The advance of photography : its history and modern applications (1911)

180 THE ADVANCE OF PHOTOGRAPHY developer alone, but which becomes more reducible when acted upon by white light, but still less so than the surrounding silver bromide which has not been affected by the oxidation process. Development of the Image. — In order to render the image visible upon the photographic plate it is subjected to the action of some kind of developing agent. It may be advisable to consider the ingredients of some well-known developer and point out the object of adding the various chemical compounds. For this purpose let us select the pyro-soda developer. This contains, as its name indicates, pyrogallol — i.e. pyrogallic acid which is the principal reducing agent present. The object of using this is to reduce the silver salts, i.e. silver bromides, to metallic silver, which is the essential process in the formation of the visible image. Were this reducing agent used alone with the necessary alkali, its action would be so rapid that it would be impossible to control and no satisfactory result could be expected. On this account some controlling agent is necessary, and potassium bromide is usually chosen for this purpose. The presence of this substance, by tending to prevent the action of the reducer from being too rapid, greatly reduces the chance of producing a general fogging of the plates, and also hinders the production of flat negatives. The following experiment,1 due to A. and L. Lumicre and A. Seyewetz, well illustrates the importance of potassium bromide in preventing fog : — Exposed and unexposed plates are placed in the same developer composed of 1000 grams water, 5 grams diamidophenol, and 30 grams of anhydrous sodium sulphite, the temperature of the developer being 18° C. In one or two minutes fog begins to appear upon the unexposed plate, while it does not commence in the case 1 Zeitschr. Wiss. Photo., o, pp. 302-3U4, Nov. 1UU7.