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It is, of course, obvious that the grain size of the emulsion must be very small. Consider, for example, the colour chart photograph, first as a negative in complementary
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colour, to have the blacks so opaque that no light can get through them.
At first sight, the most obvious type of emulsion would be the ‘‘ soot and whitewash”’ type employed by the process engraver. But such emulsions have an extremely short scale of gradation, whereas in our case we want a very long scale of gradation, because, in order to get the infinite number of delicate intermediate colours which exist in Nature, we must have mixtures in varying proportions of the three primary colours. To get such proportions we must, of course, have an equally infinite scale of variation in the density of the silver deposit.
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colours, and secondly as a positive. Any one of these colours owes its colour to the fact that only those parts of the spectrum which compose it are allowed to pass through the film. Inthe case of a green leaf in a landscape, for example, if it were pure green the blue squares and the red squares in the matrix would be completely blocked out by silver deposit, so that light could only traverse the film through the green components of the matrix. The same with the other colours. It is, therefore, indispensable in order to get a pure
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A studio shot developed half as a negative and half as a veversed transparency.
In order to get this unlimited blending of the three primaries, I have always held that the spectrum should be treated as a matter of geometry—in other words, each primary should have a maximum in the centre of its region of transmission and should taper off symmetrically on either side, as shown in the diagram on the screen. When putting forward this theory, in 1905, I was vigorously attacked by other colour technicians, and I believe that to-day the view generally
held is that the spectrum should be more or less divided into three water-tight compartments—each section, at any rate, should only slightly overlap the next. :
If the geometrical arrangement I have shown be adopted, the eye itself takes care that the various mixtures of the three primaries have the required luminosity; this is a purely physiological matter. To use that old phrase, ‘‘ the proof of the pudding is in the eating,’ and I might add that the use of such filters is the only possible way of reproducing by the additive process the full spectrum in faithful colour rendering ; I think, too, you will admit when you see the pictures we are showing presently that our colour rendering is of a very high order.
The next slide shows how very wide the overlaps of our primaries are. You will see against each of them the transmission of the usually accepted additive filters, and how very much more condensed, i.e., not overlapping, the latter are. We use these big overlaps, each primary tailing off into the next, for both taking and copying, and they
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