When bromide is present in a developer, the straight-line por tions of the characteristic curve when produced do not meet upon reproduced, and this question, generally known as the theory of tone reproduction, is fundamental to every photographic ap plication.
Psychologically, it is the apparent brightness which is of im portance, but this can conveniently be treated as the physical brightness modified and interpreted by the eye and brain, and since it can be shown that the apparent brightness is proportional the exposure axis but at a point somewhat below it, the effect of bromide being equivalent to a constant depression of density in the straight line portion of the curve. (See fig. is.) The photographic characteristics of printing papers can be measured by a procedure similar to that used for negative ma terials, but in this case it is not the mass of reduced silver which is of chief importance but its relative reflecting power. The reflec tion density is thus the logarithm of the reciprocal of the reflecting power (D=logio 1/R), the reflecting power being measured with illumination at to the plane of the paper. The characteristic curves obtained with different times of development are shown in fig. r6, and it will be seen that the effect of increase in time of development is to produce a regression of the inertia without change in y except in the very earliest stages, where the curves are usually of a distorted form. The following constants may be
derived : to the physical brightness throughout a wide range, it is generally sufficient for tone reproduction in photography to deal with the physical tones in the original and the reproduction.
The differences in brightness which occur in nature may be produced by differences either in the reflecting power of the dif ferent portions of the subject or by differences in the illumination. Since in natural scenes both the reflecting power and the illumina tion vary, some parts of a landscape consisting of clouds in sun light and others of dark rocks in the shade, the range of contrast is often very considerable. For photographic purposes a scale or contrast of I to 4, in which the brightest thing is only four times as bright as the darkest, is very low, and such a subject would be called flat ; a contrast oft to r o is a medium soft contrast ; i to Tone Reproduction.—When a photograph of a natural object is made, the form can be represented only by differences in bright ness. The accuracy with which the form is represented depends upon the precision with which the tones of the original subject are