GENERAL NOTES 368. Preliminaries. Before beginning the de velopment of a negative it is necessary to see that all solutions and utensils are at hand. The tanks or dishes should be so arranged that the negative being developed may be transferred from one to another in their proper They should be charged with the various baths required, care being taken that the temperature of these baths is about the same as that of the dark-room, which temperature should, whenever possible, be between and 65" F. 2 When the various baths, and particularly the developer, are prepared at the time of use by mixture or dilution of stock solutions, the water used for dilution should have been kept for a while in the dark-room in order to acquire its temperature. In all cases be careful not to dilute with water straight from the tap. The tap water is usually colder than the develop ing solution, and it also contains air which separates in bubbles when the mixture becomes warm ; these bubbles may adhere to the gelatine of the negatives, preventing the developer from acting at these points.' To ensure the perfect uniformity of a mixture it is necessary to stir vigorously (§ 278). On no account should the various liquids to be mixed be poured separately into a shallow dish, where it is very difficult to obtain a uniform solution even after prolonged agitation. It is still more important to avoid a direct addition to the bath during development of negatives. When de veloping in a dish, the developers must be poured back into the graduate, and any required additions then made. When developing in a tank, the negatives must be removed, and the developer stirred energetically after adding the new solutions.
It is essential that every care be taken to ensure uniformity of treatment during the various manipulations. For this, the emulsion must be wetted almost simultaneously at all points by a perfectly uniform developer, and the plates or films must be then moved several times.
It is specially essential that the solutions used should be perfectly clear, as any suspended matter produces a spot where it settles on the emulsion. All stock solutions and baths pre viously in use should therefore be poured off from any sediment.
369. Wetting Plates and Films before Develop ment. In quantity work with sensitive material in long bands (e.g. cinematograph film), the adherence of air-bubbles to the gelatine is usually avoided by wetting in water containing a little alcohol (industrial spirit), and sometimes also various substances facilitating the wetting of parts where there are traces of grease (contact of fingers or of lubricating oil from feeding mechanisms).' For this it is possible to use very
dilute solutions of the wetting agents used in textile manufacture : saponine, sulphoricinates, and sulphonated fatty alcohols (Ocenol, Lorol, Igepon, etc.) These substances can also be added to the developer (§ 357) or to the desen sitizing bath (§ 333). Except in this last case, preliminary wetting has usually no advantage in the case of sensitive material in cut Preliminary wetting accelerates development in a concentrated developer and slows it in a dilute one. With some developers it is possible to ascertain by methodical tests the strength required in order that the duration of develop ment should not be affected by the preliminary wetting. Any tanning of the reduces the speed of development only very slightly.
In every instance where the emulsion has been wetted before development it is necessary to rock the developing dish energetically for a few or to move the plate-holders or film-hangers in the solution so as to ensure the uniform replacement of the water or solution impregnating the gelatine by the developer.
370. Immersion in the Developing Bath. When developing a plate in a dish, hold it by its edges, film upwards. Large plates must be supported on the four spread fingers of the right hand and held by the thumb placed on the extreme edge. For very large plates the help of an assistant is almost indispensable. The dish is tilted to collect all the developer along one side. An edge of the plate is then rested on the bottom edge of the dish opposite to the one where the developer is lying. The dish is then let down into the horizontal position, and, at the same time, the plate is lowered to the bottom of the dish. The plate is thus swept almost instantaneously by the liquid. The dish is gently rocked, and care is taken to see that the plate is uniformly wetted. If desensitizing has not been done before development, the dish is covered with a piece of card or cover (a larger dish can serve) unless, of course, it is necessary to watch for the first appearance in order to apply the Watkins system of factorial develop ment (§ 344). Unless the worker is skilled, it is best to avoid developing several plates together in one dish, as there is a risk of damage from one plate sliding over another.' About 3 O. of developer is required in a dish of half-plate (61 x 4 in.) size, and propor tionate quantities in dishes of other sizes. It may be well for a beginner to increase these amounts by one-third in order to avoid any irregularity in the wetting of the emulsion by the developer.