(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. Arsenate 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 spaces on the globe, forming an equator of balanced hues. [Illustration: 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 quantity 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.