I90I-2.] 



Photography in Natural Colours. 



373 



that they were and are used by Ives as the basis of his method of 

 obtaining photographs in colour. These curves indicate graphically 

 what Maxwell believed to be the amount of action produced on each of 

 the colour sensations by any particular part of the spectrum. It has 

 long been known, however, that, although these curves give a fair 

 approximation, they do not exactly represent the ranges of the 

 sensations and they are now superseded by the new measurements of 

 Sir Wm. Abney, probably the most widely known authority on colour 

 photometry and on photography. He spent some nine months in 

 1898-99, in redetermining the colour-sensation curves on the Young 

 theory, and his paper treating on the subject was published in the 

 Transactions of the Royal Society for 1899. A diagram of these new 

 values can next be shown (Fig. 2) illustrating the difference between 

 the two sets of curves. It will be noticed that, although the general 



A (xBC 



Fig. 2. — Abnej 's Colour-Sensation Curves. 



forms are similar, Abney 's sensation curves include each a longer range 

 of the spectrum than Maxwell's but are not so long as those of 

 Helmholtz where each sensation embraces the whole spectrum. But 

 Abney's values, determined by the aid of more modern methods, and in 

 the light of recent researches must be regarded as giving the closest 

 approximation to the truth. From the method of their determination, 

 and as will be seen later, Maxwell's curves could more appropriately be 

 called colour-mixture than colour-sensation curves, and are very 

 essential in both the theory and the practice of the three-colour process. 



The three-colour process of photography is based on the experi- 

 mental fact, which is probably most readily e.xplained by the Young 

 theory, that all spectrum colours, and hence all the colours of nature, 

 can be imitated, to a very close degree of approximation, by mixing in 

 varying proportions colours taken from three narrow sections of the 

 spectrum. These sections or colours coincide very approximately with 



