[(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 3⅓ parts of the area, while blue-green occupies 6⅔ 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.
[(112)] A band is traced around the equator, divided in ten equal spaces, and lettered R, YR, Y, GY, G, BG, B, PB, P, and RP (see Fig. 18). This balanced red and blue-green are applied with the brush to spaces marked R and BG, care being taken to fill, but not to overstep the bounds, and the color laid absolutely flat, that no unevenness of value or chroma may disturb the balance.
