The analogy between the horse power of the automobile and the power of the developer is really very close. The high horse power automobile will start from rest very much more quickly than the machine of lower horse power, just as the elon developer forces out the image all over the film much more rapidly than the hydroquinone developer. Just as the horse power of an automobile could be measured by the effect of a hill on its speed so the power of a developer can be measured by the reduction of density produced by the addition of bromide, and just as one would not wish to have an over-powered automobile, hard to handle and always picking up speed very rapidly, so it is difficult to use the very high-power developers, and elon, for instance, is rarely used alone, but is generally adjusted by admixture with the slower hydroquinone.

Pyro is an almost ideal developer for negative making. Owing to the fact that the pyro is changed during development into a yellow colored substance, some of which remains with the silver in the image, pyro tends to give a slightly yellowish or brownish image. The yellowish stain is prevented from forming by sulphite, so that the more sulphite there is in a developer the less tendency to warmth the deposit will show. Pyro is not used for papers, for which the blue-black image obtained with elon and hy-droquinone is preferred.

When a film is developed, it is only the grains of silver bromide which have been changed by the action of light that are affected by the developer. The grains that have not been changed are not affected; at the beginning of development there are a great many exposed grains ready to be developed, and then as development proceeds, these exposed grains are turned into grains of black silver, so that the number of developable grains decreases during development until at last there are no developable grains left; all those which can be developed have been acted upon, and development ceases.

The rate at which the development proceeds can best be understood by an analogy from fishing. Suppose one went out fishing and found a pond where there were about four hundred fish. In the first day's fishing one might catch half the fish in the pond, or two hundred fish, but the second day one would not expect to catch the other half; all one could expect to catch would be the same proportion of the remaining fish, that is, half of what were left, or one hundred fish, and the third day one might catch half of what were left again, or fifty fish, and the fourth day half of what were left again, or twenty-five fish, and so on, the catch growing smaller as the number left decreased, until finally no fish were left to catch, or more probably until one got tired of trying to get the few remaining fish.

This is what happens in development. The rate at which the grains develop depends upon the number of undeveloped grains left, and as the grains are developed up and the number of undeveloped grains remaining become less, fewer and fewer grains develop in each minute, until finally, it is not worth while to prolong the development in order to get any more density. (See Fig. 54.)

If the development is prolonged beyond the point at which all the exposed grains are developed, then there is a danger of developing some of the unexposed grains, which produces a veil over the whole negative - exposed and unexposed portions alike - and this veil is known as fog.

At Beginning

After 1 Minute

After 2 Minutes

After 3 Minutes

After 4 Minutes

After 5 Minutes

Development of exposed grains in a film which is half developed in one minute.

Fig. 54.

The growth of the image during development is referred to as a growth of density, that is to say, the density is a measure of the number of grains of silver which are produced at any given point because these grains of silver, after the film has been cleared by the fixing bath, obstruct the passage of light through the film. We have seen that the density of an image is measured in units which are based on the amount of silver which will let through l/10th of the light, so that if only l/10th of the light falling on the negative gets through a certain part of it, that portion of the negative is said to have a density of 1. The blackest part of a negative may have a density of perhaps 2, the middle tones 1 or less, and the shadows, perhaps l/10th. (Fig. 55.)

The difference of density between the darkest portion and the lightest portion of the negative is called its contrast. In most negatives the shadows are nearly clear so that the contrast depends chiefly on the density of the darkest portion, but this is not necessarily so because an over-exposed negative, or one taken of a very flat subject, may have no clear portion in it and may be even very dense owing to over-exposure, and yet not contrasty at all because there is very little difference between the density of the most exposed portion and that of the least exposed portion, the negative being very dense all over. It is necessary to keep clearly in mind this difference between the density and the contrast.

Since the contrast depends chiefly upon the density of the highlights, it grows during development just as the density does. It grows rapidly at first, when there are many grains to be developed, and then more slowly until, finally, when the grains are all developed, the negative will not give any more contrast however long development may be prolonged, and a continuation of development will only result in the production of fog. (Fig. 56.)

Shadows

Half Tones

Highlights

Fig. 55. Densities of Various Parts of a Negative.

After 1 Minute

After 2 Minutes

After 3 Minutes

Fig. 56 Growth of Contrast During Development.

The final contrast which can be obtained depends upon the kind of emulsion used. The fast emulsions, such as the film emulsions, give moderate contrast, but the slow emulsions, such as those used for copying purposes or for making lantern slides, are specially made to give great contrast when development is prolonged. (Fig. 57.)

Highly Sensitive Film

Medium Speed Plate

Slow Lantern Plate

Fig. 57. Greatest Contrasts With Different Emulsions.

It would be convenient if the manufacturer could make the film so that it would be impossible to over-develop it, but this is not practicable. It would be possible if a film developed at an even rate and then stopped developing when it was correctly developed as is shown in Fig. 58, where development is supposed to go straight on for a given time and then stop altogether, the film not changing after that time. But the film does not develop like this; the growth of the image gets slower as time goes on but it takes a very long time indeed to stop completely, so that the growth of the image occurs as shown in Fig. 59. If a film were made so that we had to develop it as far as we could, it would take too long to develop, and therefore it is necessary to make a film that is capable of giving more density than is required in order that it may be developed in a sufficiently short time; this means that we must be able to stop development at the right time to get enough density and contrast, the density being the blackness of the image and the contrast.