When developing cut films in a dish it is best to choose a dish sufficiently large to accommo date two films side by side, and to have plenty of developer in order that the films be well covered in spite of their tendency to curl. The film is slid into the developer, emulsion upwards, and, while held by two adjoining corners, is drawn to the half of the dish farthest from the operator. Any bubbles that may be adhering are removed by touching them lightly with the finger. As soon as this film is well wetted with developer several other films may be introduced in succession, each being drawn to the back of the dish, leaving the front half free, until the last film of the batch has been introduced. The films become very soft, and must be handled with care. They are taken one by one and transferred in reverse order to the front half of the dish, and this transfer from one pile to the other is continued until develop ment is completed.
When single frames or hangers are used with plates or films developed in a tank, care must. be taken to see that air-bubbles are not im prisoned in the frame, as such bubbles, when rising, may adhere to the emulsion. To do this, the frames are immersed one corner first, obliquely, and they are gently moved up and down immediately after immersion. The plates or films are introduced with the emulsion facing the operator, and are put in at the back of the tank and then brought to the front so as to leave the back free for the others. By proceed ing in this way there is no danger of scratching the emulsion of one negative by the frame containing the next.
After inserting the last frame, the frames are separated as far as possible and moved up and down singly from time to time in order to bring fresh solution in contact with the emulsion. In doing this, each frame must be drawn away from the one behind it so as to avoid scratching the gelatine of the latter.
If the frames are much narrower than the tank they all must be pushed up against one side to avoid retarded development of such parts of the emulsion of a plate or film as would come too close to the vertical side of another frame. Unless the frames are very widely separated from each other it is necessary to avoid develop ing together negatives of dill sizes.
When development is done in vertical, grooved troughs, the developer should be emptied from time to time into a jug, from which it is at once poured back into the tank. Instead, the tank, if fitted with a watertight lid, may be reversed every two minutes.
Owing to the variety of the appliances in tended for the development of roll film, the only reference which can be made is that the tions issued with them should be followed.
371. Local Development. During tlic® develop ment of a negative of large size a skilful operator can modify the image by localizing the action of the developer. This is specially the case with negatives which have been desensitized, for it is obvious that the image must be very plainly visible when carrying out applications of this kind.
Development is done in a comparatively slow acting bath, and as soon as the image appears, development of the parts which tend to come up too quickly is delayed by the application of a solution of bromide. On the other hand, the appearance of the insufficiently dense parts can be hastened by applying a more energetic developer (more concentrated, warmer, or more alkaline), using for the purpose a soft brush or tuft of cotton-wool, according to the size of the image. There is no need to follow the outline of the image very closely if the precaution is taken of applying the solution in several stages, alter nated by washing in water, followed by immer sion again in the developer. In this way the various parts merge into each other, and a harsh edge is entirely avoided.
This method is particularly suitable for obtaining the correct tone value of the sky in landscapes photographed without special pre cautions. The negative is held sky downwards during the local application of the bromide, thus preventing the bromide from running on to the image of the landscape part. With practice the same method can be applied to more difficult cases, such as interiors or badly-lit portraits, where the parts to be treated are more complicated in their distribution.
It may be added that the choice of a suitable method in the first place is generally sufficient for the production of a satisfactory negative, without recourse to these dodges, which are always risky in the case of subjects which, in the event of an accident, could not be re photographed.
372. Factors Influencing the Duration of Development. It must first of all be noted that there is no single optimum time for perfect development for a fixed set of conditions (subject, type of emulsion, composition and temperature of developer). Instead, there is an infinite number of normal times of development which range between two fixed limits, each time included within those limits being the best for some conditions of printing, type of positive sensitive material (§ 503), and mode of printing, i.e. by contact or enlargement in various ways). To reproduce a complete scale of tones, every positive emulsion requires a negative of which the extreme densities transmit light in a definite ratio, which ratio varies with the method of printing employed. For example, enlargement by artificial light with a condenser needs a notably less contrasty negative than for contact printing on the same material. For contact prints, positive emulsions of great contrast yield the best results with negatives in which the extreme opacities are in the ratio of io : 1, while print-out papers give prints with a com plete tonal range, i.e. from white to black, only if the range of opacities of the negative is about 60 : I. As development progresses, the contrasts increase very quickly at first, then more and more slowly, the negative thus passing through a series of stages each suitable for a specific purpose.