[(120)] Ten meridians carry the equatorial hues across all these value zones and trace the gradation of each hue through a complete scale from black to white, marked by their values, as shown in paragraph [68]. Thus the red scale is R1, R2, R3, R4, R5 (middle red), R6, R7, R8, and R9, and similarly with each of the other hues. When the circle of hues corresponding to each level has been applied and tested, the entire surface of the globe is spread with a logical system of color scales, and the eye gratified with regular sequences which move by measured steps in each direction.
[(121)] Each meridian traces a scale of value for the hue in which it lies. Each parallel traces a scale of hue for the value at whose level it is drawn. Any oblique path across these scales traces a regular sequence, each step combining change of hue with a change of value and chroma. The more this path approaches the vertical, the less are its changes of hue and the more its changes of value and chroma; while, the nearer it comes to the horizontal, the less are its changes of value and chroma, while the greater become its changes of hue. Of these two oblique paths the first may be called that of a Luminist, or painter like Rembrandt, whose canvases present great contrasts of light and shade, while the second is that of the Colorist, such as Titian, whose work shows great fulness of hues without the violent extremes of white and black.
Total balance of the sphere tested by rotation on any desired axis.
[(122)] Not only does the mount of the color sphere permit its rotation on the vertical axis (white-black), but it is so hung that it may be spun on the ends of any desired axis, as, for instance, that joining our first color pair, red and blue-green. With this pair as poles of rotation, a new equator is traced through all the values of purple on one side and of green-yellow on the other, which the rotation test melts in a perfect balance of middle gray, proving the correctness of these values. In the same way it may be hung and tested on successive axes, until the total balance of the entire spherical series is proved.
[(123)] But this color system does not cease with the colors spread on the surface of a globe.[30] The first illustration of an orange filled with color was chosen for the purpose of stimulating the imagination to follow a surface color inward to the neutral axis by regular decrease of chroma. A slice at any level of the solid, as at value 8 ([Fig. 10]), shows each hue of that level passing by even steps of increasing grayness to the neutral gray N8 of the axis. In the case of red at this level, it is easily described by the notation R8/3, R8/2, R8/1, of which the initial and upper numerals do not change, but the lower numeral traces loss of chroma by 3, 2, and 1 to the neutral axis.
[(124)] And there are stronger chromas of red outside the surface, which can be written R8/4, R8/5, R8/6, etc. Indeed, our color measurements discover such differences of chroma in the various pigments used, that the color tree referred to in paragraphs [34, 35], is necessary to bring before the eye their maximum chromas, most of which are well outside the spherical shell and at various levels of value. One way to describe the color sphere is to suggest that a color tree, the intervals between whose irregular branches are filled with appropriate color, can be placed in a turning lathe and turned down until the color maxima are removed, thus producing a color solid no larger than the chroma of its weakest pigment ([Fig. 2]).
Charts of the color solid.
[(125)] Thus it becomes evident that, while the color sphere is a valuable help to the child in conceiving color relations, in uniting the three scales of color measure, and in furnishing with its mount an excellent test of the theory of color balance, yet it is always restricted to the chroma of its weakest color, the surplus chromas of all other colors being thought of as enormous mountains built out at various levels to reach the maxima of our pigments.
[(126)] The complete color solid is, therefore, of irregular shape, with mountains and valleys, corresponding to the inequalities of pigments. To display these inequalities to the eye, we must prepare cross sections or charts of the solid, some horizontal, some vertical, and others oblique.
[(127)] Such a set of charts forms an atlas of the color solid, enabling one to see any color in its relation to all other colors, and name it by its degree of hue, value, and chroma. Fig. 20 is a horizontal chart of all colors which present middle value (5), and describes by an uneven contour the chroma of every hue at this level. The dotted fifth circle is the equator of the color sphere, whose principal hues, R5/5. Y5/5, G5/5, B5/5, and P5/5, form the chromatic tuning fork, paragraph [117].