The history of three-color photography (1925)

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48 History of Three-Color Photography absorbed than a stronger red, with the consequent final appearance of a red transmission only, as is the case with the dark-room green dye, introduced by the Hoechst Farbwerke. Von Hubl suggested the term "density" of the dye, to express the strength of a solution or dyed film. This convenient unit was introduced to define the quantity of dye, not only in solution but also when coated as a gelatin film. In the former case it is referred to 1000 g. or ccs. of the solvent, and in the latter case to the quantity of dry dye per square meter. This has become fairly general and is very convenient in practice. One important point in the manufacture of niters of all kinds, though it is of less importance with dark-room screens, is the purity of the dye. Commercial anilin dyes are in many cases advisably broken down by the admixture of inert, as regards color, substances, such as dextrin, sodium sulfate, etc. This must not be looked upon as adulteration, but rather as an intentional and generally accepted legitimate means of weakening a color, and is actually called for by dyers. Purity of the dye, in a chemical sense, is another matter, and it should not be overlooked that the consumption of dyes by the photographic industry is comparatively negligible, when compared with the demands of cloth manufacturers and others of that ilk. On the other hand the photographer is willing to pay well for pure dyes, for the working cost is really very small. Till recently all pure filter dyes were only obtainable from Germany; but since the war other sources have become available. A still more important matter, too often neglected by the tyro in filter making, is the character of the dye itself. That is to say, its acid or basic nature. By this is not meant that the dyes are acids or bases like hydrochloric acid or sodium carbonate ; but in the case of the basic dyes, the actual coloring principle is a base, and most of the commercial dyes of this class are such bases, combined with acids, such as sulfuric, hydrochloric, etc., and the nature of the acid has absolutely no effect on the color of the dye. In the case of acid dyes, the color principle is an acid, and the dye is a compound of these color-acids with a base, such as sodium, calcium, etc. For instance, one may take such a dye as uranin, which is usually a sodium salt of fluorescein, and by the addition of an acid throw down the acid fluorescein, and obtain a colorless solution. If an acid dye be mixed with a basic in dilute solution, there may be caused a cloudiness of the solution, and a consequent weakening of the absorption. Whilst if the solutions are strong there may be actual precipitation of the compound formed between the color-base and the coloracid, and the solution thus become colorless also. There is no way of finding out whether the dyes are acid or basic, unless through information from the maker, or by actually testing them, and this is not a difficult matter. All basic dyes are precipitated by a mixture of tannin and sodium acetate, and they have little or no affinity for staining gelatin, so that one