THE PHOTOGRAPHY OF COLOUR 355 



ing waves could be produced in a photographic film, it was 

 argued that deposits of silver should be obtained starting where 

 the amplitude of vibration is greatest, with no deposit at the 

 nodes, and that these layers would reproduce the colours that 

 originated them when they were illuminated with white light 

 and viewed under suitable conditions. This, therefore, would 

 constitute a method of colour photography. It is obvious that 

 for success the film used must be sensitive to all colours, it must 

 have a good reflecting surface to send back the incident light, 

 it must be transparent so that the reflected light may be as 

 nearly as possible of its original intensity and that there may 

 be a negligible amount of scattered light in it, and the sensitive 

 material must be grainless, or very nearly so, for the layers 

 of silver to be produced would be distant from each other, 

 centre to centre, a dimension equal to onl}^ half a wave length 

 of the light producing them. Lippmann solved the problem by 

 coating glass with a suitable emulsion, and exposing it in a dark 

 slide constructed to hold mercury. The glass side of the plate 

 was turned to the light, and the sensitive film was backed up by 

 the mercury which formed the reflecting surface. The evidence 

 of his success lay in the facts that there was an obvious absence 

 of pigmentary matter, the deposit being only grey silver, but 

 that the colours of the original were reproduced as theory 

 indicated by proper illumination and inspection, and that the 

 colours changed by treating the film in any way that would 

 alter the distance between the layers of silver, as, for example, 

 by breathing on the plate and so slightly expanding the film. 

 Perhaps the chief difficulty of this method is in ensuring that 

 when finished the film shall be in exactly the same condition as 

 during the exposure, or at least that the distances between the 

 silver deposits in the finished plate shall exactly correspond with 

 the distances between the nodes of the standing waves of the 

 incident light. The chief drawback to the results is that they 

 must be illuminated and viewed at suitable angles. Many 

 workers improved the process in details, and some very excellent 

 results have been obtained. 



It still remained to actually demonstrate the existence of the 

 laminae of silver in the film. Dr. R. Neuhauss, Mr. E. Senior, 

 and others afterwards, cut transverse sections of Lippmann 

 films and obtained distinct proof of the presence of the laminae, 

 but by far the most complete and successful investigation of 



