36o SCIENCE PROGRESS 



arranged in fine lines or otherwise on one plate. In the same 

 year he showed some results at a meeting of the Societe 

 Frangaise de Photographie. By a curious coincidence, at the 

 same meeting, Charles Cros entered into great detail as to 

 the various possible methods of trichromatic photography. But 

 Cros appears to have contented himself with suggesting how 

 others might succeed, while Du Hauron put his notions to the 

 test of experiment. Moreover, it appears that Cros was wrong 

 in his colours, recommending pale tints of red, yellow, and blue 

 for the screens for getting the three negatives. 



Work of this kind was very difficult because of the insensi- 

 tiveness of the collodion plates then employed to green and 

 especially to red light. It was not until 1873 that the sensitising 

 effect of certain dyes upon photographic plates was discovered 

 by Prof. H. W. Vogel. Now by this means plates can be made 

 usefully sensitive even into the infra-red. The possibilities of 

 special sensitising, and the introduction of gelatine dry plates 

 about 1878, cleared away many difficulties that sorely hampered 

 and might well have discouraged the earlier workers. From 

 this time the development of three-colour photography was a 

 matter of perfecting details rather than evolving new principles, 

 and devising methods for converting laboratory experiments 

 into commercial methods. 



The additive method — the three coloured images being 

 separately illuminated — was made into a really practical method 

 by Mr. F. E. Ives, of Philadelphia. Mr. Ives first exhibited his 

 apparatus in London in 1892, but four years before this he had 

 demonstrated the capabilities of the process at the Franklin 

 Institute in Philadelphia. In 1893 his " photo-chromoscope,'' 

 afterwards called " Kromskop," was on the market. The appa- 

 ratus that he devised for the purpose consisted of a camera 

 for taking the three colour records, a triple lantern for projecting 

 the picture on to the screen, and the photo-chromoscope. This 

 last is a compact apparatus containing three coloured glasses, 

 one for each colour record as required, and a system of mirrors, 

 so that the three coloured images are seen superposed b}' 

 looking in at the eye-piece. Shortly afterwards the apparatus 

 was made stereoscopic. The reproductions as seen in this 

 apparatus are remarkably realistic, and I do not think that 

 any method has surpassed them. The one drawback is that an 

 apparatus has to be used for seeing the pictures. Mr. Ives says 



