NATURAL COLOURS, PHOTOGRAPHY IN This subject is of such vast extent and is divided into so many branches, which are treated of individually, that a mere sketchy outline of the whole subject can be given here. The subject may be divided into four main heads : (a) direct heliochromy, (b) interference heliochromy, (c) the bleach-out process, and (d) three-colour work.
Direct heliochromy and interference heliochromy. --Senebier, in 1785, had pointed out that when a spectrum was thrown on to silver chloride, violet and blue were reproduced, but he carried the experiments no further. However, in 181o, Seebeck, the great German physicist, sent to the poet Goethe a treatise of the action of coloured illumination (" Wirkung farbiger Be leuchtung ") which is printed in Goethe's Geschichte der Farbenlehre, Vol. II., p. 716. Seebeck details the effect of allowing " the spectrum from a perfect prism to fall for 15 to zo minutes on white damp silver chloride spread on paper and kept by a special device in the same place. I found the silver chloride to be altered in the following manner : in the violet it became reddish-brown (sometimes more violet, sometimes more blue), and even beyond the previously marked limits of the violet this coloration extended, but it was no stronger than in the violet ; in the blue of the spectrum the silver chloride had become pure blue, and this colour extended, increasing and becoming brighter right into the green ; in the yellow I found the silver chloride frequently unchanged, but occasionally it happened to be more yellow than at other times ; in the red, on the other hand, and frequently beyond the red, it had assumed rose-red." Seebeck then describes the effect of two spectral lights—red and violet—and also tests with coloured glasses. The importance of this work must not be overlooked, for it was , thirty years before the discovery of the daguerreo type, and the results were photographs on paper. But little notice seems to have been taken of this work, though this was probably due to its being published in Goethe's book, which is very poor in facts but rich in arguments. Seebeck's work also confirmed Ritter's statement (1801) that there were invisible radiations at both ends of the spectrum.
John Herschel, the son of the renowned astronomer, was the next to discover anew the curious property of silver chloride of reproducing colours (" Philosoph. Transactions," 184o, p. 28, Athenceum, 261), and followed Fox Talbot's suggestion of using alternate baths of silver nitrate and sal ammoniac, but Herschel pre ferred to sensitise his paper just before use, and pressed it in the camera obscura against a glass plate, which prevented any wrinkling of the paper.
He did not succeed in fixing these colours ; yet they were half fixed by merely washing with water, and could then be examined by diffused daylight or lamplight without deteriorating. Herschel also discovered that silver bromide gave the same colours as, though less distinct than, the chloride, whilst silver iodide gave the com plementary colours. Robert Hunt, in his Re searches on Light, details numerous experiments both on silver and other substances, and records the occurrence of colours; but the next investi gator to warrant attention was Edmond Bec querel, and to his papers (Annales de Chimie et de Physique, third series, Vols. XXII., XXV., XLII., 1849-1855) we are indebted for the real foundation of successful heliochromy ; his re searches are collected into two volumes : La Lumilre et ses Eflets. He exposed to the spectrum silver plates coated with silver chloride which gave a fairly satisfactory rendering, but the uneven film of chloride caused unevenness of colouring ; ultimately, he found it better to obtain an electrolytic deposit of chloride. He also used a solution of cupric chloride (copper sulphate I part, salt 3 parts, water io parts), and in this the silver plates assumed a violet white coloration, and gave, on subsequent ex posure, not only the spectral colours, but white also. He says : " I think that the substance formed on the surface of the metallic silver is a special chlorine compound, perhaps violet silver subchloride or a mixture of white silver chloride and subchloride." Niepce de Saint Victor carried on similar experiments to those of Becquerel ; Eder states that one of his heliochromes in his possession still shows brilliant colours, forty years after its preparation. Niepce started with the theory that those chlorides that gave a definite colour to a Bunsen flame were the best for reproducing the colour when used for chlorising the silver plate. He not only obtained the spectrum, but also copied objects in the camera and obtained excellent reproductions of the shimmer of glass and polished metal objects, and also reproduced black, which in many cases reflects the ultra violet or infra-red or both. He used a varnish of dextrine and fused lead chloride, which was a great protection, and this was further in creased by subsequently coating the plate with tincture of Siam benzoin, and heating till some of the benzoic acid was driven off ; his results are recorded in the Comptes Rendus, 1851— 1859, etc.