346 HANDBOOK OF PHOTOGRAPHY 



Sodium metaborate 

 Acetone 



Miscellaneous agents recommended: 



Sodium sulphate 



Sodium bisulphite 



Sodium nitrite 



Sugar 



Xickel chloride 



Nickel ammonium sulphate 



Benzoic acid 



Boric acid 



Salicylic acid 



Tannic acid 



Potassium bromide 



Pinakryptol green 



Isopropyl alcohol 



Two-solution Development Processes. — In the discussion just concluded, develop- 

 ing formulas mixed as a single solution have been tacitly assumed. It is not necessarj^ 

 to so compound a developer, as the ingredients may be divided into two portions. In 

 general the same compounds are used, and they fill the same roles in the development 

 process, but the reducer and preservative may be used as one bath with the alkali as 

 the other. When the bath is so divided, the emulsion is first soaked in one portion, 

 then transferred to the other for actual development. Thus, if the emulsion is soaked 

 in the reducer solution and then transferred to the alkali, onlj'^ a limited amount of 

 development is to be expected because of the hmited quantity of reducer present in 

 the emulsion. Successive, transfers back and forth from one part to the other have 

 also been suggested to secure and control the desired contrast, but none of these two- 

 solution methods have yet been widely used. Adequate control of single solution 

 baths has proved simpler. 



Change Produced by Development. — Macroscopically the change produced by 

 development is the formation of a visible silver image proportional, at least approxi- 

 mately, to the original latent image. Microscopically this consists of the reduction, 

 i.e., development, of silver halide grains. In general, except for fog, onlj^ those grains 

 develop which have been rendered developable by exposure. Exceptions to this have 

 been noted in the case of some grain clumps, where the whole group has developed from 

 an original exposure of some one grain. The silver grains produced bj^ development 

 bear a general resemblance to the parent halide grain, but there is no longer the sharply 

 crystalline structure of the original halide. Development appears to take place by 

 the deposit of silver at one or more centers or nucleuses, spreading from these until the 

 entire grain is developed. 



Closely adjacent grains frequentty appear to fuse and form clumps much larger 

 than the individual grains in size. This clumping is one of the most important factors 

 in the graininess of the final developed image, and it is through control of this factor 

 that control of graininess is frequentlj^ sought. The use of sulphite and silver halide 

 solvents in the reduction of graininess bj^ separation of the individual graininess has 

 already been mentioned in the discussion of their functions in developers. The specific 

 effect of different reducing agents will be considered in the next section discussing 

 fine-grain developers. In present-day emulsions, the largest grains have dimensions 

 of only a few ten-thousandths of an inch, and if no element larger than this existed in 

 the finished image, graininess would rarely be a trouble with current practices. 



