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230 THE ADVANCE OF PHOTOGRAPHY
were found to modify, or sometimes completely change itr so far as the action of light upon it is concerned. Thus,, the addition of the chlorides of lead and tin so alters it,, that when exposed to white light the colour becomes lighter instead of darker, as is usually the case. Again, when sodium salicylate is added, the sensitivity is greatly increased.
An interesting fact with respect to those early attemptsof E. Becquerel is that the colours which he succeeded in obtaining were produced by the action of stationary light waves upon the silver salts in the sensitive film, although the phenomenon was apparently not so interpreted by Becquerel.
In 1868 Zenker developed the theory that the polished metal surface on which the sensitive film was placed formed a reflecting body by means of which the incident light was thrown into a series of stationary undulations. In the loops of these waves the chemical action due to light is a maximum, the silver salts are decomposed and an extremely thin layer of silver is deposited.
The thickness of the film will in all cases be many times the wave-length of the light, hence many of these layers will occur at definite distances apart throughout the substance of the film ; the distance apart of the various layers will, of course, depend upon the wave-length of the incident light.
When such a film is made to reflect light, the colours visible will be the same as those to which the plate was originally exposed — that is, provided white light is used to illuminate the plate when it is used as a reflector. The theory of this has also more recently been developed by Lord Rayleigh.1
Lippmanns Colour Photographs. — The first real practical application of this theory is due to Lippmann, and the method of obtaining photographs in natural colours which
1 Phil. Mag., 5, 26, p. 256, 1888.