1. To perceive and to imitate promptly and surely the modifications of the light on the model. 2. To harmonize those colours of a composition which are essentially inherent to the nature of the objects to be produced. 316. We learn by the law of _simultaneous contrast of colours_, that when we regard attentively two coloured objects at the same time, neither of them appears of the colour peculiar to it; that is to say, such as it would appear if viewed separately, but of a tint resulting from the peculiar colour and the complementary of the colour of the other object. On the other hand, if the colours of the objects be not of the same tone, the lightest tone will be lowered, and the darkest tone will be heightened. 317. The first conclusion from this is, that the painter will rapidly appreciate in his model the colour peculiar to each part, and the modifications of tone and of colour which they receive from contiguous colours. He will also perceive and be prepared to imitate modifications in them, which, if they had not always escaped him because of their feeble intensity, might have been disregarded, because the eye is peculiarly susceptible of fatigue when it seeks to disentangle modifications, the cause of which is unknown, and which are not very prominent. 318. Let us now return to _mixed contrast_ (77 _et seq._), in order to make it evident that the painter is liable to see the colours of his model inaccurately. As the eye, after observing one colour for a certain time, has acquired a tendency to see its complementary, and as this tendency is of some duration, it follows, not only that the eyes of the painter thus affected cannot see correctly the colour which he had for some time looked at, but also whatever colour he sees while this modifying influence lasts. So that, conformably to what we know of mixed contrast, he will see,—not the colour which is before him,—but the result of this colour, and of the complementary of that seen previously. It must be remarked, that besides the want of clearness of view which will arise, in most cases, from the want of exact coincidence of the second image with the first—for example, when the eye has seen a sheet of green paper A (Plate III., fig. 4), in the first place, and, in the second place, a sheet of blue paper, B, of the same dimensions, but placed differently, this second image, not being coincident in all its surface with the first, A, as represented in the figure, the eye will see the sheet B violet only in the part where the two images coincide. Consequently, this defect of perfect coincidence of images will affect the outline of the second image, as well as the colour which it really possesses.