Suppose several bands in juxtaposition, 1, 2, 3, 4, (Plate 1, fig. 3), of different tones in flat tints of the same scale, to form part of an object: to imitate it perfectly, it is evident that it must be painted in flat tints; but this object will appear to the eye a channeled surface, the lines where the two bands touch will appear like a relief by the effect of contrast of tone (9-11); therefore, if the painter is ignorant of this, he will reproduce, not an absolute copy of the model, but an exaggerated one. I the more willingly cite this example, because it gave me an opportunity of enabling a most skilful paperstainer to appreciate the utility of the law of simultaneous contrast. In going with him over his factory, he showed me a chimney-board representing a child whose figure stood out from a ground formed of two circular bands in grey flat tints; 1 and 2 (Plate I., fig. 7); the first was higher than the second; the phenomenon of contrast of tone was manifested at the borders, _a a_, of the two bands, so that the part of the band 2, contiguous to the band 1, was darker than the rest, as the part of the band 1, contiguous to 2, was lighter than the rest, conformably to what has been stated above (11). This effect, not being what the skilful artist wished to obtain, he inquired of me how it was to be avoided. I replied, that the grey of the band 2 must be reduced with white, in proportion as it approached the border _a a_; and, on the contrary, the grey of the band 1 must be strengthened with black, in proper gradations, beginning at the same border. And I proved to him, _that to imitate the model faithfully, we must copy it differently from what we see it_. 322. From the above we educe the six following principles:— 1. Put a colour upon a canvas, it not only colours that part of the canvas to which the pencil has been applied, but it also colours the surrounding space with the complementary of that colour. Thus, a red circle is surrounded with a green areola, becoming weaker as it extends from the circle:— A green circle is surrounded with a red areola. An orange ” ” blue ” A blue ” ” orange ” A yellow ” ” violet ” A violet ” ” yellow ” (See Plates 3, 4, 5.) 2. White placed beside a colour heightens its tone; it is as if we took away from the colour the white light which enfeebled its intensity (44-52). 3. Black placed beside a colour weakens, and in some cases impoverishes, its tone, as upon certain yellows (55). It is, in fact, adding to black the complementary of the contiguous colour. 4. Put grey beside a colour, the latter is rendered more brilliant, and at the same time it tints this grey with its complementary (63).