In over-exposure, the image makes its appearance very rapidly all over the plate, and unless checked will soon become black and lost. To prevent this, the solution is returned to the measure, and about ten to . twenty drops of a 10 per cent solution of potassium bromide is added to it, then returned to the dish. This, acting as a restrainer, will slow down the action, and development is proceeded with until the image appears at back. A negative thus produced will probably be very dense, and require reducing. See "After-Treatment of the Negative."
In under-exposure, the highest lights of the subject will appear first, and will gather density out of proportion to the darker or shadow portions of the picture. To assist this, to the solution poured back into the measure, add two more parts of B, return to dish, and gently rock. Development is then carried on until all obtainable is seen on the back. Care should be taken not to over-develop; that is, as soon as the detail ceases to appear development is as complete as can be got under the circumstances, and keeping the plate in the solution will only tend to increase contrasts without any gain in detail. Although the negative may be thin and weak, it may possibly be improved by intensification. See "After-Treatment, etc."
Some workers find it useful to work to a factor. This depends upon the time at which the image first shows itself after the plate has been placed in the developing solution. Thus, supposing the image appears in just 1/2 a minute, the complete development of the image to suit the worker's idea may take 5 times as long - 2 1/2 minutes - in this case the Factor 5 is taken. The worker then uses the figure 5 when developing with the same solution and if possible at the same temperature, and multiplies the length of time elapsing before the appearance of the image by it.
When development is complete the image has to be fixed. The plate is rinsed with water and then placed in the "Fixing bath," which is made up of four ounces of hyposulphite of soda, commonly called "Hypo," to a pint of water. It is kept in the fixer until the creamy-looking substance to be seen at the back is dissolved away. Fixation must be complete, and to ensure this the plate should be left in the fixer for a short time longer, say half as long again as it has taken to clear; or better, transfer to a fresh fixer.
When the plate has been thoroughly fixed, it has to be well washed, to completely remove all traces of the fixing salt "Hypo." If only one or two plates are under treatment the washing may be done in the developing dish, by changing the water about every 10 minutes for an hour or so. The elimination of the Hypo is materially assisted by giving the plate about half a dozen quick changes of water as soon as fixed, and then proceed as directed. When there are more than a couple or three plates to be washed, a washing tank, Fig. 39, will be found the most useful. The plates are first put in the rack, B, by sliding them into the grooves. The rack is then placed in the tank, A, which has been previously filled with water. The tank works on the siphon principle, that is, the water runs in at the top and as it rises in the tank, as soon as it reaches the level of the curved tube on the outside, the water rising up the left tube from C, immediately passes down the right tube and out at the bottom, D. The plates being in an upright position, as the fixer is dissolved out of the gelatine it falls to the bottom and passes out through the siphon.
Alter washing, the plate is put away to dry, in a place where no dust can settle on it. The plate, while drying, should stand on its end, to allow the water to drain off. A "Draining rack" is easily made by driving some long nails into a piece of wood at such a distance apart that two will support a plate with one corner pointing downwards; this allows the water to drain down to that corner. When the plate is dry we have a "NEGATIVE."