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DEVELOPMENT OF THE NEGATIVE
in which each tone is truly proportional to the light-intensity (or light reflected by that part of the subject) which it represents. In other words, the truthful representation of the light and shade of the subject demands that the tones or luminosity contrasts in the positive shall range from light to dark in geometrical progression, i.e., as 64, 32, 16, 8, 4, 2, 1.
For example : let us suppose that we are photographing three houses — a white one, a gray one and a black one — and that their light-intensity values (or relative visual luminosities) are, respectively, 5 for the black house, 20 for the gray one, and 80 for the white one. Here the progression of light-intensities is geometric, viz., as 1:4:16. The truthful representation of tone in such a case demands that the relationship between the three houses in the positive shall be proportional to the relative luminosity of the three houses as seen by the eye — i.e., as 1 4:16.
This applies in every instance. Whenever we see a photograph wherein the tones are true to nature, we may be sure that this relationship of proportionality exits. On the other hand, when we fail to secure this vital relationship between the lightintensities of the subject and the tones in the positive, our photographs are necessarily untruthful in their representation of light and shade. As the gradations of tone in the photograph result from the opacities in the negative, it is plain that a similar proportionality between light-intensities and opacities must pre-exist in the negative. Here we have the key to the truthful representation of light and shade in photgraphy. With this in mind we can go a step further.
When we expose a film in the camera, the light-intensities at all parts of the subject begin at once to work a change in the sensitive film. The amount of work done (or light action) is, of course, determined by the intensity of the light at the same part of the subject. Thus, keeping aside for the moment all thought of the form of the thing photographed, the result of exposure is to impress on the sensitive film a latent range of gradations, distributed throughout the film and forming the latent picture image. On development, this latent range of gradations becomes a visible range of gradations, consisting of metallic silver deposited in the film by the reducing action of the developer. This is the negative.
Here we come to the parting of the ways. According to the old
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