344 HANDBOOK OF PHOTOGRAPHY 



stop or fixing bath. This need most frequently arises in processing emulsions under 

 adversely high temperature conditions, and accordingly the term "tropical developer" 

 is frequently used to designate a formula in which particular provision is made to 

 avoid dangerous swelling even when processing at temperatures of 85°F. or over. 

 Sodium sulphate is the usual addition to accomplish this result, as it reduces the 

 swelling of the gelatin when present in relatively high concentration, 100 g. per 1. or 

 above. It tends to slow development somewhat, perhaps due to reduction of diffusion 

 rate, but that is a rather small price to pay for the additional safety from damage by 

 warm solutions. The sulphite of the developer itself has a similar tendency to 

 restrain swelling of the gelatin, though it appears less marked. However, the quan- 

 tity of sulphate required is less in the presence of high sulphite than with low sulphite 

 concentrations. The protection afforded the emulsion by sulphate-sulphite com- 

 binations is not permanent, hence hardening baths designed to impart permanent 

 hardening to the emulsion should be used after these "tropical developers." 



Silver Halide Solvents. — The use of silver halide solvents has been proposed for a 

 number of purposes, the most extreme of which is combined development and fixation. 

 To secure this action a rather vigorous developer is required, as development must 

 proceed in spite of the simultaneous fixation. In concentrated fixing solutions, 

 fixation is often complete in 1 to 3 min. Such times are too short for development 

 under these conditions, so low hypo concentrations are used to permit sufficient time 

 for development. These methods, while possible, have not supplanted the use of 

 developer and fixer separately and are rather rarely used. 



When less drastic effects are sought weaker solvents may be used. The effect of 

 the solvent action of sulphite was mentioned in the discussion of that chemical. 

 Attempts have been made to secure fine grain by the addition of other weak silver 

 halide solvents such as ammonium chloride and potassium thiocyanate. These have 

 met with some success, but the presence of a silver halide solvent does not by itself 

 insure fine-grain development. 



Sometimes these silver halide solvents are used to modify the tone characteristics 

 of the final image. This image is normally made up of finely divided silver which does 

 not have any inherent color of its own but appears in tones of nearly neutral gray, as 

 xisually viewed. However, there is frequently a residual tone which depends upon 

 the particle sizes from which the image is built up. Fine-grain deposits usually tend 

 to show greater departures from neutral grays than coarser deposits, but the color of 

 the deposit has not proved an accurate measure of graininess for coarse grains maj^ be 

 mixed with others fine enough to give a tone to the deposit. This discussion of image 

 tone is given to suggest the probable mechanism of the action of silver halide solvents 

 in affecting image tone. The image tone is not primarily important in negatives, 

 though it has an effect there, as colored deposits frequently print as of greater contrast 

 than their visual appearance or densities would indicate. It is primarily important in 

 positives, paper prints, lantern slides, motion-picture prints, and the like. 



In reversal work it is usually necessary to secure a pleasing tone in the final image, 

 and silver halide solvents are sometimes used to improve the tone quality by changing 

 it from a brownish-black to a neutral or blue-black quality. When used in the first 

 developer of a reversal process, the solvent probably dissolves the extremely fine grains 

 first. These grains are usually slow photographically and would otherwise remain to 

 form part of the final image, thus giving foggy or plugged high lights and a brownish 

 tone. 



Blue-black Agents. — Other compounds have been found which affect the tone 

 quality of the final image. Presumably this is through an effect on the aggregation 

 of the silver of which the deposit is made. 



