as the hues of the soap-bubble or mother-of-pearl. That such is their origin is demonstrated by the fact that the tints of a negative change, when damped, by reason of the gelatine of the film swelling slightly, and thus altering the structure of the silver deposit.
The colours produced by his method, Professor Lippmann holds to be perfectly true if exposure and development be properly conducted. They can, moreover, be completely fixed, so as to resist the action of light and time. Development and fixing are affected in the usual way. But, al-though Lippmann’s process is a decided advance, up to the present it has resisted all attempts to obtain prints from the negatives, and hence, so far as practical colour photography is concerned, we are much where we were.
The three-colour process, therefore, is the only one by means of which photographs in natural colours can be obtained, and, as we have already noted, this process cannot be called in the exact language of science colour photography. In recent years the Lumiere process has been modified and developed by several investigators and it must be admitted that the results obtained are very beautiful, although no means of obtaining prints from the original plates has been devised.
One of the most interesting of recent developments in photography has been the introduction of motion pictures in colour by what is known as the ” Kinemacolour ” process. The results are most graphic and true. The process in a general way may be explained as follows : the moving objects are taken in somewhat the usual manner, but at the same time the exposures are being made, screens of green and red pass before the lens coincident with the various exposures, with the result that automatically negatives are made for colour values, the record being not in colour, but in a scale of grays. For projection on the viewing screen, the positive films are rotated with the colour screens revolving before them in the same sequence as originally taken ; the grays of the colour-value record are thus retranslated into the original tints, which appear on the screen.