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.]

This habitual interpretation of the flat in art as answering to objects