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Historical and Theoretical Data 9
underlying principles, we have now arrived at the strict lines on which to work.
If further progress is to be made in the future, it will be rather in minor details than in fundamentals. We may yet hope to see improvements in the sensitive surfaces ; in the obtaining too of printing inks that more nearly conform to the strict theoretical requirements, for here would appear to lie the field of the greatest advance. The improvements in color-sensitizing of the past few years, as exemplified in the discovery of the isocyanins for this purpose, would seem to present but little further chance in this direction. The emulsion maker can and does supply us with all that the most exacting devotee may demand, though possibly the theorist would still require a higher speed than is already obtained with all the advantages of freedom from fog and fineness of grain concomitant with the slower emulsions. Theoretically perfect inks are still a desideratum, though much has been done for us by the ink makers since the first three-color prints were commercially issued about 1889.
Exactly what the fundamental axioms of three-color work are, and how we have arrived at them, may be gathered from the following pages, wherein some attempt has been made to gather the opinions of many, with references, as being preferable to the laying down of mere personal opinion, though this would not differ from the main result.
Ducos du Hauron. — Clerk Maxwell does not appear to have used or suggested the application of his process to the production of subtractive prints or transparencies, contenting himself entirely with the additive method. Du Hauron, on the other hand, clearly outlines the possibility of doing so,17 and the fullest exposition of his views is to be found in his English patent, in which in defining the particular shades of his filters, he gives the main principles of his method.
H. W. Vogel's Theory. — After describing du Hauron's work, Vogel says : "Another defect of Ducos' process is the comparatively arbitrary choice of the printing inks. The rule that the plate photographed through a colored glass must be printed with a color complementary to that of the colored glass is not sufficiently precise and allows considerable latitude ; for statements as to the colors which are complementary to one another are very doubtful. Thus it is said that the complementary color to red is green, which green is not determined. Actually the complementary color to many reds is rather a blue than a green. The author (Vogel) proposes below a modification of the processes which is free from the above described faults. This is as follows: — 1. That instead of one optical sensitizer several should be used and actually each with a special plate, thus for instance, a sensitizer for red, one for yellow, one for green, one for blue-green ; for blue one is not required as silver bromide is itself sensitive to blue. 2. That the optical sensitizers should be also the printing inks for the plates thus obtained, or if the sensitizers themselves can