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.