The judging of contrast solely by visual inspection of the image leads only to misleading results, except in the case of very long practice, limited to the photography of subjects of similar character. Judgment is apt to be deceived by the presence of the residual silver bromide, unequally distributed over the image, and by the coloured light of the dark-room. Red light, especially, tends to exaggerate the contrasts, and beginners nearly always tend to stop development too soon. When developing by examination by transmitted light, it is difficult to avoid the error of over-developing an under exposed negative and under-developing an over-exposed one, because the eye can more easily estimate the average density of the negative than the contrast between the extreme densities. 1 The beginner is sometimes advised to stop development when the portions of the emulsion which have been protected by the rebates against the action of light during exposure begin to veil over. Or he is advised to stop when the image begins to show at the back of the plate or film. Both these methods are most misleading.
If, with a correctly compounded developer, the development of a plate or film in a good state of preservation (on which only the light from the lens has acted) does not proceed on being continued beyond the point where fog appears, the latter being capable of increasing more rapidly than the image, it is quite possible for an image to fog before its development is complete. This occurs if the developer is exces sively active, and especially if it does not con tain enough bromide to avoid chemical fog with the emulsion used, or if the emulsion has been fogged when loading the camera or during the first instants of development.
On the other hand, an emulsion coating which is thin and poor in silver will always allow the image to appear more quickly at the back than one thick and rich in silver. This will lead to development being stopped before a sufficient degree of contrast has been attained. Even when the same emulsion is in constant use, so that the richness in silver and consequent translucency of the coating remain almost the same, the appearance of the image at the back can at most indicate only that the high-lights have reached a certain density. It is not, however, a given density in the high-lights which must be sought for, but a given difference between the extreme densities. If the subject is weak in contrasts, or if the exposure was full, the image of the shadows will be much denser when the high-lights reach a given stage than in cases where the subject is contrasty or the exposure on the short side. But it should be noted that if a little silver has been reduced in the protected portions, when the image has appeared on the back, the negative (unless considerably over-exposed) is probably suffi ciently contrasty to be usable, at least with papers suited for negatives of poor contrast.
It may be said that this indication is of some value for the beginner, a value more or less similar to that of other recommendations made to him with the same object. 1 When examining plates by transmitted light, it is necessary to be careful to hold them by the edges only, and films by two corners, avoid ing as much as possible placing the fingers on the surface. The negative must not be held near the lamp longer than is needful (especially when the lamp is hot), in order to avoid melting the gelatine. Unless the emulsion has been desensitized, examination must not be too protracted, especially at the beginning of development, otherwise the image may be fogged.
375. Rinsing the Negative after Development. When development is considered to be complete, the negative must be transferred quickly to a tank or dish of water, or held under a jet of water of moderate pressure, in order to free it from the greater part of the developer adhering on its surface, prior to fixing. As a rule, a few seconds suffice for this brief rinse. In the more usual case, where fixing is done in an acid fixing bath, this rinsing in pure water is sometimes replaced by placing the negative for a few minutes in water rendered slightly acid (by per cent of hydrochloric acid or 5 per cent of liquid sodium bisulphite), so as to neutralize the alkalinity of the developer, and thus to avoid the latter neutralizing the fixing bath. This acid bath is often called a because it arrests development.' The negative i then fixed (Chapter XXIX).
When development has been carried out under conditions which render washing difficult after fixing (as on expeditions), the actual fixing may be postponed by adopting deferred fixation with an acid bath followed by a brief rinse. It is then preferable to use a volatile acid (acetic acid of 2 per cent strength) or a weak acid (boric acid of 5 per cent strength), as a strong non-volatile acid would involve the risk of disintegrating the gelatine in the course of time if it has not been completely eliminated. After soaking in this bath for at least five minutes, the negatives are rinsed in clean water and put to dry_ 376. Critical Examination of Finished Nega tives. Although, in the normal course of opera tions, negatives cannot usefully be examined until fixed, and preferably only after washing and drying, it seems inadvisable to postpone to a subsequent chapter considerations so inti mately connected with development.