DEVELOPERS AND THEORY OF DEVELOPMENT 347 



The distribution of developed grains within an emulsion layer is frequently far 

 from uniform, though the layer is usually 0.001 in. or less in thickness. Variation of 

 exposure with depth is one factor affecting this distribution, and variation of develop- 

 ment with depth is another. This latter effect is probably less than the former, with 

 formulas in common use, but separation of the two is difficult. 



The thickness of the final dry developed emulsion layer often varies because of the 

 removal of the bulk of the original silver halide in regions of low density and the reten- 

 tion of the silver grains in regions of high density. Thus the final surface may show a 

 relief pattern due to the volume differences introduced by processing. 



Fine-grain Developers. — The various efforts to produce fine-grain images center 

 primarily around control of grain clumping during processing. This is not the only 

 aspect of graininess, as three general phases are qualitatively recognized, though rarely 

 quantitatively separated. In ascending order of unit size, they are: 



1 . The effect of the individual silver grains. These are usually but slightly larger 

 than the elementary silver halide grains from which they were produced and conse- 

 quently become visible only at the higher magnifications. 



2. The effect of clumping of the particles. Some of the factors affecting this very 

 important aspect of graininess have been mentioned, but they will be reviewed here to 

 bring them together. 



3. The effect of nonuniform distribution of clumps. This is, perhaps, the least 

 definite of the effects. 



Such separation into classes as has been practically attempted appears to be based 

 upon some measure of magnification at which graininess disappears, or appears to 

 match a standard graininess. At different levels of magnification the various effects 

 become apparent. It is the second effect, clumping of grains, which appears as of 

 greatest importance in current procedures. 



In the discussions of sulphite and silver halide solvents in developers, it was pointed 

 out that an effect on graininess was sought through solvent action on the surfaces of 

 the grains which would tend to keep them separate and prevent clumping. This 

 effect is suggested as the primary mode of action of the common borax developer 

 formulas as fine-grain developers. 



Much work has been done in the attempt to discover any specific fine-grain charac- 

 teristics which may inhere in various reducers. These attempts have been quite 

 generally unsuccessful, and no reducer has received wide credit as having fine-grain 

 characteristics except paraphenylenediamine. This has been recognized for many 

 years as having specific fine-grain characteristics. Two disadvantages have been 

 found to its use. It produces a dermatitis on some skins, but this is not a very serious 

 objection, since it is readily possible to avoid contact with the powder or the solution 

 by careful handling and use of gloves if needed. The second is more serious. Para- 

 phenylenediamine, used alone under conditions to produce minimum grain, does not 

 produce such high effective emulsion speeds as some other developers, e.g., the borax- 

 type developers. Development times are rather long, and attempts have been made 

 to obviate both disadvantages by use of additional compounds in the developer. The 

 greatest success appears to have been obtained by the addition of glycine as a second 

 reducer with the paraphenylenediamine. The rate of development is increased and 

 the effective emulsion speed is higher with practically no change in graininess. 



The extremely wide range of additions which have been tried with paraphenylene- 

 diamine was noted in an earlier paragraph. 



Some success has attended the efforts to secure fine-grain results with other 

 reducers. Various means of prolonging development have been tried. These have 

 included dilution of developer and reductions of pH and in a few cases, finer grain 

 has resulted. Greater success appears to have been obtained by the addition to 



