If the artist abandons himself to his feeling, colour presently announces itself. Black no sooner inclines to blue than the eye demands yellow, which the artist instinctively modifies, and introduces partly pure in the light, partly reddened and subdued as brown, in the reflexes, thus enlivening the whole.--Note G G. 864. All kinds of _camayeu_, or colour on similar colour, end in the introduction either of a complemental contrast, or some variety of hue. Thus, Polidoro in his black and white frescoes sometimes introduced a yellow vase, or something of the kind. 865. In general it may be observed that men have at all times instinctively striven after colour in the practice of the art. We need only observe daily, how soon amateurs proceed from colourless to coloured materials. Paolo Uccello painted coloured landscapes to colourless figures.--Note H H. 866. Even the sculpture of the ancients could not be exempt from the influence of this propensity. The Egyptians painted their bas-reliefs; statues had eyes of coloured stones. Porphyry draperies were added to marble heads and extremities, and variegated stalactites were used for the pedestals of busts. The Jesuits did not fail to compose the statue of their S. Luigi, in Rome, in this manner, and the most modern sculpture distinguishes the flesh from the drapery by staining the latter. KEEPING. 867. If linear perspective displays the gradation of objects in their apparent size as affected by distance, aërial perspective shows us their gradation in greater or less distinctness, as affected by the same cause. 868. Although from the nature of the organ of sight, we cannot see distant objects so distinctly as nearer ones, yet aërial perspective is grounded strictly on the important fact that all mediums called transparent are in some degree dim. 869. The atmosphere is thus always, more or less, semi-transparent. This quality is remarkable in southern climates, even when the barometer is high, the weather dry, and the sky cloudless, for a very pronounced gradation is observable between objects but little removed from each other. 870. The appearance on a large scale is known to every one; the painter, however, sees or believes he sees, the gradation in the slightest varieties of distance. He exemplifies it practically by making a distinction, for instance, in the features of a face according to their relative position as regards the plane of the picture. The direction of the light is attended to in like manner. This is considered to produce a gradation from side to side, while keeping has reference to depth, to the comparative distinctness of near and distant things. 871.