Manchester Memoirs, Vol. Ix. (191 6), No. 9. 3 



out film, and we shall obtain a reproduction of the 

 coloured original in its own colours direct and not from 

 an intermediate negative as in ordinary photography. 



The accompanying coloured plate {Fig. a) shows the 

 action of light upon a bleach-out film containing three 

 dyes, not of the primary colours as Liesegang suggests, 

 but of the secondary colours yellow, blue-green and pink, 

 as these are necessary on account of the greater lumin- 

 osity of the colour mixtures ; the bleach-out process being 

 based upon the subtractive process of colour mixtures 

 like three-colour printing. The three dyes in the film are 

 supposed to be applied in three separate layers in order 

 to simplify the demonstration of the process. The same 

 principle holds good in the case of dyes mixed up in one 

 layer, the process more generally employed. The coloured 

 original to be copied may be a natural spectrum, coloured 

 glasses, a coloured window transparency, natural leaves or 

 flowers pressed flat, a photographic three-colour trans- 

 parency or a Lumiere Autochrome photograph. In the 

 last-named subject, the most difficult of all to print, the 

 starch granules employed in the manufacture of the plate 

 are coloured in the three primary colours orange-red, 

 green and blue-violet, allowing only the light of the 

 visible spectrum to pass of approximately 7,000 — 5,900, 

 5,900 — 5,000, and 5,000 — 4,000 wave length respectively 

 as shewn in Fig. a. The position of the secondary colours 

 yellow and blue-green have been indicated by narrower 

 bands, while the pink not being a spectrum colour has 

 been shown outside. Narrow bands of black and of 

 transparency to the whole visible spectrum (white) have 

 also been added to show the effect of these also on the 

 bleach-out film. 



In the case of the transparent band all three dyes are 

 bleached out and we obtain a colourless band in the copy. 



