W. Huggins, R. S. Newall, W. Abney 173 



shadows, but his picture was evanescent, as he was unable 

 to fix it. Rudimentary as this procedure was, it contained 

 the germ of the future contact printing. Next came the 

 work of Daguerre and Niepce in France, resulting in the 

 well-known daguerreotype. In 1840 Sir John Herschel 

 introduced hyposulphite of soda as a fixing agent, and in 

 1841 Fox Talbot greatly improved Wedgwood's original 

 process, using silver iodide on paper sensitized by " gallo- 

 nitrate of silver." The introduction of collodion as a con- 

 venient vehicle holding the silver salts was first suggested 

 by G. le Gray, and put to practical use by Frederick Scott 

 Archer and P. W. Fry. In the subsequent development of 

 the dry plate important progress was due to R. Manners 

 Gordon, W. B. Bolton, and B. J. Sayce. The gelatine 

 emulsion process was used by R. L. Maddox in 1871 and by 

 J. King in 1873, but first introduced in a workable form by 

 R. Kennett in 1874. The merit of giving rapidity of action 

 to dry plates belongs to C. Bennett (1878). Further progress 

 was made by Colonel Stuart Wortley and by W. B. Bolton 

 in 1879. 1 



The modern theory of photography almost entirely 

 depends on. the investigations of Sir William Abney. He 

 introduced scientific methods in the measurement of the 

 sensitiveness of plates, investigated the effects of tempera- 

 ture, and showed the important influence which the size of the 

 sensitive particles had on their behaviour in different parts 

 of the spectrum. He was thus able to obtain a silver bromide 

 sensitive to the red light, and was the first to photograph 

 the infra-red rays of the solar spectrum. 



A few words should be said about the history of colour 

 photography. Lord Rayleigh pointed out in 1887 how 

 particles of silver might be deposited in layers half a wave- 

 length apart. A film containing such layers would have the 

 power of reflecting copiously that special kind of light which 

 had served to form it. This process was actually employed 

 to reproduce natural colour effects by M. Lippmann, of 

 Paris ; but it suffers from the disadvantage that the correct 



1 For a fuller account of the history of photographic processes, 

 see the article on " Photography," by Sir Wm. Abney, in the 

 Encyclopaedia Britannica," Xlth ed. 



