we must look at it with one eye; for, in looking with two eyes, the duplication of the visual lines that define the position of each point as it is perceived, must cause more or less of the impression of the really plain surface to persist. In this case the corporeal projection becomes confused with the superficial conception, and is more or less modified by it. The effect of perspective is also heightened by looking at the painting through a tube, by means of which it is abstracted from the frame and from its surroundings.
We observe, also, in looking at a picture, the curious phenomenon that, whatever position we may take toward it, it always appears projected, but in aspects which are varied not only in respect to the absolute position of its constituent parts, but also in respect to their mutual relations with one another, to such an extent that from an extremely unfavorable position it appears distorted.
Whenever an artist can not produce the perspective effects he desires in a picture drawn with absolute correctness, he does not hesitate to exaggerate the perspective if he thinks he can thereby enforce a better conception of his design. lie represents distant objects on a smaller scale than strict accuracy demands, and causes us, by unconsciously taking account of the reduction, to see them farther off than we otherwise would do; and he knows how to call our imagination to his help in other ways.
Although the application of perspective devices is usually enough for the purpose, the projection of the design is heightened to a considerable degree, facilitated and brought nearer the appearance of reality by the introduction of effects of light and shade. This also is borrowed from Nature. The shading may, indeed, sometimes define and fix the effect which mere perception fails to convey positively. A square with two diagonals drawn across it may mean simply that, or it may be intended to represent a pyramid. If proper shading is added, the figure is made to mean a pyramid unmistakably.
If we assume that the light is evenly diffused through any space, then, other conditions being equal, those things in the space which are nearer to us will be clearer in appearance in a proportion determined by the rule that the intensity of light diminishes as the square of the distance increases. A point twice as far from the eye as another appears four times as obscure. This normal diminution of intensity is augmented by the darkening which objects undergo in consequence of the interposition of strata of air, or by the effect of "air-perspective," as it is called, in distinction from linear perspective. The painter must take both of these phenomena into account. By the application of a suitable coloring, the effect of air-perspective may be produced in so striking a degree that objects may, by means of it, be made to appear wonderfully remote. This effect, also, is dependent upon our unconscious application of knowledge we have gained from previous experience.
