t and dark grays, but the value of colors which are at
the same level in the scale. Thus R7 (popularly called a tint of red) is
neither lighter nor darker than the gray of N7. A numeral written above
to the right always indicates _value_, whether of a gray or a color, so
that R1, R2, R3, R4, R5, R6, R7, R8, R9, describes a regular scale of
red values from black to white, while G1, G2, G3, etc., is a scale of
green values.

(67) This matter of a notation for colors will be more fully worked out
in Chapter VI., but the letters and numerals already described greatly
simplify what we are about to consider in the mixture and balance of
colors.

+Mixture of light hues with dark hues.+

(68) Now that we are supplied with a decimal scale of grays, represented
by divisions of the neutral axis (N1, N2, etc.), and a corresponding
decimal scale of value for each of the ten hues ranged about the equator
(R1, R2,-- YR1, YR2,-- Y1, Y2,-- GY1, GY2,-- and so on), traced by ten
equidistant meridians from black to white, it is not difficult to
foresee what the mixture of any two colors will produce, whether they
are of the same level of value, as in the colors of the equator already
considered, or whether they are of different levels.

[Illustration: Fig. 10.]

(69) For instance, let us mix a light yellow (Y7) with a dark red (R3).
They are neighbors in hue, but well removed in value. A line joining
them centres at YR5. This describes the result of their mixture,--a
value intermediate between 7 and 3, with a hue intermediate between R
and Y. It is a yellow-red of middle value, popularly called "dark
orange." But, while this term "dark orange" rarely means the same color
to three different people, these measured scales give to YR5 an
unmistakable meaning, just as the musical scale gives an unmistakable
significance to the notes of its score.

(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 hu