n of a chromatic score.+

[Illustration: Fig. 21.]

(132) The last chapter traced a series of steps leading to the
construction of a practical color sphere. Each color was tested by
appropriate instruments to assure its degree of hue, value, and chroma,
before being placed in position. Then the total sphere was tested to
detect any lack of balance.

(133) Each color was also _written_ by a letter and two numerals,
showing its place in the three scales of hue, value, and chroma. This
naturally suggests, not only a record of each separate color sensation,
but also a union of these records in series and groups to form a _color
score_, similar to the musical score by which the measured relations of
sound are recorded.

(134) A very simple form of color score may be easily imagined as a
transparent envelope wrapped around the equator of the sphere, and
forming a vertical cylinder (Fig. 21). On the envelope the equator
traces a horizontal centre line, which is at 5 of the _value scale_,
with zones 6, 7, 8, and 9 as parallels above, and the zones 4, 3, 2, and
1 below. Vertical lines are drawn through ten equidistant points on this
centre line, corresponding with the divisions of the _hue scale_, and
marked R, YR, Y, GY, G, BG, B, PB, P, and RP.

(135) The transparent envelope is thus divided into one hundred
compartments, which provide for ten steps of value in each of the ten
middle colors. Now, if we cut open this envelope along one of the
verticals,--as, for instance, red-purple (RP), it may be spread out,
making a flat chart of the color sphere (Fig. 22).

+Why green is given the centre of the score.+

(136) A cylindrical envelope might be opened on any desired meridian,
but it is an advantage to have green (G) at the centre of the chart, and
it is therefore opened at the opposite point, red-purple (RP). To the
right of the green centre are the meridians of green-yellow (GY), yellow
(Y), yellow-red (YR), and red (R), all of which are known as _warm
colors_, because they contain yellow and red. To the left are the
meridians of blue-green (BG), blue (B), purple-blue (PB), and purple
(P), all of which are called _cool colors_, because they contain blue.
Green, being neither warm nor cold of itself, and becoming so only by
additions of yellow or of blue, thus serves as a balancing point or
centre in the hue-scale.[31]

[Footnote 31: To put this in terms of the spectrum wave lengths,
long waves at the red