DEVELOPMENT. It is somewhat difficult to give an exact defi nition of this term, as used in photography. If it is defined to be the production of a visible picture from an invisible impression, then the term would not include the case of the printing processes in which a faint visible image produced in the pressure frame is intensified or developed with gallic acid. If, on the other hand, we consider deve lopment to be merely an intensifying process, then all the various methods of toning would come under the head of development, which would lead to confusion, and be objectionable. Suppose, then, we define development to be the process which immediately follows exposure, and which renders the picture visible in all its details.
In all the common development processes the image acquires fresh material. In the case of the daguerreotype process that material is mercury ; in the negative processes with the salts of silver, it is silver, or silver combined with organic matter; in the chrysotype process it is gold ; and so on. But the principle on which development pro ceeds is different in different processes. This will be understood by A few examples.
If a design be traced with the finger on a window pane, and we then breathe on the glass; the dew condenses on those parts which have not been touched by the finger. The design is therefore deve loped by steam, and rendered visible, dew being deposited on some parts of the glass and not on others ; in other words, material is added to some parts and not to others. If an iodized silver plate is exposed in the camera, and then submitted to the vapour of mercury, the vapour adheres to those parts of the plate which have been modi fied in some way by the action of light, but not to the parts where light has not acted. The lights of the picture are therefore " brought out " or " developed " by the mercury ; that is to say, the lights acquire fresh material, which renders them visible.
In the case of a collodion negative, light affects in some way those parts of the film on which it acts ; a solution of a deoxydizing body, such as proto-sulphate of iron, or pyrogallic acid, is then poured over the plate ; this mixes at once with the free nitrate of silver on the plate, and would at once decompose it and precipitate silver indiscriminately all over the film, were not that effect prevented for a time by acidifying the developing solution with acetic acid. We have then an acidulated mixture of the proto-salt of iron or pyro gallic acid with nitrate of silver floating upon a film, parts of which have been acted on by light. Now these parts have the property of decomposing the unstable fluid in contact with them, and of attract ing in this way a precipitate of silver (or of silver combined with organic matter), which is thrown down upon them only ; so that a visible picture is produced by the addition of fresh material to the parts where light has acted.
The same thing happens in the chrysotype process, although in this case the developer is not a deoxydizing but an oxydizing sub stance. In this process a sheet of paper is first impregnated with a persalt of iron or uranium, (the per-oxalate, or ammonio-citrate of iron, or the nitrate or tartrate of the sesquioxide of uranium,) and then exposed to light. The metallic salt is deoxydized by light, and reduced to a proto-salt, a faint visible image being produced. The picture is then immersed in a solution of chloride of gold, which is an oxydizer, for it parts with oxygen from the water to the reduced iron salt. The liberated hydrogen unites with the chlorine, and metallic gold is precipitated of a dark purple colour. Here then again the picture is produced by the addition of material to the parts where light has acted.
Should, however, any process be discovered in which a developer acts by withdrawing material from the sensitive film, that would not invalidate the definition we have given of development.
In the collodion process, both positives and negatives may be pro duced by development. In the case of a positive the picture is looked at, and the precipitated metal which forms the lights must be white. This is effected by acidifying the nitrate bath and deve loper with nitric acid, which causes pure white silver to be thrown down uncontaminated with organic matter. In the case of a negative the picture is not to be looked at but printed through, and the pre cipitated material must possess sufficient density to stop the light from passing through too readily. In this case it is found that organic matter must be eombined with the silver thrown down. This is accomplished by using an organic acid, such as acetic or citric, instead of an inorganic acid, as nitric. In the negative pro cess the dark parts of the picture should at first be red, and should then slowly acquire density and pass ultimately to a brown or purple black. This red tint, in the early stage of the development, indi cates the presence of a reduced organic salt of silver, which acts powerfully in attracting more material to itself. When the picture comes out grey at first instead of red, this indicates that the mate rial is more metallic and contains less organic matter ; it cannot then be easily intensified to a proper degree. When organic matter is introduced too freely in the positive collodion process the lights of the picture are brown or drab, instead of white, which interferes greatly with its beauty, although in detail and gradatien of shade it may leave nothing to be desired.