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Historical and Theoretical Data 19
useless in all cases where an extended range of gradation has to be rendered, as in oil paintings, for this purpose fast plates have to be employed, and with these there is scarcely any region of proportionality at all, as was proved by Messrs. Hurter & Driffield. Another consideration is that such curve filters, even if possible, would be difficult to make, and we think we are safe in assuming have never yet been made (although approximations have), and if they were, they would certainly break down in use. The fact that the exposure on the lower slopes of a curve will often fall below the minimum of the effective stimulus (so-called photographic inertia) causes those to be lost entirely, restricting the recorded regions to the higher parts of the curve. On the other hand, in practice it is found that they are not necessary, since the spectrum can be tolerably imitated and all natural colors fairly well reproduced by using filters and plates recording three even bands, the blue-violet extending to wave-length 5000, the green record extending from wave-length 4600 to 6000, and the red record from wave-length 5800 to the end. The ultra-violet should be cut out, since it can have no possible utility, and does in certain cases exercise a degrading effect on some colors. Such filters appear to us to give the best results throughout a range of contrast within the limits of the plate.
"The influence of the sensitiveness and the capacity for rendering gradation of the plates is very great, thus no plate records the extreme red, and with some so-called panchromatic plates used for three-color work, even a light red is recorded as black. However, in the case of pinachrombathed plates and collodion emulsion dyed with ethyl violet, the red is recorded much further than usual. With regard to gradation it is obvious that with a plate having a fairly steep range, the light tones of the picture will be very fully exposed before the deeper ones have time to fully impress themselves, and this becomes of the utmost importance when the complementary colors are strongly contrasted in the opposite way to the record desired, thus a dark green and a light crimson, a dark blue and light yellow, a deep orange and pale blue ; in each of Jhese pairs the dark colors should be more or less recorded and the light ones more or less stopped, but with normal filters and normal exposures on a plate having a steep gradation this is not done, as we have proved in the case of wet collodion and collodion emulsion. The ideal conditions of filter and plate would be that colors should always record evenly throughout, even with the longest exposure, not extending beyond the limits laid down, and even with the shortest, going all the way, though the deposit were less opaque. It should not be forgotten that in an ideal three-color negative the selected colors should be at least as well recorded as white itself, because if there is any less opacity, then it means the subsequent printing there of some part of the antagonistic complementary. Thus, if a white and a pure green do not record equally well through the green filter, then