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 the five sets of opposite hues described in Chapter III., paragraphs 61–63:—

(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.

(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