In a process so complicated there is clearly ample room for error, and wrong estimates of distance whenever unusual circumstances are present are familiar to all.  Thus, the inexperienced English tourist, when in the clear atmosphere of Switzerland, where the impressions from distant objects are more distinct than at home, naturally falls into the illusion that the mountains are much nearer than they are, and so fails to realize their true altitude.

Illusions of Art.

The imitation of solidity and depth by art is a curious and interesting illustration of the mode of production of illusion.  Here we are not, of course, concerned with the question how far illusion is desirable in art, but only with its capabilities of illusory presentment; which capabilities, it may be added, have been fully illustrated in the history of art.  The full treatment of this subject would form a chapter in itself; here I can only touch on its main features.

Pictorial art working on a flat surface cannot, it is plain, imitate the stereoscope, and produce a perfect sense of solidity.  Yet it manages to produce a pretty strong illusion.  It illustrates in a striking manner the ease with which the eye conceives relations of depth or relief and solidity.  If, for example, on a carpet, wall-paper, or dress, bright lines are laid on a dark colour as ground, we easily imagine that they are advancing.  The reason of this seems to be that in our daily experience advancing surfaces catch and reflect the light, whereas retiring surfaces are in shadow.[38]

The same principle is illustrated in one of the means used by the artist to produce a strong sense of relief, namely, the cast shadow.  A circle drawn with chalk with a powerful cast shadow on one side will, without any shading or modelling of the form, appear to stand out from the paper, thus: 

[Illustration:  FIG. 1.]

The reason is that the presence of such a shadow so forcibly suggests to the mind that the object is a prominent one intervening between the light and the shaded surface.[39]

Even without differences of light and shade, by a mere arrangement of lines, we may produce a powerful sense of relief or solidity.  A striking example of this is the way in which two intersecting lines sometimes appear to recede from the eye, as the lines a a’, b b’, in the next drawing, which seem to belong to a regular pattern on the ground, at which the eye is looking from above and obliquely.

[Illustration:  FIG. 2.]

Again, the correct delineation of the projection of a regular geometrical figure, as a cube, suffices to give the eye a sense of relief.  This effect is found to be the more striking in proportion to the familiarity of the form.  The following drawing of a long box-shaped solid at once seems to stand out to the eye.

[Illustration:  FIG. 3.]