October 24th, igig.l Proceedings. xxix. 



colour-filters respectively, and pointed out that the component 

 colours could be reproduced by superposing the three images, 

 each illuminated by appropriately coloured light. Maxwell 

 remarked that " by finding photographic materials more sensi- 

 tive to less refrangible rays the representation of the colours 

 of objects might be greatly improved." Long after Maxwell's 

 day it was discovered that the ordinary photographic plate can 

 be rendered sensitive to green, yellow and red light by 

 incorporating certain dyestuffs with the material of the sensitive 

 film; previous to the war all the various methods of colour 

 photography — the first of which was devised by Professor Joly, 

 of Dublin — the modern processes of photographic colour- 

 printing, and the present-day panchromatic photographic 

 methods for obtaining a correct rendering in monochrome of 

 parti-coloured objects, were based upon the success which had 

 been attained in imparting sensitiveness throughout the visual 

 spectrum to the ordinary blue-sensitive photographic plate. 

 By staining the plate with erythrosine it becomes sensitive to 

 green and orange ; plates so treated are termed orthochromatic. 

 A number of dyestuffs belonging to the class of cyanine dyes 

 discovered by Greville Williams, in 1856, are capable, however, 

 of sensitising a photographic plate throughout the whole range 

 of the visible spectrum. These substances are difficult to make 

 and but little concerning their preparation and chemical 

 behaviour has been published ; their photographic applications 

 were carefully studied by several of the large German coal-tar 

 colour firms, and those most suitable for use in colour photo- 

 graphy were selected and put on the market under trade names 

 which disguised their chemical identity. At the outbreak of 

 war the Allies were entirely dependent on Germany for these 

 so-called photographic sensitisers ; such substances had never 

 been made in this country and but little information as to the 

 identity of the chief sensitisers and their methods of preparation 

 was available. A little acquaintance with scientific photography 

 suffices to show that this situation not only affects such aesthetic 

 industries as those of artistic photography and colour-printing 

 but is also vital to aeroplane photography. 



Thus, a distant scene in bright sunlight is always seen to 

 be obscured by a slight haze ; this haze is enormously intensified 

 when the scene is photographed on an ordinary blue-sensitive 

 plate, intensified to such an extent that the photographic 

 reproduction of the haze may blot out the whole distant view. 

 The reason for this is that a clear atmosphere is less penetrable 

 by blue light than by light from the less refrangible end of 

 the spectrum. The blue-sensitive plate cannot see far into the 

 distance because of the comparative opacity of the clear 



