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PHOTOGRAPHY IN NATURAL COLOURS 231
we shall now consider is very generally known as Lippmann's Colour Photography.1 This process was placed before the French Academy of Science in 1891.
Lippmann makes use of a film of mercury as the reflecting surface in the place of the polished silver surface as used by Becquerel. The photographic plate he uses is of the ordinary kind, but having a fine-grained film. This plate is placed in an especially constructed dark slide, in which an arrangement has been made in order that the mercury can be poured in so as to be in contact with the sensitive film while the exposure is taking place. Since this thin layer of clean mercury is to act as a reflecting surface for the incident light, it can readily be seen that the film side of the plate must be turned towards the camera back — i.e. away from the lens. Great care must be taken to ensure the absolute cleanliness of the glass surface before exposing, and when the slide is removed from the camera it is also as well to dust the film with a camel-hair brush which has been moistened with alcohol, as this will tend to remove any adhering mercury. Develop and fix in the ordinary way, but be careful not to carry either process too far. After well washing the plate, it is bleached with mercuric chloride, and then either re-developed with amidol or else blackened with a 10 per cent, solution of sodium sulphite, after which it should be washed and dried. A systematic series of researches on this subject have recently (1908) been carried out by H. E. Ives, and a brief mention will now be made of some of the more interesting of his results.
1 Those wishing to trace the historical development of this process will find the following list of references useful : — (1) Lippmann, Comptes Rendus, 112, 274, 1891 ; (2) Lippmann, Journ. de Physique, 3, 97, 1894 ; (3) Wiener, Annalen der Physik, 69, 488, 1899 ; (4) Neuhaus, Die Farbenphotographie nach Lippmann' s verfahren, 1898 ; (5) Valenta, Die Photographie in Natiirlichen Farben, 1894 ; (6) Lehmann, Beitrdge zicr Theorie und Praxis der directen Farbenphotographie, 1906 ; (7) Ives, Astrophysics Journal, 27, 5. 1908.