Density.—The density of a negative has very little effect on the printing value, except to increase the time actually occupied in printing, whereas the contrast is the governing factor for a good print. Developing negatives to a certain density is a fallacy, although photographers have been invariably taught that this is the correct method. It is far better to develop for a certain contrast. Many have the idea that density and contrast are synonymous, whereas they are totally different ; the density of a negative is practically the thickness of the deposit, while contrast is the distance between the high light and shadow. It is possible to have a dense negative with little contrast, or a thin negative with plenty of contrast. If four plates of the same batch were exposed on a subject each receiving a different exposure (within the latitude of the plate) and developed together in the same developer for the same length of time, the result would be four negatives of different density, but all would have the same contrast. They would all give equal prints, but the time of printing would vary according to the density. The negative which received the longest exposure would be denser than the others and take longer to print. On the other hand, if these negatives were developed in the dark room, and development had been stopped when each reached the same density, they would all have different contrasts. And in this case it would be impossible to get equal prints. Some negatives would give flatter prints than the others, although each would take about the same time to print.
Therefore the exposure does not only decide the relations of the tones, but the density of the negative, and the development decides the contrast between the tones. And as the determination of this contrast is the only means of control the photographer has, there is no necessity to watch the process of development, which can be just as well carried out by calculation.
The necessary factors are a standard developer, a standard quantity of water, a standard temperature, and a standard time. Any variation in the first three factors can be corrected by altering the last. A strong or a warm developer acts more quickly than a cold or weak one, but this can be corrected by altering the time of development. The amount of contrast is also regulated in the same way, so that every thing required can be obtained by altering the time of development.
Time development can be carried out in an open dish ; but a tank is far preferable, as the dark room need only be used for loading the tank. With films and the Kodak developing tank, the dark room can be done away with entirely, as this system allows of the film being placed in the developer while the photographer is in ordinary day light.
If all the factors are carefully adjusted when developing in the tank by time, the best possible result will be obtained, whether the exposure has been correct or incorrect ; and this system entirely removes the temptation to over-develop under-exposures and to under-develop over-exposures, which is usually done when development is carried out by ocular observations. With under-exposures photographers continue the development, hoping to get more detail in the shadows, and make the negatives so hard that they are spoilt and will only yield prints of a soot and whitewash character.
Over-exposures are dense and opaque in the high lights long before development is complete. Hence there is a tendency to take them out of the developer too soon ; and this practice is so general that it is usually understood that the thin negative is the result of over-exposure, whereas it is due to under-development. Errors in development are often confused with errors in exposure : which leads to doubt where the fault really lies. But when all negatives are developed for the same time, and the other factors kept normal, there will be no error in development. Con sequently, the errors in exposure are easily located, and time development teaches correct exposure in the simplest way.
Developers.—Nearly all developers are available, but it must be remembered that variations of temperature have not the same effect on all. The normal time is usually calculated for Fahr. ; the time given must be increased by one-half in the case of pyro, and about doubled with rodinal, metolquinol or glycin if the temperature of the solutions has fallen to 55'. The Kodak formula is as follows : Stock Solutions.
A. Pyrogallic Acid . . . . . . . I oz.
Sulphuric Acid . . . . . . . 20 minims.
Water . . . . . . . . 28 oz.
B. Sulphite of Soda (cryst.) . . . . . 6 oz.
Carbonate of Soda (cryst.) • 4Water . . . . . . . to 28 „ For development take IA oz. of each and make up to zo oz. with water. Time of development is to minutes at temperature 65' Fahr.
Glycin is a great favourite, and Fuerst's formula gives a very clean negative, allowing for reasonable latitude before the half-tones become unprintable.
Stock Solution.
Hot water . . . . . . . . . to oz.
Sodium Sulphite . . . . . t „ Potassium Carbonate . . . . . . 3 „ Glycin . . . . . . . . . 24o gr.
Use a zo oz. measure in mixing, and add the glycin slowly to prevent excessive effervescence. For use make each ounce up to four ounces with water. The tank must be carefully washed before use with this developer, as any trace of hypo will produce bad stains. An alternative is the metolquinol formula in the last chapter. Some of the original advocates of time development advocated rodina] and similar formulw so diluted that twelve or more hours were required ; in fact, that plates should be put into the tank overnight and found in the morning correctly Such prolonged development has no good effect on the gelatine of the plate, and will often result in silver fog. Fot the benefit of experimenters we give Bothamley's formula lot one-hour development at 65°.
Glycin . . . . . . . . . 46 gr.
Sodium Sulphite . . . . . . . . 46 gr.
Water . . . . . . . . . 7o oz.
Develoj5ing and Fixing Combined. (V. Cremier).
Diamidophenol . . . . . . . 15 gr.
Sodium Sulphite . . . . . 15o gr.
Hypo (2o per cent. solution, 4 oz. to the pint) . 22 drams.
Water . . . . . . . . . 31, oz.
Fixation is said to be complete in about fifteen minutes but with many varieties of plates a second fixing bath wil be necessary and may be given in daylight.