This corrective is applicable to an image which is being developed in a concentrated bath. It is sufficient, as soon as the excessive nature of the contrasts is noticed, to place the negative in a dish containing an ample quantity of clean water, rocking the dish once or twice in order to distribute uniformly in the water most of the developer adhering to the film. The develop ment of the high-lights will stop almost at once, but that of the shadows will continue.
It must be added that the effects of local exhaustion of the developer are often supple mented by the formation of chemical fog, favoured by long development in a dilute bath (§ 339 and § 34o).
342. Effect of the Temperature of the Devel oping Solution. In a given developing bath, development takes place more quickly as the temperature is higher, but developers differ considerably in the range of variations from this cause. A rise in temperature not only acceler ates the development of the image, but it accelerates still more the production of fog, thus compelling the photographer to compensate to a certain extent for the effect of a comparatively high temperature (above 70° F.) by adding bromide to the developer in proportion increas ing with the temperature. it is, of course, well known that all chemical changes are accelerated by heat, and also diffusion in the gelatine is more rapid at a high temperature than at a low one, which effect is, however, partially compensated by the swelling of the gelatine which makes the liquid travel a longer distance.
The temperature of the (lark room is often higher than F. in summer, and frequently lower than 45° F. in winter. Thus, plates from the same box, exposed under identical conditions and developed in identical baths, may require to be developed for three minutes in summer and 10 minutes in winter to give results as similar as possible. The best results are obtained by maintaining the temperature of the baths (and as far as possible that of the dark room) as evenly as possible, between F. and 65° F.
The term temperature coefficient of a developing solution is applied to the ratio between the times of development which, at temperatures differing by 10° C. r8° F.) yield equivalent results, all other conditions being identical. This tem perature coefficient may be said to be practically constant for different emulsions. 2 It is not affected by dilution of the developing bath but may undergo slight variations according to the composition of the bath.
The above table gives the average values of the temperature coefficients for baths prepared with various developers. 3 It must be pointed out that, in a developing bath contain:rig two different developers, the notion of temperature coefficient loses all signi ficance as soon as there is a definite departure from the optimum temperature, for each devel oper retains, in fact, its own temperature coefficient. For example, in the case of the mixture of metol and hydroquinone, at low temperatures it is the metol, almost alone, which is active, the hydroquinone being active only at comparatively high temperatures.
A rise in the temperature of the developing bath favours the development of under-exposed negatives both by increasing the reduction potential of the developer and by accelerating the production of chemical fog. Development in a very cold bath often yields negatives of a kind produced by under-exposure, and thus favours the correction of over-exposure.
343. Effects of Waste Products ; Effect of Agitation of the Developing Bath, In a devel oping solution that portion which is partly exhausted by the reduction of the silver bromide in parts of the emulsion representing the high lights, is richer in bromide than other portions of the bath and is also charged with oxidation products. It is therefore denser than the fresh portions, as may be shown by the existence of convection currents, rendered visible by the movements of particles of cotton fibre (E. R. Bullock, 1922), or by local variations in the refraction of the liquid which are shown in a silhouette formed in a suitable manner by a point source of light (A. Haelsig and F. Ltift, When a sensitive film is developed in a vertical position the currents of exhausted liquid flowing from the portions which have had the most exposure may retard the development of the parts through which they pass, with the production of streaks of less density. These are most apparent when a strongly exposed part of the negative is adjacent to a region which has received uniform but much less intensive exposure. Moreover, the products of develop ment tend to accumulate at the bottom of the tank, and development there soon becomes somewhat slower than at the top, unless the depth of the tank below the lower edges of the plates or films is sufficient to allow of the waste products accumulating there.