DEVELOPER (Pr., Revelateur ; Ger., Ent wickler) Any agent used to render visible the latent image, or, in other words, to reduce to silver or other metal the latent image produced by the action of light upon any sensitive salt. In ordinary photographic phraseology the term is applied to the solutions generally ; whereas strictly speaking it should be applied only to that chemical or agent which actually reduces the exposed silver salt.
The composition of the developing solutions varies considerably not only with each make of plates, but frequently with each worker ; but there is a definite quantity of developing agent which should at least be used, and this is un doubtedly largely dependent on the number of molecules in the active group, and the amido groups are more active than the hydroxyl groups. Von Hiibl has given the following table based on this fact, which shows the best con centration of the actual developing agent and also the strength usually employed, assuming that a 5 per cent. solution of potassium carbon ate is used as the alkali.
Weight in every ioo parts of developer Calculated Generally used Metol . . . o•6 o•6 Pyrocatechin . o•6 0.6 Hydroquinone . 0r6 Amidol • • 0'4 • • Paramidophenol . o•5 • • Pyrogallol . . 0.5 .. 0-3--o•6 Eikonogen . . 0•9 .. Adurol . . . i•o •o Diogen . . . 1.2 .. Glycine 1 -o The calculated quantities will give the maximum rapidity of development.
A second table, also due to Von Hiibl, is that at the foot of the preceding page. In column I. is given the duration of development to yield a cer tain density, in column II. the relative rapidity of development, in column III, the relative density giving power, in column IV. the slowing of
development in seconds by the addition of 2 per cent. of potassium bromide, in column V. the retardation in seconds due to cooling the developer to 5o° P. in column VI. the retarda tion in seconds by F., with an equal volume of water, in column VII. the action of bromide on the density, and in column VIII. the keeping power of the mixed developer, I being taken as that which keeps longest.
A developing solution should contain a certain quantity of alkali to form the actual developing salt or to increase the reducing power of the developer proper ; and for this reason it is called the accelerator. A preservative is required to prevent too rapid oxidation or the deposition of an organic stain due to the oxidation of the developing agent.
The alkalis generally used are sodium car bonate, potassium carbonate, and caustic soda and caustic potash. Some alkalis act better than others with given agents, and on the Euro pean Continent potassium carbonate is generally used, whilst in England the corresponding sodium salt is used. The preservative is usually sodium sulphite, whilst potassium metabisulphite is occasionally used also. Only a few years ago an alkaline bromide was recommended in almost all cases, but this was partly due to the fact that the plates then did not work quite free from fog. Of late years the use of bromide has become much less general. Formula' for the various developing agents are given under the respective chemicals.