It is important to know how long to continue development, and some experiments at the cost of 71 few plates will teach more than will many pages of printed matter. A negative that is taken out of the developer too soon is very thin, and will not give a good, clear picture ; whereas a negative that has been left in the developer too long will be dense or harsh. As a general rule, when the negative is sufficiently developed, the dark parts in the negative, such as the sky, will show through the negative, and can be seen when the plate is examined from the glass side. The plate may also be taken out of the developer once or twice, and examined by holding it up to the red lamp, and its density judged by looking through it.
In cases of under-exposure, the image comes up very slowly, or only the high lights appear and not the half-tones and shadows. Develop ment should be complete in ten minutes or even less. If after, say, fifteen minutes nothing, or very little, appears on the plate, try breathing on it, or warm the developer, but only very little, or the film will melt. If after a long time nothing appears, the plate may be destroyed as being useless. On the other hand, if the image appears
extremely quickly, and the plate goes black all over in a minute or two, the plate has been over exposed, or fogged by light other than that through the lens. A little extra potassium bromide added to the developer will sometimes save over-exposed plates if it is known that they are over-exposed before the developer is poured on ; but after development has started it is a waste of time to add the bromide.
Subsequent processes, assuming that all has gone well, include a minute's rinsing in cold water, and transference, film uppermost, to the fixing bath, in which the plate remains for a period twice as long as that occupied by the whiteness in disappearing. Thus, if the whiteness disap pears in ten minutes, allow the negative to remain for a further ten minutes. (See also " Fixing.") Finally, the negative, which may now be brought out into the daylight, is washed for at least thirty minutes in running water (see also " Washing "), while it stands on edge.
The beginner is recommended to adopt the factorial system of development. (See " De velopment, Factorial.")