2 J. H. Smith, Bleach-out Process of Colour Photography. 



Becquerel (3) confirmed in 1848 in a general way 

 Herschel's conclusions. 



Draper's law, which affirms that chemical action by 

 means of light can only result from the light rays 

 absorbed by a body, and not by those it transmits or 

 reflects, must lead one also to the same conclusions. 

 This law, according to Luther, should be attributed to 

 Grothuss, as the latter affirms (4) that " this absorption in 

 the substance of the body must be the cause ; at any rate 

 in many cases, of a considerable chemical action." 



Liesegang (5) in 1889 was the first to propose a pro- 

 cess of colour photography based upon the bleaching out 

 of dyes in complementary coloured lights. He writes : 

 "The theoretically most correct method of colour pho- 

 tography is that which is based upon the following 

 principle. It is already known that a dye is decomposed 

 only by that light which it absorbs or by its comple- 

 mentary colour. Certain aniline dyes bleach very rapidly 

 in light. If we mix pigments of the three primary 

 colours, red, green and blue, we obtain black. If we pro- 

 duce this black upon paper by means of very sensitive 

 dyes, and place a red coloured picture upon it ; light 

 green, i.e., blue and yellow, will bleach out so that only 

 red remains. In the same way blue only remains under 

 blue, green under green, etc. Naturally the dyes must 

 correspond fairly well with the colours of the spectrum 

 and be of the same sensitiveness. The production of 

 such a print would occupy too long to be practically 

 useful. It is possible, however, to obtain the same result 

 with more sensitive materials. The aniline coloured print 

 must be protected from the further action of light by 

 varnishing, etc." 



According to this proposition of Liesegang, each 

 colour in the original will be reproduced in the bleach- 



