_Tone._—"Each step in a color scale is a tone of that color."[20] The term tone cannot, however, be properly applied to a step in the spectrum scale, in which each contiguous pair of the six distinct spectrum or "fundamental" _colors_ are connected by _hues_. Hence _tone_[21] is exclusively applicable to the steps in a scale of a single color or hue, comprising the full color (in the center) and graduated tints and shades leading off therefrom in opposite directions; or of neutral gray similarly graduated in tone from the darkest shade to the palest tint. Each one of the colored blocks in the vertical scales of the plates in this work represents a separate tone of that color. _Scale._—A linear series of colors showing a gradual transition from one to another, or a similar series of tones of one color. The first is a _chromatic scale_[22] (or scale of colors and hues) and in the plates of this work is represented by each horizontal series; the second is a _tone scale_, on the plates running vertically, growing from the full color, in the center, to a pale tint (at the top) and a dark shade (at the bottom). For clearer comprehension of these two distinct scales, each plate of this work may be compared to a sheet of woven fabric; the chromatic scale (horizontal) representing the warp, the luminosity or tone scale (vertical) the woof. A third kind of color scale is represented by adding progressive increments of neutral gray to any color. This is shown by the several series of Plates, of which the first (Plates I-XII, with colors numbered 1-71) represents each step in the spectrum scale unmixed with gray, followed by five other series in which the same colors[23] are shown dulled by gradually increasing increments of neutral gray, the first (Plates XIII-XXVI, colors 1′-71′) containing 32 per cent., the second (Plates XXVII-XXXVIII, colors 1′′-71′′) 58 per cent., the third (Plates XXXIX-XLIV, colors 1′′′-69′′′) 77 per cent., the fourth (Plates XLV-L, colors 1′′′′-69′′′′) 90 per cent., and the fifth (Plates LI-LIII, colors 1′′′′′, 15′′′′′, 23′′′′′, 35′′′′′, 49′′′′′, 59′′′′′ and 67′′′′′) 95.5 per cent. of gray, the last being in reality colored grays. Finally scales are shown (on Plate LIII) of neutral gray (in which all trace of color is wanting), and of carbon gray, a simple mixture of lamp-black and chinese white. It is not easy to find a suitable name for these scales of reduced or "broken" colors, but they may, for present convenience, be termed _reduced_ or _broken scales_. _Full Color._—A color corresponding in intensity with its manifestation in the solar spectrum. _Pure Color._—A color corresponding in purity with (or, in the case of material colors, closely approximating to) one of the spectrum colors.