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]