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PHOTOGRAPHY IN NATURAL COLOURS
Historical Development. — The first attempts to make coloured photographs date a long way back. Professor Seebeck of Jena, as early as 1810, found that chloride of silver, when exposed to the solar spectrum, became coloured in a corresponding manner. This observation, published in Goethe's " Farbenlehre/' ii. p. 716, passed unnoticed, until, in the year 1841, after the discovery of the daguerreotype, experiments in the same direction were made by Sir John Herschel. He took paper saturated with chloride and nitrate of silver, let a powerful solar spectrum fall upon it, and obtained immediately, like Seebeck, a coloured image of the spectrum, agreeing, however, only imperfectly with the original. Becquerel, in 1848, was more successful. He ascertained that the solution of nitrate of silver in Herschel's experiments had a disturbing effect, and he worked with chloride of silver alone. He employed silver plates, which he plunged in chlorine water. A white film of chloride of silver was thus formed on the plates, and, on exposure to the solar spectrum, an image was obtained, the colours of which agreed very closely with those of the spectrum. Becquerel observed that the duration of the action of the chlorine water was very important, and he preferred at a later date to chlorinate the plates by the electric current. For this purpose he suspended them in hydrochloric acid from the copper pole of a galvanic battery. The current decomposes this acid into chlorine and hydrogen. The chlorine passes to the silver plate, and forms chloride of silver. This method enables the operator to produce a film of chloride