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 (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.

(68) Now that we are supplied with a decimal scale of grays, represented by divisions of the neutral axis (N$1$, N$2$, etc.), and

a corresponding decimal scale of value for each of the ten hues ranged about the equator (R}, R$1$,—YR$1$, YR$2$,—Y$1$, Y$1$,—GY$2$, GY$1$,— and so on), traced by ten equidistant meridians from black to white, it is not difficult to fore- see 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.

(69) For instance, let us mix a light yellow (Y$7$) with a dark red (R$3$). They are neighbors in hue, but well removed in value. A line joining them centres at YR$5$. 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 YR$5$ an unmistakable mean- ing, 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 Y$9$ and R$5$ be chosen for mixture, we know at once that