THE PHOTOGRAPHY OF COLOUR 357 



whatever, as consisting of a mixture of these three colours 

 (or two or one of them), and by restricting the light that falls 

 upon a sensitive plate to one of them at a time, a photograph 

 can be made of each light-constituent reflected by the object. 

 The photographs are themselves colourless, but they are colour 

 records, and it only remains to bring together the three 

 colours as indicated by the photographs to produce a picture 

 that appears of exactly the same colours as the original — 

 because it has exactly the same effect upon the retina. In 

 this case no colour is produced by photography or by the 

 photograph : the colours are pigments or dyes, and the photo- 

 graphy merely locates them. 



This is the principle of "three-colour" processes, and, 

 speaking practically, all indirect colour photography comes 

 under this description. It must not be thought that 

 three-colour work is dependent upon the Young-Helmholtz 

 theory of colour perception — it is founded rather upon the 

 experimental facts that underlie that theory. The theory and 

 three-colour processes are better regarded as two independent 

 products of the same set of facts, and these facts will remain 

 whatever fate awaits the colour-vision theory. Indeed the 

 connection between this theory and three-colour photography 

 may not be so intimate as it appears to some to be. The colours 

 that represent the three sensations are fairly definite, and 

 Colonel von Hiibl maintains that the number of sets of three 

 colours that will serve for three-colour work is indefinite. He 

 arranges the spectrum tints in a circle with complementary 

 colours diametrically opposite each other on a plan that would 

 take too long to describe here, and states that the best three 

 colours will be at the angles of an equilateral triangle drawn 

 in the circle, and that the triangle may be in any position. 

 At the same time he allows that practical considerations — chiefly 

 the difficulty of getting a bright yellow by the mixing of 

 pigments — obliges one to use the three colours which, if they 

 do not exactly match the three primary sensation colours, 

 approximate to them. Sir William Abney, who has done a 

 great deal of work in this connection, and is a very successful 

 producer of colour photographs, accepts absolutely the 

 Young-Helmholtz theory of colour vision as his guide, and 

 has redetermined the spectrum curves of the primary sensations. 

 Mr. F. E. Ives, who has worked as a pioneer with very notable 



