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110 PRINCIPLES OF CINEMATOGRAPHY
some time by developing a quantity of waste film thus removing the excess sulphide which has collected whilst the developer was out of use. Sulphide fog may be eliminated if the solution container is sterilized with sodium hypochlorite before use.
uneven development. Continuous streaks, parallel to the films' travel through the machine, usually suggest poor solution circulation and can be eliminated by increasing the agitation and, if possible, the speed of the film through the machine. The introduction of agitation by air bubbles is also found to be effective in some cases.
yellow stains. When these are fairly transparent and 'clean' they are usually caused by developer oxidation but, when darker and dirty in colour, they are caused by silver left in the film due to inadequate fixing.
Functions of the Processing Solutions
From the foregoing very brief survey of some of the possible troubles which may occur as the film passes through the solutions, it will be apparent that two main systems of control must be applied if continuously high quality results are to be achieved. The first of these concerns the analysis of the processing conditions as they vary throughout the useful life of the developer and the second, which is only possible after the first has been established, concerns the adjustment of the developing conditions to maintain a relatively constant condition.
The analysis of processing conditions is made by a system of production control known as Sensitometry and will be discussed in detail at a later stage. Assuming for the moment that such control is available, it is obviously necessary to provide the means to hold the developing conditions stable. Because of this a system of developer replenisher is used whereby a known quantity of exhausted developer may be removed from the solution and replaced by a proportionate amount of fresh concentrated 'replenisher'. The amount of solution which is intentionally removed from the bath does not represent the total amount by which the original quantity has been reduced. A certain amount of developer will usually be carried over by the film from the developing bath into the rinse bath and, naturally, this quantity must also be determined and replaced.
Once the quantity lost due to 'carry-over' from one tank to the next and the amount of replenisher necessary to overcome exhaustion has been determined by experimental observation over a period of constant machine conditions and also by chemical analysis of the solutions -it is then usual practice to calculate an average rate of flow at which the replenisher should be introduced into the main solution. It is obviously desirable for replenishment