Part 8 (1/2)

It has been shown that the neutral centre of the system is a balancing point for all colors, that a line through this centre finds opposite colors which balance and complement each other; and we are now ready to make a practical application, carrying out these ideal relations of color as far as pigments will permit in a color sphere[27] (Fig. 16).

[Footnote 27: Patented Jan. 9, 1900.]

(103) The materials are quite simple. First a colorless globe, mounted so as to spin freely on its axis. Then a measured scale of value, specially devised for this purpose, obtained by the daylight photometer.[28] Next a set of carefully chosen pigments, whose reasonable permanence has been tested by long use, and which are prepared so that they will not glisten when spread on the surface of the globe, but give a uniformly mat surface. A gla.s.s palette, palette knife, and some fine brushes complete the list.

[Footnote 28: See paragraph 65.]

(104) Here is a list of the paints arranged in pairs to represent the five sets of opposite hues described in Chapter III., paragraphs 61-63:--

_Color Pairs._ _Pigments Used._ _Chemical Nature._

Red and Venetian red. Calcined native earth.

Blue-green. Viridian and Cobalt. Chromium sesquioxide.

Yellow and Raw Sienna. Native earth.

Purple-blue. Ultramarine. Artificial product.

Green and Emerald green. a.r.s.enate of copper.

Red-purple. Purple madder. Extract of the madder plant.

Blue and Cobalt. Oxide of cobalt with alumina.

Yellow-red. Orange cadmium. Sulphide of cadmium.

Purple and Madder and cobalt. See each pigment above.

Green-yellow. Emerald green See each pigment above.

and Sienna.

(105) These paints have various degrees of hue, value, and chroma, but can be tempered by additions of the neutrals, zinc white and ivory black, until each is brought to a middle value and tested on the value scale. After each pair has been thus balanced, they are painted in their appropriate s.p.a.ces on the globe, forming an equator of balanced hues.

[Ill.u.s.tration: Fig. 17.]

(106) The method of proving this balance has already been suggested in Chapter IV., paragraph 93. It consists of an ingenious implement devised by Clerk-Maxwell, which gives us a result of mixing colors without the chemical risks of letting them come in contact, and also measures accurately the quant.i.ty of each which is used (Fig. 17).

(107) This is called a Maxwell disc, and is nothing more than a circle of firm cardboard, pierced with a central hole to fit the spindle of a rotary motor, and with a radial slit from rim to centre, so that another disc may be slid over the first to cover any desired fraction of its surface. Let us paint one of these discs with Venetian red and the other with viridian and cobalt, the first pair in the list of pigments to be used on the globe.

(108) Having dried these two discs, one is combined with the other on the motor shaft so that each color occupies half the circle. As soon as the motor starts, the two colors are no longer distinguished, and rapid rotation melts them so perfectly that the eye sees a new color, due to their mixture on the retina. This new color is a reddish gray, showing that the red is more chromatic than the blue-green. But by stopping the motor and sliding the green disc to cover more of the red one, there comes a point where rotation melts them into a perfectly neutral gray.

No hint of either hue remains, and the pair is said to balance.

(109) Since this balance has been obtained by _unequal areas_ of the two pigments, it must compensate for a lack of equal chroma in the hues (see paragraphs 76, 77); and, to measure this inequality, a slightly larger disc, with decimal divisions on its rim, is placed back of the two painted ones. If this scale shows the red as occupying 3? parts of the area, while blue-green occupies 6? parts, then the blue-green must be only half as chromatic as the red, since it takes twice as much to produce the balance.

(110) The red is then grayed (diminished in chroma by additions of a middle gray) until it can occupy half the circle, with blue-green on the remaining half, and still produce neutrality when mixed by rotation.

Each disc now reads 5 on the decimal scale. Lest the graying of red should have disturbed its value, it is again tested on the photometric scale, and reads 4.7, showing it has been slightly darkened by the graying process. A little white is therefore added until its value is restored to 5.

(111) The two opposites are now completely balanced, for they are equal in value (5), equal in chroma (5), and have proved their equality as complements by uniting in equal areas to form a neutral mixture. It only remains to apply them in their proper position on the sphere.