The American annual of photography . of stannous chloride will reduce unexposed silverbromide, while a reducing agent like sulphurous acid does notaffect it at all. Developers therefore lie between these two ex-tremes, an ideal developer being one which completely developsall the exposed but does not begin to develop the unexposedcrystals in the emulsion. Most developers fall short of this ideal condition and developa small proportion of the unexposed emulsion, producing auniform veil or layer of silver over the entire plate which hasthe effect of obliterating fine shadow detail and diminishin
The American annual of photography . of stannous chloride will reduce unexposed silverbromide, while a reducing agent like sulphurous acid does notaffect it at all. Developers therefore lie between these two ex-tremes, an ideal developer being one which completely developsall the exposed but does not begin to develop the unexposedcrystals in the emulsion. Most developers fall short of this ideal condition and developa small proportion of the unexposed emulsion, producing auniform veil or layer of silver over the entire plate which hasthe effect of obliterating fine shadow detail and diminishing thecontrast. This fog produced by undesirable chemical action isknown as chemical fog, and it may be measured in terms ofdensity in the same way as the silver image. The amount of chemical fog produced in any particular in-stance depends on the following factors:/. The Nature of the Emulsion A. The Type of Emulsion Photographic emulsions may be divided broadly into the fol-lowing classes: High and low speed negative emulsions. 20. THE OPEN DOOR. WM. F. KRIEBEL. 21 High and low speed positive emulsions (lantern slides).High and low speed paper emulsions (bromide and gas-light).Chemical fog occurs most frequently during the develop-ment of high speed negative emulsion, because in view of theirhighly sensitive nature and the infinitesimal amount of lightaction required to render the silver halide developable, if thedeveloper is too energetic or not compounded correctly it willdevelop the unexposed crystals also. Low speed emulsions usually give images free from chemicalfog providing a suitably restrained developer is employed,though it is somewhat of an anomaly that some developers willgive clean images on high speed emulsions but will fog a slowemulsion like that on a lantern slide or positive motion picturefilm. It is therefore desirable in all cases to adapt the de-veloper to the emulsion. B. The Age of the Emulsion On keeping, all sensitive photographic emulsions becomemore
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