It is an interesting fact that some of the first recorded experiments in photography were directed to obtaining results in natural colours. More than fifty years before photography, as we know it, was invented, such experiments were being made; and when the researches of Daguerre and Fox-Talbot culminated in the production of permanent light-pictures in monochrome, the scientists of the day, while giving them due credit for their epoch-making discoveries, declined to regard photographs without colour as the realisation of their dreams; and the search for a means of photographically reproducing the colours as well as the forms of objects was pushed forward with redoubled vigour. For, after all, nature and art alike appeal to us as much by beauty of colour as by beauty of form, and no method of pictorial representation can be considered as perfect, which, while reproducing the one, fails to convey any adequate impression of the other.
The aim of the early experimenters was to discover how to prepare a photographic surface that would not merely darken in varying degrees under the influence of light, but would directly reproduce the colours of the image thrown upon it. The substance they were seeking proved as elusive as the philosopher's stone of the ancients. Some faint resemblances to the colours of the spectrum were obtained on a surface prepared with chloride of silver; but these results were very imperfect, and no means could be found of fixing the tints.
Other investigators, quite apart from photography, had in the meantime been studying the nature and properties of colour; and in 1860 Prof. James Clerk Maxwell demonstrated conclusively that all colours could be produced from three simple or primary colours - red, green and blue-violet. This discovery suggested a solution of the problem of photography in natural colours. The search for a substance that would permanently reproduce the colours of the light rays thrown upon it had failed, but perhaps it would be possible to photograph separately the three primary colours, and by combining them produce a complete colour photograph. Prof. Clerk Maxwell himself experimented on these lines, and in 1861 displayed on the lantern screen at the Royal Institution, in the course of a lecture, a crude colour-photograph obtained by the method indicated. He did not follow the matter up, however, and it was left for an exceedingly able French experimenter and inventor, Louis Ducos du Hauron, to elaborate a practical system of three-colour photography, which he published in 1869. Ducos du Hauron, in his work, anticipated all the really successful methods of tricolour photography of the present day, including the easy and popular screen-plate processes, and may justly be called the father of practical colour photography.
It may be well, at this point, to state briefly the scientific facts underlying these methods of producing colour photographs.
Colour is purely a sensation produced upon the eye by rays of light. Without light there can be no colour, nor can we be conscious of any colour that is not contained in the rays of light falling upon the object we term coloured. It follows that, as our natural source of light is the sun, all the colours of nature are contained in sunlight. These infinitely varied colours, as Clerk Maxwell showed, are equivalent to mixtures of the three primaries - red, green and blue-violet - so it is convenient for our purpose to consider sunlight as being composed of red, green and blue-violet rays in equal proportions. These three, so combined, produced the sensation of white light, and a sheet of white paper, or a whitewashed wall, appears white because it reflects all the rays equally. Some surfaces, however, do not reflect all the rays, but absorb some of them. For instance, a red rose absorbs the green and blue rays, reflecting only the red ones; consequently it produces on the nerve fibrils of our eyes the sensation of red. The grass absorbs the red and blue rays, leaving only the green to be reflected, so it appears green to us; while the blue corn-flower absorbs the red and green, and we therefore say it is blue. The same may be said of transparent objects; for instance, one piece of glass is blue, because it absorbs the red and green rays and only allows the blue ones to pass through it; another is red, because it absorbs the blue and green rays; a third is green, because it absorbs the red and blue rays.
It is this property of transparent substances to absorb some rays and pass others that is utilised to produce colour photographs. If before exposing our photographic plate we place before the lens, or between the lens and the plate, a transparent medium which absorbs the blue-violet and the green rays, we shall get a photograph of only those objects or parts of objects before the camera that are reflecting red rays. If we take another photograph through a transparent medium absorbing the red and blue-violet rays, it will represent only those objects or portions of objects which are reflecting green rays; and if we make a third exposure through a medium which absorbs the red and green rays, the result will be a photograph of those objects or portions reflecting blue-violet rays.
But although the theory of three-colour photography was practically complete before the year 1870, many difficulties had to be overcome before practical success could be achieved. Endless experiments had to be made before the exact colours necessary for the light-filters could be arrived at, but the chief obstacle was that all the photographic plates of those days were far too sensitive to blue-violet rays, very little sensitive to green, and quite insensitive to red.