TECHNIQUE OF DEVELOPMENT 353 



Many professionals and most amateurs must work with less elaborate installations. 

 The refinements of properly connected refrigeration with thermostatic control are 

 things to dream about but not to use. Ice cubes from the refrigerator take the place 

 of heat interchangers and thermostats. When cooling solution by such means, a water 

 bath should be used, with the ice in the bath, not in the developer. Of course, it is 

 somewhat quicker to cool with ice directly in the solution, but the dilution produced 

 by the ice as it melts is an uncertain and undesirable quantity. 



Water baths are occasionally cooled by the cooling effect of dissolving crystalline 

 hypo. This is a hazardous method for photographic processes, as accidental splashes 

 of the hypo solution can spoil the developer or make clear spots in the emulsion. 



Influence of Agitation of Developing Solution. — The influence of agitation on the 

 course of development can most readily be visualized by consideration of the develop- 

 ment process as it actually takes place. The silver halide grains are embedded in a 

 binding agent, usually gelatin, which hinders diffusion of chemicals to and from the 

 solution. Thus, as development proceeds, there is a tendency for development prod- 

 ucts to collect locally and hinder development. At the surface of the gelatin layer in a 

 region of high density there is a corresponding increase in concentration of develop- 

 ment products due to diffusion from the layer when the body of the fluid is undis- 

 turbed. At a low-density region there is low concentration of these products as there 

 has been little development. If now the solution is agitated, the development prod- 

 ucts adjacent to the gelatin layers will be washed away and evenly distributed 

 through the body of the solution. This facilitates diffusion of fresh solution into the 

 layer and development products out of the layer. In the region of high density a 

 considerable increase of development is the result, but at the low density little change 

 results. This nets an increase in y., for agitated development. 



Practically, it requires great care to secure reproducible agitation. Completely 

 stagnant development is practically impossible due to the mechanical requirements of 

 placing the film or plate in solution. Violent agitation is the other extreme, and it 

 has proved more practical to secure uniformity of development through agitation 

 than through either complete stagnation or specified intermediate agitation. This is 

 probably due to the fact that development does not increase indefinitely as the rate of 

 agitation increases but reaches a maximum or constant value, beyond which further 

 increase in rate of stirring is no longer effective. A few cases have been noted where 

 the most violent agitation tested even resulted in a decrease of development. No 

 entirely satisfactorj^ explanation of such a decrease of development with high agitation 

 has yet been found, but the initial increase of development with rate of agitation is 

 probably due to the difference in rapidity of removal of development products from the 

 surface of the layer into the body of the liquid. The limit for this effect would appear 

 to be that which would result from continuously supplying fresh developer to the 

 emulsion surface. 



One factor influencing results under conditions of low agitation is this: The prod- 

 ucts of development are usually of greater density than the developing solution and 

 hence tend to sink under the influence of gravity. Accordingly it has sometimes 

 been suggested that plates or films should be supported face down in the developing 

 solution to secure optimum conditions for development: some mechanical devices have 

 been made to do this, and at the same time create high-circulation velocities close to 

 the emulsion surface. This procedure, carried to the limit by proper agitation, prob- 

 ably produces the maximum development which can be obtained from a given devel- 

 oper formula and film at any one temperature, since it appears that advantage is fully 

 taken of all the physical factors affecting the result. This system is rarely used, as 

 adequate results can usually be obtained by simpler means, a number of which are 

 discussed on pages 354-357. 



