The Development of the Negative in

bromide, bath, containing, developing, solution, developed, developer, dissolved and effect

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It is known that electrolytic phenomena manifest themselves when a couple formed of two bodies in contact, differing from each other in their chemical or physical properties, is immersed in an electrolyte. And it is also known that the grain of silver bromide affected by light is not homogeneous, the germs of the latent image differing in some way from the remainder of the grain. When an emulsion is placed in the developing solution which is always an electrolyte, electrolysis, limited to each grain under consideration, may therefore be produced. In order that the reaction shall continue, it is necessary for the electrolyte to contain a de polarizer, capable of fixing the oxygen which would tend to accumulate at the anode (J. Desalme, 1910), and it is precisely this part that is played by the deveoper proper.

336. Effect of Dissolved Bromide in the Developing Solution. In the early days of the gelatino-bromide plate, development almost always required to be done in a solution contain ing soluble bromides (usually potassium bromide) to delay and slow down the development of chemical fog. The progress made in the manu facture of emulsions, the fact that they are always protected by traces of bromide, added when they are re-melted, and the possibility of avoiding oxidation fog by selecting suitable desensitizers, all render the addition of a bromide to the developing solution unnecessary in many cases. Indeed, it will be seen that the develop ment of a negative in a bath containing bromide gives very nearly the same result as would be obtained by developing in a bath containing no bromide a negative of the same subject taken under the same conditions on a less sensitive emulsion (or with a shorter exposure on the same emulsion). A number of modern emulsions only show their extreme sensitiveness when developed in a bath made up without bromide, or, if a very high-potential developer be used, with only the merest trace of bromide, such a developer being less susceptible to the action of bromides.

The presence of dissolved bromide lowers the already poor solubility of silver bromide in a developing bath, and it would seem that this property is responsible, at least in part, for the slowing of development, it being known that the reaction between dissolved salts take place more speedily as the concentrations are higher.' The various developers differ very consider ably as regards the effect of dissolved bromide. In the same way that the power of a motor-car may be measured by the effect of a hill on its speed, so the reduction potential of a developer can be measured (at least in relative terms) by the effect of a given content of bromide on the speed of development.

Contrary to an opinion sometimes expressed, the chlorides cannot, no matter in what quanti ties, play the part of bromides, even on emul sions of silver chloride ; the chlorides are abso lutely ineffective in a developer either containing bromide or not (E. Weyde, 1933).

Citrates, tartrates, or boro-tartrates, either dissolved as such in the developing solution or formed in it by the addition of the corresponding acids, have, at equal doses, an action differing very little from that of bromides (L. Lobel, and some allied sub stances, when introduced into negative de velopers in the proportion 1/25,000, reduce fog as effectively as potassium bromide at a strength of I 5,ouo without causing the drop in speed dealt with below (A. P. H. Trivelli and E. C. Jensen, 1930).

337. The effect of bromide in a developer cannot be exactly described except by comparing the characteristic curves 202) corresponding with negatives developed respectively in a bath containing no bromide and in the same bath with addition of bromide. In Fig. 170 there are shown the characteristic curves corresponding with the plates, exposed behind a step-wedge, which have been given the same exposure and been developed, some for two minutes and the others for eight minutes, in a solution contain ing no potassium bromide (solid lines), and in two other lots of the same bath, one containing a small addition of bromide (dash lines) and the other a larger quantity of bromide (dotted lines). For the same time of development in all three solutions the parts of the image which are fully developed show the same degree of contrast, but the parts which have received the least light (corresponding with the shadow detail in the negative of a natural subject) are incompletely rendered in the negatives developed in the baths containing bromide, and this to a greater degree as the proportion of bromide is greater or the time of development is shorter.

As development proceeds, the straight lines forming the middle (straight) parts of the curves change their angular position around the points A, B, C. Each of these points corresponds with a given proportion of bromide, the depression of the curves being greater as the bromide content is larger.' (A. H. Nietz, 1922.) It will be readily understood that the horizontal dis tance between the curves corresponding to the same periods of development in baths with and without bromide decreases in proportion to the progress of development. Now, this horizontal distance is a measure of the ratio of the exposures required to yield equal densities. It will there fore be seen that the apparent loss of sensitive ness due to development in a bath with bromide decreases progressively (regression of inertia).

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