(70) Evidently, this way of writing colors by their degrees of value and hue gives clearness to what would otherwise be hard to express by the color terms in common use. (71) If Y9 and R5 be chosen for mixture, we know at once that they unite in YR7, which is two steps of the value scale above the middle; while Y6 and R2 make YR4, which is one step below the middle. Charts prepared with this system show each of these colors and their mixture with exactness. (72) The foregoing mixtures of dark reds and light yellows are typical of the union of light and dark values of any neighboring hues, such as yellow and green, green and blue, blue and purple, or purple and red. Next let us think of the result of mixing different values in opposite hues; as, for instance, YR7 and B3 (Fig. 11). To this combination the color sphere gives a ready answer; for the middle of a straight line through the sphere, and joining them, coincides with the neutral centre, showing that they _balance in neutral gray_. This is also true of any opposite pair of surface hues where the values are equally removed from the equator. [Illustration: Fig. 11.] (73) Suppose we substitute familiar flowers for the notation, then YR7 becomes the buttercup, and B3 is the wild violet. But, in comparing the two, the eye is more stimulated by the buttercup than by the violet, not alone because it is lighter, but because it is stronger in chroma; that is, farther away from the neutral axis of the sphere, and in fact out beyond its surface, as shown in Fig. 11. The head of a pin stuck in toward the axis on the 7th level of YR may represent the 9th step in the scale of chroma, such as the buttercup, while the “modest” violet with a chroma of only 4, is shown by its position to be nearer the neutral axis than the brilliant buttercup by five steps of chroma. This is the third dimension of color, and must be included in our notation. So we write the buttercup YR 7/9 and the violet B 3/4,--chroma always being written below to the right of hue, and value always above. (This is the invariable order: HUE {VALUE/CHROMA}.) (74) A line joining the head of the pin mentioned above with B 3/4 does not pass through the centre of the sphere, and its middle point is nearer the buttercup than the neutral axis, showing that the hues of the buttercup and violet _do not balance in gray_. +The neutral centre is a balancing point for colors.+ (75) This raises the question, What is balance of color? Artists criticise the color schemes of paintings as being “too light or too dark” (unbalanced in value), “too weak or too strong” (unbalanced in chroma), and “too hot or too cold” (unbalanced in hue), showing that this is a fundamental idea underlying all color arrangements.