19. More than once I have been agreeably struck with this appearance, produced unintentionally when I have been writing by candle-light on a winter's morning; upon the daylight being let in, the shadow of my pen and fingers in the orange-light of the candle, were beautifully blue. 20. I suppose there is such a thing as the harmony of colours, of which painters speak so much; according to the explanation here given, our key to the solution of every case of harmony and of contrast, is to consider what is the other colour, simple or compound, which, joined to a given one, simple or compound, will constitute white. Thus red, requires green; yellow, purple; blue, orange; and vice versa, the mixtures in proper proportions will be white.