experience and association, have become the
signs of differences of depth in space. Among these are the degree of
indistinctness of the impression, the apparent or retinal magnitude (if
the object is a familiar one), the relations of linear perspective, as
the interruption of the outline of far objects by that of near objects,
and so on. 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
li