The strict style in Classical painting limited its palette to yellow, red, black and white. This singular fact was observed long ago, and, since the explanation was only sought for in superficial and definitely material causes, wild hypotheses were brought forward to account for it, e.g., a supposed colour-blindness in the Greeks. Even Nietzsche discussed this (_Morgenröte_, 426). But why did this painting in its great days _avoid_ blue and even blue- green, and only begin the gamut of permissible tones at greenish-yellow and bluish-red? It is not that the ancient artists did not know of blue and its effect. The metopes of many temples had blue backgrounds so that they should appear deep in contrast with the triglyphs; and trade- painting used _all_ the colours that were technically available. There are authentic blue horses in archaic Acropolis work and Etruscan tomb- painting; and a bright blue colouring of the hair was quite common. The ban upon it in the higher art was, without a doubt, imposed upon the Euclidean soul by its prime symbol. Blue and green are the colours of the heavens, the sea, the fruitful plain, the shadow of the Southern noon, the evening, the remote mountains. They are essentially atmospheric and not substantial colours. They are _cold_, they disembody, and they evoke impressions of expanse and distance and boundlessness. For this reason they were kept out of the frescoes of Polygnotus. And for this reason also, an “infinitesimal” blue-to-green is the space- creating element throughout the history of our perspective oil-painting, from the Venetians right into the 19th Century; it is the basic and supremely important tone which _supports_ the ensemble of the intended colour-effect, as the _basso continuo_ supports the orchestra, whereas the warm yellow and red tones are put on sparingly and in dependence upon this basic tone. It is not the full, gorgeous and _familiar_ green that Raphael and Dürer sometimes—and seldom at that—use for draperies, but an indefinite blue-green of a thousand nuances into white and grey and brown; something deeply musical, into which (notably in Gobelin tapestry) the whole atmosphere is plunged. That quality which we have named aerial perspective in contrast to linear—and might also have called _Baroque_ perspective in contrast to Renaissance—rests almost exclusively upon this. We find it with more and more intense depth- effect in Leonardo, Guercino, Albani in the case of Italy, and in Ruysdael and Hobbema in that of Holland, but, above all, in the great French painters, from Poussin and Claude Lorrain and Watteau to Corot. Blue, equally a perspective colour, always stands in relation to the dark, the unillumined, the unactual. It does not press in on us, it pulls us out into the remote. An “enchanting nothingness” Goethe calls it in his _Farbenlehre_.