320 HANDBOOK OF PHOTOGRAPHY 



.accumulation of a halogen compound. Thus used developer contains oxidation 

 products of the reducing agent and greater halide content than fresh solutions. 



Not all reducing agents are developers. Many such compounds lack the power to 

 discriminate between exposed and unexposed silver halide and thus lack that prime 

 requisite of a photographic developer — ^the ability to transform a latent to a visible 

 image. 



While considering differentiations of this type, the role of the gelatin or other 

 carrier of the silver halide should be mentioned. Silver halide precipitated from 

 aqueous solution can be reduced by ordinary developers, independent of light exposure 

 of any kind. Thus the characteristics normally associated with the latent image do 

 not control such reactions even with reducers which are developing agents in the 

 generally accepted sense of that term. It appears, therefore, that the gelatin or other 

 carrier of the silver halide is of basic importance and pla^ys a cooperative role in pro- 

 viding the conditions favorable to development of the latent image to a visible image, 

 as distinct from indiscriminate reduction of the entire silver halide present. 



In many cases, this differential action is primarily a rate or time effect. If per- 

 mitted to act long enough, virtually all the silver halide will eventually be reduced. 

 Normal developing conditions are designed to avoid such effects, but even within these 

 regions there is a marked difference between the rate of development of fog and 

 density resulting from exposure. Commonly the increase of exposure density with 

 time is less rapid as development progresses. On the other hand, it is quite generally 

 true that fog builds up more and more rapidly within corresponding limits of develop- 

 ment times. 



From these considerations, the developer appears in its true role as a differential 

 reducer, acting prefei'entiallj' to produce a visible image from an original latent image 

 before the entire pattern is lost in fog. 



In 1851 pyrogallol, or pyro, was discovered to be capable of developing the latent 

 image. In 1880 the second organic compound to have developing action was dis- 

 covered in hydroquinone. From then on the developing action of a large number of 

 organic compounds has been recorded. 



Generally speaking, it was observed that photographic developers were, to a large 

 degree, the colorless or reduced forms of dyes or dye intermediates. A compound 

 which would oxidize these compounds to their colored state would itself be reduced. 

 Thus silver halide would oxidize pvrogallol to its yellow-brown oxidation product, and 

 the silver haUde would at the same time be reduced to metallic silver. 



Almost without exception up to very recent times, all organic photographic 

 developers have contained benzene as the nuclear structure. The structure of benzene 

 is thought to be the ring 



H 



h 



/ \ 

 H— C C— H 



H— C C— H 



I 

 H 



which is represented as 



for the sake of simplicity. 



