CHAPTER XI 



DEVELOPERS AND THEORY OF DEVELOPMENT 



By D. R. White and J. R. Weber 



The development of the latent photographic image is a necessary and important 

 step in current practical photography. The origin of photography antedates the 

 knowledge of developers and processes of development, but the original methods have 

 been practically completely supplanted by procedures using development as a step on 

 account of the greatly increased photographic efficiency so obtained. A few photo- 

 graphic materials are still used without developers, notably printing-out papers, 

 which, as their name suggests, are exposed until the image is visible. The reactions 

 involved are directly induced by the action of the exposing light. Greater photo- 

 graphic efficiency results, in general, from smaller exposures, designed to produce 

 only a latent image, itself invisible, with subsequent development and fixation to 

 transform it into a permanent visible image. 



There has been much speculation concerning the nature of the latent image, but 

 experimental work to determine its nature is very difficult, and no theory yet advanced 

 has secured imopposed acceptance. Study of development has had to proceed in 

 spite of this lack of knowledge of the latent image developed. Much empirical 

 data have been obtained and some theoretical relationships have resulted from years 

 of experimental work devoted to this study. 



Two classes of procedures have resulted and are recognized under the names 

 "chemical" and "physical" development. The final images produced are not 

 chemically distinguishable, but the course followed is primarily different in the source 

 of the silver finally deposited as the image. In chemical development, the more 

 important method, the silver halide of the emulsion supplies the silver for the final 

 image. This is probablj^ through an initial solution of the halide bj^ the developing 

 solution, followed at once by reduction and deposit of the silver on nucleuses forming 

 the latent image. In physical development, the developing solution itself contains a 

 soluble silver salt which deposits or plates out, as reduced by the developing agent, 

 with the nucleuses from the latent image guiding the location and magnitude of the 

 deposit. This type of development can be carried on even after fixation, though it is 

 quite obvious that chemical development is impossible then. 



With either tj^pe of development, the fundamental problem is to obtain a differen- 

 tial deposit of silver in such manner that at each point it is at least approximately 

 proportional to the intensity of the latent image at that point. If this were achieved, 

 there would be no deposit where there had been no exposure. In practice this clear- 

 cut case is not attained, as there is in general an over-all veil or fog not accounted for 

 by the exposure given. Developers differ much in their differential action. Only 

 those developing agents and formulas are of general importance which are capable of 

 a high degree of discrimination in their action. 



In addition to the basic role of development just discussed, i.e., the production of a 

 visible image from the latent image, developing solutions are sometimes used which 

 produce other effects at the same time. For some purposes it is desired to harden 

 the gelatin of the emulsion localh' where the silver is deposited. This has been 



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