338. The action of bromide is therefore greatest on the shadow detail and on the dark half-tones, which in a bath containing bromide appear distinctly more slowly as compared with their appearance in a bath without bromide, but this effect falls off progressively, becoming practically negligible when development is carried very far.
This property of bromide provides a means for correcting over-exposure. To obtain the best advantage of this corrective, development must not be carried at all far, the negative being intensified afterwards, if necessary.
A negative, the development of which has started in a bromided bath and then continued in one without bromide, or vice versa, gives results intermediate between those obtained when using only one or the other of these developers. The effect of the soluble bromide is therefore not permanent, as would be the case if it partially destroyed the latent image ; this effect seems to be a manifestation of the law of mass action, the dissolved bromide decreasing the solubility of the silver bromide or reducing its ionization, thus slowing down development (j. I. Crabtree, H. Parker, and H. D. Russell, 1033).
By reason of its action in preventing or considerably retarding the formation of chemical fog, bromide enables development to be pushed farther than would be possible in a bath without bromide, thus obtaining the maximum contrast obtainable with the emulsion used. In this case bromide is generally used in fairly large doses.
It should be mentioned that, owing to the reduction of the silver bromide in the negatives developed in it, a used developing solution always contains some potassium bromide or sodium bromide, and in quantity increasing with the area of image which has been developed. It also contains oxidation products of the developer, the effect of which we shall study on a later page.
339. Chemical Fog. We have seen that during ripening (§ 192) some grains reach a state in which they are more easily reducible than others and can actually be developed without exposure to light. While chemical fog is often more marked in very rapid than in slow emulsions, there are exceedingly fast emulsions which are also extremely clean. Fog generally increases
with the age of an emulsion. A very clean emulsion will fog in a badly compounded developer (excess of sulphite or of caustic alkalis), or one prepared with impure chemicals, or used at too high a temperature, unless a more or less large dose of bromide be added to such a bath.' Contrary to a fairly widespread opinion, it is not, as a rule, the developers with a high reduction potential which give the greatest chemical fog. As a matter of fact, the following developers, Hydroquinone, Pyrogallol, Metol, Amidol, here placed in the order of increasing potential, stand in the reverse order as regards tendency to fog (A. H. Nietz, 1922). With high potential developers the addition of bromide in moderate quantities retards the fog more than the image.
If two negatives which have had identical exposures are developed to the same degree of contrast, one in a concentrated developing solution, and the other in some of the same bath greatly diluted, it will nearly always be found that the negative developed in the weak bath is more fogged than the one developed in the strong one. 2 Finally, chemical fog is markedly increased each time the sensitive film is taken out of the developer and exposed to contact with air, as when examining it. This aerial fog (which occurs in a most marked manner in the development of cinematograph films on a drum which is only partly immersed) seems to be (Inc to a phe nomenon of chemico-luminescence ; it may be completely avoided by desensitizing (E. Fuchs, 340. Chemical fog appears more quickly and becomes more dense on an unexposed sensitive film placed in the developer than on one bearing a latent image, unless the bath contains a large proportion of bromide. In the case of an actual image, the fog is densest in the parts which have received the least light (shadow detail), its effect being almost nil in the high-lights. As a matter of fact, the development of the image gives rise to bromide in quantity proportional, at each point, to the density of the image already developed, and this bromide counteracts the formation of fog according as it is present in greater or less quantity at the particular point.