These differences in the behaviour of developing agents are due to a property of the developer which is commonly known in photography as the reduction potential.
The reaction of development may be represented by the follow ing general equation:— The sodium salt of the reducing agent symbolized by R reacts with silver bromide and forms metallic silver, the oxidation product of the developing agent, which is symbolized by R', and sodium bromide.
The reaction will go in either direction according to the driving force available, and, in practice, quinone, the oxidation product of hydroquinone, together with sodium bromide, will bleach a silver negative, transforming the metallic silver into silver bromide. Now, the driving force in the direction from left to right depends upon the reduction potential of the de veloper. This is opposed by the reaction in the opposite direction, which is dependent on the concentration of soluble bromide and of developer oxidation product in the solution, so that if the bromide be increased, the rate of the reaction will be lowered. It is thus possible to measure the reduction potential of a de veloper by the effect of potassium bromide on development, as was originally suggested by S. E. Sheppard. A careful study of this was carried out by A. H. Nietz, who gives what are probably the most reliable data for the developers as follows :— A developer much used for negative making was pyrogallol. Owing to the fact that pyro is changed during development into a yellow substance, some of which remains associated with the sil ver in the image, pyro tends to give a slightly yellowish image, which has very much greater printing power than the plain silver image. The yellowish stain is prevented from forming by sulphite, so that the more sulphite there is in a developer, the less colour the deposit will show. Pyro is, therefore, used only for the de velopment of negatives, prints on paper or positive films in motion picture work being developed with metol and hydro quinone. Recently there has been a tendency to replace pyro by metolhydroquinone also for negative development.
When a film is developed, it is only the grains of silver bromide which have been changed by the action of light that are affected by the developer. The grains that have not been changed are not affected.
At the beginning of development, there are a great many ex posed grains ready to be developed, and then as development proceeds, these exposed grains are turned into grains of black silver, so that the number of developable grains decreases during development until at last there are no developable grains left, all those which can be developed have been acted upon, and the development of the image ceases. The rate at which the grains de
velop depends upon the number of exposed but undeveloped grains left, and as the grains are developed and the number of unde veloped grains remaining becomes less, fewer and fewer grains develop in each minute, until finally it is not worth while to pro long the development in order to get any more density. (See fig. 6.) The growth of the image during development is referred to as a growth of density ; that is to say, the density is a measure of the number of grains of silver which are produced at any given point. These grains of silver, after the film has been cleared by the fixing bath, obstruct the passage of light through the film.
The density of an image is measured in units which are based on the amount of silver which will let through of the light, so that if only 3 of the light falling on the negative gets through a certain part of it, that portion of the negative is said to have a density of i. Similarly, when only a 0 of the light incident is transmitted, that part of the negative is said to have a density of 2. The blackest part of a negative may have a density of perhaps 2, the middle tones i or less, and the shadows, perhaps (See fig. 7.) The difference of density be tween the darkest portion and the lightest portion of the nega tive is called its contrast.
Since the contrast depends chiefly upon the density of the highlights, it grows during devel opment just as the density does. (See fig. 8.) It grows rapidly at first, when there are many grains to be developed, and then more slowly until, finally, when the exposed grains are all developed, the negative will not give any more contrast however long development may be prolonged, and a continuation of development will result only in the pro duction of fog. The final contrast which can be obtained depends upon the kind of emulsion used. The fast emulsions, such as the negative emulsions, give moderate contrast, but the slow emulsions, such as those used for copying purposes or for making lantern slides, are especially made to give great contrast when develop ment is prolonged. (See fig. 9.) The temperature coefficient of development is about 2 for i5° F, although it varies somewhat with the developer. Thus, if the development time at 65° be 4 min., it will be about 2 min. at 8o° F, and about 8 min. at 5o° F. Tables and curves showing crease of the swelling of the gelatine and consequent difficul of penetration.