Development and Developers
taking photographs in positions which are likely to cause derision of, or annoyance to, anybody ; they are merely portable cameras which should be used to obtain mementos of rapidly moving scenes and persons, and not for universal picture-making.
Development and. Developers. The production of a visible
picture from an invisible or latent impression. A design traced on a window pane will often be invisible, but developable for weeks afterwards by breathing on the glass. Development may be divided into two distinct classes - physical and chemical. If we have a wet collodion plate which has been exposed on a sub-ject, we shall have practically a film of silver salt, part of which has been affected by light and which is covered with adherent solution of nitrate of silver. On adding a solution of ferrous sul-phate to nitrate of silver we get a precipitate of metallic silver, this precipitation, however, being delayed by the presence of organic acid such as acetic or citric. If a piece of clean silver wire be placed in the acidified ferrous-sulphate and silver solution, the silver is slowly deposited from the solution on to the silver. In the case of the wet plate the sensitive salt of silver which has been affected by light acts precisely in the same way, and the nascent silver from the adherent nitrate solution is deposited on the light-affected places forming the image. At present there is no proof that the sensitive salt is itself reduced. This is called physical development. At the present time when dry gelatino-bromide plates are so much in use we have practically another state of things. In this case the sensitive salt affected by light is itself reduced to the metallic state by certain chemicals. We have no silver nitrate slowly depositing silver on the film, but the sensitive silver salt itself is reduced, hence this is called chemical development. In a developer there are four essential ingredients: (i) the developer proper ; (2) the accelerator, which hastens the action of the developer ; (3) the restrainer; and (4) the solvent or water. The developing agent may be pyrogallol, hydroquinone, eikonogen, amidol, metol, etc. The accelerator is usually an alkali, ammonia, or the hydrates or carbonates of potassium or sodium. The restrainers are the bromides of , potassium or sodium, citrates, etc. If a developer of normal strength is applied to a plate which has never been exposed to light and allowed to act for a long time, it will be found that