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 Nov., 1913 211 A MNEMONIC DEVICE FOR COLOR-WORKERS Based on a consideration of Ridgway's "Color Standards and Nomenclature" By WILLIAM LEON DAWSON E HAVE ALL caught ourselves making plctt..res, geometrical designs or graphs, out of mental concepts. Thought relations of all sorts tend to arrange themselves automatically into spatial groups. Thus, the .days of the week to our minds are segments of a closed circle, or steps of a ladder, or links in a chain, as the case may be. If an eighth day were added to the week by statutory decree we should chop open our mental circle, change the curve and insert the new segment, or we should add another rung to our mental image of a ladder, or add an eighth link to our chain. But of all mental graphs I venture to say that our color schemes have been least perfectly organized, least logical, least related. Following the analogy of the chart we have sometimes pictured color groups in two dimensions, but the charts thenselves rem/ined dissociated, unorganized, arbitrary. What may be the extent of Ridgway's indebtedness to other color theorists I do not know--he hints at such indebte.dness in his "pro- logue"--but so far as zoological color-workers are concerned it remained for the orderly mind of Robert Ridgway to so present color relations their we may con- ceive them in three dimensions, to fix it indeed so that we must so conceive then. To be sure the limitations of book making still necessitate the use of dissected charts serially presented. But even with this handicap the sequence 'is so logi- cal that we are able to reconstruct a mental cube er visualized color-file having length, breadth and thickness. Color-file is perhaps the best name for this new piece of mental furniture. Let us conceive it as made up of prisms, cubes, of colored glass. To understand its order, therefore, let us examine its 'first or facing wall--thirty-seven cubes, or colmnns of cubes, wide, and nine tiers, or rows of cubes, deep. The central tier reading from left to right comprises the pure colors of the spectrum, red, orange, yellow, etc., together with carefully selected intergrades, orange-red, orange orange-red, red-orange, etc.,--thirty-six colors to be known as hues (with red repeated at the extreme right to give meaning to the violet-red series). The bottom tier of our wall is pure black, the limit, or asymptote, of the suc- cessively deepening shades produced by mixing the pure color of the central member of each column with increasing percentages of black. Th e top tier of our wall is pure white, the limit, or asymptote, of successively lightening tints of the central color produced by increasing dilutions of white. In Ridgway's scheme three steps are made in each direction, so that we have seven colored tiers. separating the black and the white boundary tiers. So much is commonplace; the next step is inspiration--Ridgway's. The third dimension of our color file is secured by progressive dilutions of neutral gray, additions of a uniform amount in a given wall, each block differing from its neighbors in the same wall in precisely the same degree in which the pure color blocks differ from their neighbors. It is obvious that if progressive addi- tions of gray differed only by one percent, we should have one hundred walls, bounded on the rear by a wall whose central tier was pure neutral gray and whose successive tints approached the top layer of pure white, and whose suc- cessiv.e shades approached the bottom layer of pure black, as in the first wall. As a matter of convenience only five such progressively grayed intermediate walls are found necessary to cover for practical purposes the whole range.