PHOTOGRAPHY. 1537 Developmcnt.—After exposure in the camera, the plate is carried to the dark room, and there taken out of the slide. No visible change will be observed on the plate.
The image is impressed, but it is latent, and may be evoked by pouring over the film a solution containing 1 oz. iron prutosulphate, 1 dr. copper sulphate, 1 oz.
baryta nitrate, oz. glacial acetic acid, 20 oz. water.
Filter out the white precipitate, and it is ready for th-e.
Another formula oz. iron protosulphate, -} oz.
glacial acetic acid, I oz. alcohol, 8 oz. water.
Either of the above solutions may be successfully em ployed, but the proportions can be varied, and different reagents used in developing the latent image. After washing off sll trace of the developing solution, the negative may be intensified by flowing over the plate 3 gr.
pyrogailic acid, 3 gr. citric acid, 1 oz. water. To this is added, just before using, one or two drops of a mixture of 30 gr. silver nitrate and 1 oz. distilled water.
The object of intensifying is to confer tile degree of opacity upon the lights necessary to yield a brilliant positive print.
If the subject be a portrait, the extreme high-lights ehould appear quite opaque when the plate is held up and viewed by transmitted light. When fully intensified, the plate may be washed under the tap, and fixed either by lowering iuto a bath containing a nearly saturated solution of sodium hyposulphite in water, or pouring over the film a solution of 100 gr. potassitun cyanide to 10 oz. water.
In any case, after the negative has been cleared of the unaltered opalescent bromo-iodide ot silver, it must be thoroughly washed under the tap, and reared up to dry. When dry, the negative should be slightly heated, and the following varnish (which has been filtered) applied :— 1 oz. sandarac, 4 dr. seed-lac, 1 dr. castor-oil, 9 oz. alcohol. The varnish must be poured over the collodion film, and drained off at one corner of the plate, which must agaio be heated to drive off the alcohul, and harden the surface.
In this wet collodion process, there are distinctly tnarked stages of progress. The bromo
iodized collodion is in itself insensitive to light, and may be exposed to sunshine without detriment. So indeed is the silver nitrate solution. But a great transition takes place when the collodionized plate is plunged into the silver bath. The iodide and bromide in the collodion form a union with the silver salt, and produce a highly sensitive film of hromo-iodide of silver. On the successful formation of this powerful compound, hinges the entire result of the process. Should the silver bath prove too acid, the negative will be hard black and white. On the other hand, if the bath is alkaline, the negative will fog over, and lack contrast. Should white light fall upon the plate when it is sensitized, and before it is developed, the negative will fog hopelessly. If the glass plate has not been thoroughly cleaned before coating with collodion, stains will show after development. Dust flying about in the dark room and settling on the plate will cause pinholes in the negative. These may also be caused by sediments in the collodion, or in the silver bath.
Dry Collodion Processes.—The dry collodion processes are falling into disuse before their formidable rival dry gelatine emulsion. Even were they not destined to become processes of the past, they are too numerous to catalogue.
Plates prepared in the way described under the head of Collodion Process may be dried and preserved for an indefinite length of time. After leaving the silver bath, it is only needful to wash them thoroughly under the tap, and flow over them 20 gr. tannin in 1 oz. water, after which, they may be dried in a dark place, and preserved for use.
Preservatives other than tannin may be used without number, but one will serve as a type of all the others. Washing, preserving, and drying greatly reduce the sensitiveness of wet collodion plates. But this defect is to some slight extent counteracted by employing an alkaline developer : 5 F —(a) 3 gr. pyfogallic acid, 1 oz. water ; (b) 30 gr. potassium bromide, 1 oz. water ; (c) 30 gr. ammonium carbonate, 1 oz. water.