23. Every one knows that red colours and yellow colours mixed together, in different proportions, produce orange colours of various kinds: also that reds and blues produce purples and violets; and, lastly, that blues and yellows produce greens in great variety; but it is not so generally known that green, purple, and orange colours, are, as it were, almost annihilated by mixture, and much improved by contiguity with red, yellow, and blue colours respectively. The little diagram [fig. 123], suggests all these things to the memory, and a great many more of the same kind; and, therefore, must be extremely useful to the artist who is endeavouring to produce certain effects by contrast, harmony, &c., but it should always be carefully remembered, that it contributes nothing to the proof of any of the truths here advanced; the proof rests upon the reasons given for each of them respectively.
