igoi-2. ] Photography in Natural Colours. 371 



PHOTOGRAPHY IN NATURAL COLOURS. 



13Y J. S. PLASKETT, B.A. 



(Read i^th March, igo2.) 



The subject to be discussed in this paper has, ever since the 

 discovery of photography some sixty years ago, excited the keenest 

 interest and attention, not only among photographers and scientists, but 

 among the lay public as well. From time to time, results have been 

 obtained which have led many people to believe that the problem was 

 approaching a solution. Again and again glowing reports have been 

 published stating that the long-looked-for process had at last been 

 discovered. But in nearly every case it has proved that the colours 

 obtained were either unlike, or, if like, were not dependent upon the 

 colours of the light waves which produced them ; and it is very 

 doubtful whether any real progress towards realizing a practical solution 

 of the problem of obtaining a direct photograph in colours has been 

 made. 



The nearest approach to such a solution is reached by the 

 Lippmann process, in which the colours are produced by the inter- 

 ference of light, this interference giving rise, in the taking process, to 

 what are known as standing waves in the photographic film. These 

 standing waves cause a peculiar, laminated structure in the deposit of 

 silver on the plate, the position of the laminae corresponding to the 

 lengths of the waves, and hence to the colours, that give rise to them. 

 The explanation of the colours seen, when such a plate is viewed by 

 reflected light, is quite similar to that accounting for the colours of thin 

 films such as soap bubbles. The theory is not, however, perfectly 

 complete and satisfactory as the cause of certain abnormalities in the 

 process is not evident. The true colours can only be seen when the 

 heliochrome is viewed by reflected light at normal incidence, and are 

 hence not very easy to observe. Probably the most satisfactory way of 

 viewing it is to strongly illuminate the surface and, by means of a lens, 

 form an image of this surface upon a screen. The technical difficulties 

 of the process are very great, so great, indeed, that, during the ten years 

 it has been discovered, only comparatively few good examples of 

 interference heliochromy, as it is termed, have been produced. 



