8 J. H. SMITH, Bleach-out Process of Colour Photography. 



Sufficient and equal natural sensitiveness to light. 

 Amenability to the influence of known sensitisers. 

 Bleaching in regular intervals from the full colour 



always through the same shade of colour to 



white. 

 No recuperation of the colour after printing. 

 The possibility of fixing the remaining dyes 



thoroughly after printing. 



The first and last of these conditions are hardly com- 

 patible with each other : a compromise has to be made. 

 I know of some fugitive dyes which can be sensitised to 

 such an extent as to give prints from Autochromes in 

 5 — 10 minutes in strong sunlight, but it seems so hopeless 

 to attempt to find a satisfactory method of fixing these 

 dyes as to preclude their use altogether. I prefer to 

 employ dyes of fair natural stability, amenable to being 

 strongly sensitised, and to eliminate the sensitisers well 

 after printing. 



In order to obtain sufficiently regular bleaching of a 

 dye there are some conditions which must be fulfilled 

 which have not been taken fully into account by writers 

 on this subject. On referring again to the coloured plate 

 it will be seen that the theory of the process requires that 

 a dye should be stable in the light of about half of the 

 visible spectrum and bleach-out in the other half of com- 

 plementary colour. Now this is far from being an ordinary- 

 property of dyes. They usually absorb light in narrow 

 bands and this tends to produce degraded colours in 

 bleaching. There is a spreading out of the absorption 

 bands on prolonged exposure so that we get an approxi- 

 mation to the desired conditions. This spreading out of 

 the absorption bands on increased exposure occurs, how- 

 ever, in many cases only on one side of the spectrum, a 

 solid wall being built up on the other side beyond which 



