THE PHOTOGRAPHY OF COLOUR 367 



is completely covered. As each ink absorbs one of the three 

 fundamental colours, we may describe the grey roughly as one- 

 third black and two-thirds white. If now the three colours are 

 superposed they will produce black, but only one-third of the 

 area of the spot of white paper concerned will be covered, so 

 that the result will be actually one-third black and two-thirds 

 white — practically as before. 



In all the processes so far considered the colour has been 

 produced by light, or its distribution has been controlled by it. 

 But there is a radically different method that has been in the 

 minds of those interested in these matters for nearly thirt}^ 3"ears, 

 and latterly has been worked upon by many investigators with 

 more or less, but on the whole gradually growing, success. 

 The three necessary colours are put on the paper to begin with, 

 and the light destroys or bleaches those that are not wanted. 

 The method depends upon the fact that light can affect a sub- 

 stance only when it is absorbed, and therefore when a mixture 

 of unstable coloured substances is exposed to coloured light, 

 there is always a tendency for those substances that are of the 

 same colour as the light to survive the longest, because they 

 reflect more of the light than the others. The difficulties in 

 working out such a method are many. The three dyes must be 

 of the right colours, they must be about equally bleachable 

 by white Hght, they must be readily bleached or the exposure 

 would be too prolonged, and it is desirable that they shall be 

 rendered more stable after the exposure. Dr. J. H. Smith of 

 Zurich about a year ago put this method on a commercial footing 

 and issued the paper ready for exposure beneath the coloured 

 original under the name of " Uto " paper. The colours are 

 made more sensitive by the addition of anethole, and after the 

 exposure the print is soaked in benzene or acetone to remove 

 the sensitiser. This paper gives surprisingly vivid reproduc- 

 tions of the colour of the original, but obviously the prints 

 are not very stable to light. 



Thus the present practical methods of colour photography are 

 "three-colour" methods, and they do not aim at reproducing 

 colour, but only at imitating it. If the imitation is perfect by 

 daylight it will probably be faulty by artificial light, and a 

 spectroscopic analysis of the two colours will at once show their 

 differences. The perfection of the imitation depends upon the 

 skill of the worker as well as upon the characteristics of the 



