Of two objects equally light, one will appear less so if seen upon a whiter ground; and, on the contrary, it will appear a great deal lighter if upon a space of a darker shade. So flesh colour will appear pale upon a red ground, and a pale colour will appear redder upon a yellow ground. In short, colours will appear what they are not, according to the ground which surrounds them.
Chap. CCLVI.—The Mode of composing History.
Amongst the figures which compose an historical picture, those which are meant to appear the nearest to the eye, must have the greatest force; according to the second proposition [62] of the third book, which says, that colour will be seen in the greatest perfection which has less air interposed between it and the eye of the beholder; and for that reason the shadows (by which we express the relievo of bodies) appear darker when near than when at a distance, being then deadened by the air which interposes. This does not happen to those shadows which are near the eye, where they will produce the greatest relievo when they are darkest.
Chap. CCLVII.—Remarks concerning Lights and Shadows.
Observe, that where the shadows end, there be always a kind of half-shadow to blend them with the lights. The shadow derived from any object will mix more with the light at its termination, in proportion as it is more distant from that object. But the colour of the shadow will never be simple: this is proved by the ninth proposition[63], which says, that the superficies of any object participates of the colours of other bodies, by which it is surrounded, although it were transparent, such as water, air, and the like: because the air receives its light from the sun, and darkness is produced by the privation of it. But as the air has no colour in itself any more than water, it receives all the colours that are between the object and the eye. The vapours mixing with the air in the lower regions near the earth, render it thick, and apt to reflect the sun’s rays on all sides, while the air above remains dark; and because light (that is, white) and darkness (that is, black), mixed together, compose the azure that becomes the colour of the sky, which is lighter or darker in proportion as the air is more or less mixed with damp vapours.
Chap. CCLVIII.—Why the Shadows of Bodies upon a white Wall are blueish towards Evening.
The shadows of bodies produced by the redness of the setting sun, will always be blueish. This is accounted for by the eleventh proposition[64], which says, that the superficies of any opake body participates of the colour of the object from which it receives the light; therefore the white wall being deprived entirely of colour, is tinged by the colour of those bodies from which it receives the light, which in this case are the sun and the sky. But because the sun is red towards the evening, and the sky is blue, the shadow on the wall not being enlightened by the sun, receives only the reflexion of the sky, and therefore will appear blue; and the rest of the wall, receiving light immediately from the sun, will participate of its red colour.