DEVELOPMENT.
In the first place it is necessary to understand what is the meaning of a negative. We may define it as a picture where all the shades of nature are reversed, where what is in reality black is shown as white, whilst what in reality is white is shown as black. When we hold up a photographic negative between us and the light this is what we see. For example, if the subject be a man dressed in a black coat, with white collar, etc., his collar will appear quite black, and his face nearly so, whilst his coat will appear white.
We here give a print which will explain the appear ance better than words can do. The first cut shows a negative, the second the positive, which may be produced from it by the printing process.
What we wish to produce thus by the camera is a negative representation of the picture on the ground glass. By the exposure which we have given we have got this negative, so to speak, in is to say, there is on the plate what is called a "latent image," that is to say, although there is as yet nothing visible, yet there is such a change of the silver bromide particles that, when the operation of development is performed, the negative will appear. We must further explain what is the effect if we have continued the exposure for a time either too short or too long, and how we can tell by the appearance of the negative whether or not the exposure has been sufficient.
Let us examine closely the landscape which we have chosen for our first experiment. We shall see that different parts of the subject reflect very different quantities of light ; in other words, there are different degrees of brightness. Possibly, to take an extreme case, there has been included a glaring whitewashed wall, or a line of clothes hung out to dry, whilst near this there is a widespreading tree with deep shadows under the branches. If we look with attention we shall see that certain small portions of the deepest shadow appear to be quite black. Apparently they send no light at all to the eye.
Now let us consider what takes place when the cap is removed from the lens. The image, which, as we know, is an exact counterpart of the landscape in front of the camera, and shows all the same gradation of light, falls on the plate. The brighter shades, or, as they are usually called, the " high lights," naturally impress themselves first, and were the exposure stopped at a very early stage nothing but these would be visible in the negative. All the half-tone and the darker tones, or, as they are usually called, the "detail in the shadows," would be quite un represented. To take the case that we have supposed, a very brief exposure would show nothing hut the white wall or line of clothes. As the exposure is continued, however, darker and darker shades find their representa tion until at last every detail which is visible to the eye is impressed on the film,—not visibly, be it understood, but in a form which may be made visible by the after process of development. When this stage is reached the correct exposure has been given. It might be supposed that no harm could come of giving a much longer exposure, and it is true that less harm results from over exposure than from under-exposure, yet excessive ex posure has its evils as well as under-exposure. It must be understood that there is always in the camera, besides the light forming the image, a certain amount of diffused light which arises from reflection from the sides of the bellows. If the exposure be much protracted this faint light will act on those portions of the plate which repre sent the blackest parts of the subject, and which ought to be represented by transparency, or a near approach to it.
Assuming that the student understands this explana tion of what we may call the rationale of the matter, we pass on to the practical development of a plate.
The chemicals which we require are the following : Pyrogallic acid.
Strongest liquor ammonia •HO).