demonstrated that thirty-six is the practicable limit, and accordingly that number has been adopted.[1] If the number of intermediate hues were equal in all cases there would, in this scheme, be five between each two adjacent fundamental colors of the spectrum; but a greater number of recognizably distinct hues is obviously necessary in some cases than in others; for example, spectrum orange is decidedly nearer in hue to red than to yellow, and therefore the number of intermediates required on each side of the orange is different, being in the proportion of four for the red-orange series to five for the orange-yellow, and similarly six are required for the violet-red series, while four suffice for the blue-violet hues.
There is no known means by which we can measure the proportion of two or more pigments in any given mixture, "because color-effect cannot be measured by the pint of mixed paint or the ounce of dry pigment;"[2] but, fortunately, we have a very exact method, in the color- wheel and Maxwell disks, by which the relative proportions of two or more colors in any mixture may be precisely measured. This method has been used in the painting of every one of the 1115 colors of the present work, by means of one disk to represent each one of the thirty-six colors (both pure and "broken"), together with a black, a white, and a neutral gray disk, the last being a match in color to the gray resulting from the mixture of red, green and violet on the color-wheel;[3] the neutral gray disk, however, being used only for the making of disks for the broken series of colors (′, ′′, ′′′, ′′′′, and ′′′′′) and for the scale of neutral grays (Plate
