The Optical Magic Lantern Journal (February 1901)

23 The Optical Magic Lantern Journal and Photographic Enlarger. The Optics of Trichromatic Photography.—Part III. THe Traitt Tayntor MemoriaL LECTURE. Continued from page 10. + Du Hauron displayed extraordinary ingenuity, but, like Collen, he was missed by Brewster’s theory of | colour, aud there were fatal defects in every method which he proposed. One of his mistakes was in the assumption that pure colour screens should be used in the photographic analysis intended for positive synthesis, and that the same screens should ba used again in the synthesis. Another was that negatives of a totally different character would be required for the purpose of negative synthesis by superposed transparent colour prints.* He recognised that the opacities of each of the three negatives intended for positive synthesis should represent a single colour element only, but not that they should be made through ‘‘colour curve’' screens for synthesis with ‘pure colour’? screens. With respect to the character of negatives required for colour priat synthesis, he made exactly the same theoretic mistake as Collen. He assumed that the opacities in each negative employed to make colour prints should represent two of the colour elements instead of one only; but, instead of superposing two single element negatives, he proposed to allow two colour elements to act together in producing each negattve.t For instance, he would allow both the blue and the yellow to act in the production of the negative for making red prints. The principle ie exactly the same as that enunciated by Collen, and has the sama fatal defect. Du Hauron himself saw that two colour elements could not be fully represented in a single vegative in their relation to the whites and greys, but declared that in practice the defect was not as great as would naturally be expected, and made an ingenious but wholly inadequate argument to explain away the difficulty. No amount of argument can nullify a fact. It has been asserted that Du Hauron’s ‘‘binary” spectrum colours in reality correspond to the true ‘‘ primaries,” the descriptive term aod not the colours themselves being erroneous. Writers who have made this assertion imply that I have misunderstood and misrepresented Du Hauron’s work, but they are mistaken. Du Hauron not only plainly specified the ‘sifting of the rays of the two other colours,” in contradistinction to exclusive sifting of the simple colours, but he distinctly *“ When the reader has read this book entirely through, he } will recognise that, in the method described in Chapter IV., and which constitutes the direct method [for positive synthesis] . . each of these images is obtained by the exclusive passage, or sifting of the rays of the simple colour corresponding, through a space of the same colour. Proceedings are not the same for the method of inversion" |for colour print svnthesisj.--From Louis Ducos Du Hauron’s treatrwe of 1869. t “Bach of the images, instead of being obtained by a sitting ot the rays of the corresponding simple colour through a space of that colour, is obtained by a sifting of the rays of the two other colours through a space of the double colour compleinentary to its own.’—Du Hauron in 1869. said that yellow and blue objects must act alike (and as | white) through the “green” screen, and while admitting that a cobalt blue glass is better than a ‘‘ violet” * one for the blue-red negative, he adds that ‘‘ thera is here presented a physical fact not in accord with the theory." This error of Collen and Du Hauron dies very hard, and is perpetuated with supreme assurance by nev writers every little whils. One fact which must not be ignored is that, although Du Hauron’s theory was that the printing colours should correspond to his idea of the primary spectrum colours, red, yellow, and blue, the colours which he actually racommended are not very far removed from the minus colours of the true ‘‘ primaries.’ His blue (Prussian blue) is a greenish blue; his red (carmine) is a crimson red; and his yellow is a minus blue. The fact is, that sheer experience led him to depart very considerably in the right direction from the requiremants fixed by his theories ; but his conclusion, most positively stated, that the photographs for positive synthesis should be made by “exclusive passage of the single colour corresponding,” while the photographs for negative synthesis should be made by ‘‘ rays of the two other colours,"’ is proof positive that he utterly failed to grasp the true relation between the two kinds of synthesis—the plus and minus character of the synthesis colours themselves—which certainly demands identical colour records for both purposes, and also that they shall be ‘‘single element,” ‘colour curve "’ records. Du Hauron’s most important new contribution to the development of trichromatic photography at this period was his suggestion of a photochromoscopic device, of which I shall speak in another part of my lecture. This may seem scant treatment of Du Hauron's early exploitation of the principles of trichromatic photography, in view of the fact that many people have regarded his early writings a3 the fountain head of tbi: sciencs; but I must take the facts‘as i find them, and his more or less natural errors do not in any way detract from my admiration of his prophetic spirit, and ingenuity, and fertility of imagination, which should be an inspiration to everybody who can read his original treati:e on trichromatic photography. Du Hauron's first patent application was dated November 23, 1868, but his first printed publication appeared in le Gers, March 11, 18, 16, 18, 20, 23, and April 1 and 6, 1869, and shortly afterwards in pamphlet form. According to his brother, Alcide Ducos Du Hauron, he had communicated his ideas very fully, except in the matter of negative synthesis, in a mémoire transmitted to a French scientist and Member of the L[ustitute, M. Lélut, July 14, 1862. It is a curious fact that this mémoire, said to have been intended for formal presentation to the Institute, but which was not published unti) 1897,t discloses several important ideas which are not to be found in any of Louis Ducos Du Hauron’s writings which were published prior to 1897, and which had meanwhila been published or patented by others. I shall have occasion to refer to this mémoire again. After Du Hauron applied for his patent, but before there had been any publication of his method, Chas. Cros, of Paris, proposed a system of trichromatic photography ia a communication which appeared io Les Mondes, February 25, 1868, and it is stated that he had described the same system two years before in a sealed mémoire deposited in the Academy of Sciences. (To be continued.) * He describes “violet” as a colour containing a large proportion of red, showing that lhe means purple, which is called for by his theory. + “La Triplice Photographique des Couleurs et l'Imprimerie.” Alcide Ducos Du Hauron, Paris, 1897, p. £50.