The first essential to safety in development is to standardise the developer. It is obvious that if we are to develop a particular plate or film for a certain definite time, we must always use a developer which is constant in composition, strength and activity. We cannot be sure of this if we employ stock solutions made up at home or bought ready-made. Even if they are accurate and fully active when first made up, the nature of photographic developers is such that in solution they must lose activity on keeping. With some solutions, and under some conditions, this deterioration takes place rapidly ; with others, and when great care is exercised, it may proceed more slowly. But it always takes place to some extent, and the amount of deterioration cannot be estimated, much less judged, with accuracy.

Consequently, if five minutes is found to be the time when the solution is freshly made or newly bought, six, seven, eight or more minutes may be necessary after the solution has been kept for some while.

The Effect Of Temperature

It is well known to photographers, even of limited experience, that development takes place more slowly in cold weather than in hot. In other words, developers are more active in warm weather than in cold. Few, however, recognise how great the difference is, especially with some developers.

In ordinary tank development of a roll of film, a difference of five degrees of temperature, say from 60°F. to 65°F., may alter the right time for development as much as four minutes. Fortunately, there is no difficulty in allowing for this factor if we recognise its importance. We can always take the temperature of the solution with which we are working, and so prevent climatic variations putting us in the wrong.

Influence of the Plate or Film used.

-Some people have assumed that if one plate takes five minutes to develop under standard conditions every plate will take the same time. Here they are wrong, and the error is an extremely serious one.

As a matter of fact, some plates take four times as long as others to develop under precisely the same conditions, and it is quite a common occurrence to have three kinds of plates, of the same speed, and exposed exactly alike, one of which requires five minutes, another seven and a half minutes, and another ten minutes' development in order to produce negatives of the same character.

Time Development

Now, how are we to determine how long to develop ? The old methods were two :-

(1) ;To keep holding the negative up to the dark-room light until it was thought to be right.

The difficulty here is that modern fast plates and especially colour sensitive plates resent this treatment, and will fog even in a very dim red light. Further, owing to the presence of the unaltered silver bromide, it is very difficult to judge how the plate will look when it is fixed, even if we are quite sure how it ought to look-a point on which even experts make mistakes.

(2) ;A second method is to look at the back of the plate from time to time and to stop development when the main details are plainly visible there.

This is a very misleading method, because the appearance on the back varies tremendously with the different plates and different subjects.

The modern scientific and safe system is that of development for a fixed time settled on beforehand.

It is really the outcome of the Hurter and Driffield investigations into the subject of exposure.

As already stated, there are two definite rules for this scientific method. They are so important that they will bear repetition :

(1) Correct exposures should be developed for a definite period of time in a standard developer of constant activity. This time varies according to the temperature, the plate or film, and the printing process to be employed.

(2) Incorrect exposures must be developed for the same time as correct exposures, the other conditions being the same.

The first objection generally made to this method is that it must mean very weak negatives from under-exposures, very dense negatives from over-exposures. The answer is that it does, but that it preserves the gradations in each as correctly as is possible.

It is no good going on developing an under-exposed negative after you have got out all the detail in the shadows which the scanty exposure has impressed upon the plate. You cannot develop what is not there. You can only produce fog in the shadows which masks the faint detail already present.

At the same time, in the high lights, where exposure has been full, you develope up density, w;hich is a positive nuisance.

The sane way is to develop for the standard time and get a thin but perfectly-graded negative free from fog. This will point with care on a slow gas-light paper, or can be intensified.

The next argument is : How am I to know how long to develop ? The answer to that is that you must depend on the materials and figures supplied by scientists who have the time and knowledge to devote to the subject.

The whole subject of time development in dish or in the tank is treated very fully, and very explicitly in the " Wellcome " Photographic Handbook.

Remember that the length of the exposure governs the density of the negative ; the length of development governs contrast. Therefore, if the negatives are too thin, the cause is under-exposure; if too dense, the cause is over-exposure. If the negatives yield flat prints, development has been too short for the printing process ; if the prints are too contrasty, development has been too long.