Manchester Memoirs, Vol. Ix. (1916)", No. 9 



IX. A resume of work on the bleach-ouitvprofeeiis 

 of colour photography. V *, 



By John H. Smith, Ph.D., F.I.C., A.R.CSc.I 



( Communicated by Mr. R. L. Taylor, F.C.S., F.I.C.) 



( Read February 22nd, igi6. Received for publication March 2jrd, 191 6.) 



The bleach-out process of colour photography is 

 based primarily upon the instability of certain natural 

 and artificial colouring matters to light. The fading 

 action of light upon many coloured bodies was observed 

 long before artificial dyes were discovered. 



Grothuss (1) in 18 19 seems to have been the first to 

 have attempted to formulate the nature of the action of 

 coloured light upon bodies. He writes : " Coloured light 

 seeks to destroy, in bodies upon which it acts, those 

 colours which are opposed to its own while it endeavours 

 to retain its own or another analogous colour." If instead 

 of " opposed " we substitute complementary colours, we 

 have the first reference to the fundamental principle 

 underlying the bleach-out process of colour photography. 



Sir John Herschel (2) writing on the same subject in 

 1842 says : " The rays effective in destroying a given tint 

 are in a great many cases those whose union produce a 

 colour complementary to the tint destroyed, or at least 

 belonging to that class of colours to which such comple- 

 mentary tints may be referred. For example, yellows 

 tending towards orange are destroyed with more energy 

 by the blue rays, blues by the red, purples and pinks by 

 yellow and green rays." 



July 5th, igi6. 



