COMPENSATION FOR UNDER AND OVER EXPOSURE We have so far been considering merely a case of "nor mal exposure" and "normal development," but we must go farther than this and give examples of abnormal exposures and the corresponding variations which require to be introduced in development to correct these.
One of the greatest advantages of dry plates is that, although there certainly is for every subject a correct exposure, any considerable deviation from which is to be as far as possible avoided, still it is quite possible to correct or compensate for considerable differences from this " normal exposure," as it is generally termed.
Thus if the normal exposure for a certain subject were ten seconds—that is to say, if ten seconds were the exposure which, with the developer as commonly mixed, would give the very best result—it would be quite possible, by somewhat varying the manner of mixing the solutions, to get an equally, or almost equally, good result with an exposure as short as seven or eight seconds or as long as sixty seconds, or even possibly longer.
This power to compensate gives what we term "lati tude of exposure," and is one of the most useful powers which we possess in working dry plates, because it is impossible to judge with certainty of the intensity of light, and could we not compensate in some way for a slight error it would only be on rare occasions that we could have a perfect negative.
This compensating for under or over exposure is the chief reason for ever altering the proportions of the solutions used in development ; but there is still another. The nature of the subject to be treated varies very much. We have some subjects which tend to give very strong contrasts of light and shade, whilst others exhibit no such strong differences of brightness. As an example of the first we may return to our old illustration, where we had side by side a whitewashed wall and a tree with widespreading branches and deep shadows. As an ex ample of the other we may take an open landscape, a scene with possibly a river in the foreground, and beyond that roads and houses all brightly lighted. Now, were the plates exposed on these two very different subjects developed in precisely the same manner, the result would probably not be satisfactory. The first would make a chalky picture, in which the wall came out in a pure white mass showing no detail, the tree a deep black mass showing also almost no detail. The second would not
show contrast enough, but would be flat and wanting in sparkle.
It is necessary then, at any rate occasionally, to vary the developer to suit the subject, even if the exposure have been correct.
Before going into these questions of variation of developer we must say a few words on the functions of the various chemicals of which the developer is made up.
The three constituents are : (1) pryogallic acid ; (2) ammonia ; (3) bromide of ammonium. The function of the nitric acid is merely to preserve the pyrogallic acid.
This latter is the true developer. It is possible, if sufficient exposure be given to the plate, to develop with a solution of pyrogallic acid only, but not with either of the other chemicals.
The greater the portion of pyrogallic acid used, the denser will be the negative ; but the addition of pyrogallic acid beyond what is necessary to make the image appear does not tend to increase the amount of detail which can be brought out on the plate.
The ammonia greatly increases the energy of the action of the pyrogallic acid. A certain quantity of it is necessary to secure sufficient density with normal exposure, but any increase beyond this has the effect of bringing out more detail until a point is reached where the developer has so much energy that it will reduce or blacken a film even when it has had no exposure to light.
The effect of the bromide of ammonium is to restrain the action of the developer. It may at first be thought strange that there should be added to the pyrogallic acid, first, a chemical to increase the energy of its actions, then another to restrain it, and it might be supposed that these two would merely counteract each other. This is not the case, however. The chief effect of the am monium bromide is to make the action of the developer slower merely, but not to prevent it from ultimately doing its work. When in considerable quantity, how ever, it does actually counteract the effect of the am monia. If a developer be used without bromide as a restrainer the action is so rapid that it cannot be watched, and it is consequently not under control. If very little ammonia be used with the idea of preventing this very rapid action, then sufficient density is not gained.