First let us repeat: volume and space are registered by the human retina as a
mosaic of surfaces. Every pictorial representation is concerned with arranging surfaces on a plane. The manifold means of perspective can recreate the illusion that these flat surfaces enclose bodies or that they surround space. Lines, in the mathematical sense, are invisible. Properly speaking, even the finest stroke is an unusually narrow surface. It is, however, felt as a line and can serve either as a means of delimiting the surface before it is given substance with pencil or paint, or as an outline which stimulates the beholder to imagine the substance of the surface.
The flat surface is thus the foundation of all pictorial art. Disregard of foundations always leads to catastrophe or to cheating. It would be reasonably consistent to consider as cheating those Baroque and Rococo frescoes which pretend to be plastic modeling. As you already know from working with a viewer, a picture can reproduce only the impression received by one eye. If we look with both eyes, their different viewpoints give us the feeling that we can see slightly around to the right and left of objects. This phenomenon is repeated in a stereoscopic photograph, which takes the appropriate picture for each eye.
It is impossible for a painter or draftsman to reproduce this sensation. He draws as a one-eyed man sees. Hence, no picture assumes depth of space or a sense of volume until it is looked at from some distance. The stereoscopic photograph, too, shows two practically identical pictures when the object photographed lies' so far away that the difference in position of the two eyes or viewpoints is insignificant.
If you look at a wide landscape in which a telegraph pole stands about 15 feet away from you, and look at it alternately with one eye and then the other, the pole will seem to move back and forth against the background. If you move 100 yards from the pole and repeat the experiment it will move much iess, and at greater distances it will not move at all. Thus, at great distances we do not see things in the round. It is only constant experience which gives us the impression of space, and it can give it equally when we look at a picture.
In pictures which are seen close up - near in comparison to the distance, say, of a ceiling fresco - the illusion of depth is most stimulating when the plane of the picture is not too drastically broken up. This breaking up is the result of a strong use of spatial shadow-and-color perspective, heavy dark areas which look like holes and light areas that jump out. If these resources are used only very sparsely so that they merely make suggestions to the beholder's imagination, then the almost unruffled plane of the picture does not distract from its conent or theme.
If, in addition, the plane of the picture is deliberately emphasized, it will at a cursory, unconcentrated glance, seem devoid of any depth and arrest all the dynamic of color and movement. But if you really submerge yourself into a picture of this kind it can assume depth and volume as if by magic and perhaps only then be really understood. It would be interesting for physiologists and psychologists to study why this is so, For the practice of painting and drawing it must be simply accepted as a phenomenon to be reckoned with.
Even if the beholder is concerned primarily with naturalistic representation and responds first to the illusion of space and depth, he will be unconsciously aware of the picture plane. Excessive naturalism and the illusory depiction of what is known not to be there very soon becomes tasteless and tedious. Moreover, it is rare to find spiritual richness combined with exceeding virtuosity.
Emphasis of the picture plane is obtained by a number of so-called "artistic" means. The epithet is justified, for it is not enough here to follow the technical recipes unless they be combined with the extra "something" of real talent. The talent, however, can find no outlet without a mastery of the technical means. This is the precondition for any kind of art, if it is not to look amateur. How the means are mastered, whether in school or self taught, is a matter of little importance. Talent alone is not enough to produce a work of art.