The history of three-color photography (1925)

18 History of Three-Color Photography J. Joly. — In describing his linear screen-plate process Joly said :31 "Maxwell curves are not color-sensation curves (Abney, 'Color Vision, Tyndall Lecture,' 1895) and it is misleading to speak of the foregoing method as effected on color-sensation curves. Maxwell curves represent, in fact, the subjective synthesis of the spectrum out of three chosen wavelengths, a red, a green and a blue-violet." Joly stated that accuracy of the curves was not essential and that he actually followed Konig's curves. The particular passage to which Joly referred, runs as follows : "As a matter of fact Clerk Maxwell chose colors which do not best represent the color sensations. The red is too near the yellow, as is also the green. The blue should be nearer the violet end of the spectrum than the position which he chose for it. We may take it, then, that except as a first approximation, Clerk Maxwell's diagram need not be seriously taken into account. The diagram itself shows that the color-sensations are not represented by the colors he chose. Supposing anyone, in whom the sensation of green is absent when examining the spectrum, there would, according to the diagram, be no light in the green at E. Anticipating for a moment what we shall deal with in detail shortly it may be stated that in cases where it is proved that a green sensation is absent, there is no position in any part of the spectrum where there is absence of light. Had he chosen any other green, the same criticism would have been valid. The diagram as it stands is really a diagram of color mixtures, in terms of three arbitrarily chosen colors, and not of color sensations. It merely indicates what proportions were needed of the three colors, which he took as standards, to match the intermediate spectrum colors." Newton and Bull's Work. — These two experimenters published one of the most important papers32 on this subject and laid down in such clear language the requirements for practical work, that but little else can be said on the subject. The paper deals with the practical performance of tri-color filters, and included an examination of many recommended at that date, with color prints of the spectrum obtainable with the same. They stated : "While many think that there are valid theoretical reasons for the use of filter curves, they must suppose at the same time perfect photographic plates with unlimited power to render gradation, that is to say, a straight line law for photographic action, which it can never follow, the plate to be perfectly exposed and the printing to be done in perfect inks ; but the spectrum reproductions with graded exposures prove conclusively that the action of a photographic plate is such that they can not be made to follow any definite set of curves, such as have been suggested in various hypotheses of tri-color reproduction. This result was to be expected, if the facts are taken into account when theorizing, for only in the case of very slow plates is there any proportionality between the light stimulus and the opacity produced in the negative, and in this case over a small part of the total range of action. Now slow plates are