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DRY PLATES, FILMS AND PAPERS 183
Metol is often used also in combination with pyrogallol, especially in the development of hand camera plates. This reducer is found to be a very rapid one and to give good results so far as detail is concerned.
A very favourite developer in use with bromide papers is the ferrous oxalate. This developer, which is produced by the reaction of ferrous sulphate and potassium oxalate, has no fogging effect upon plates or paper — i.e. it only reduces the silver bromide in those parts which have been exposed to light. On this account it is extremely good for very slow developing, as it does not tend to produce any marks on the prints or plates. The slowness of this developer is often looked upon as one of its chief drawbacks, the other being its want of keeping properties.
The papers or plates developed with ferrous oxalate must not be placed into plain water directly after removal from the developer, or brown stains will almost certainly appear, owing to the precipitation of oxide of iron on the papers or films.
In order to overcome this difficulty, a clearing bath is used. This generally consists of a very weak solution of acetic acid, although oxalic acid, which is much more effective but has the disadvantage of being poisonous, is also sometimes used for this purpose.
This developer should not be used in dishes which have been employed for any of the pyro developers, otherwise black stains will be sure to occur on the papers.
Among the other well-known developers are ortol. rodinal, glycin, eikinogen, and rytol. Each of these have their own special qualities which recommend them to the user, but it would be out of place to enter into details here.
There is perhaps no branch of practical photography in which such great improvements and changes have taken place during the last twenty years as in the development of the negative. During the infancy of photography the worker had to prepare the developer for himself,