in relief, or having depth, can only be understood when it is remembered
that our daily experience gives us myriads of instances in which the
effect of such flat representations answers to solid receding forms.
That is to say, in the case of all distant objects, in the perception of
which the dissimilarity of the retinal pictures and the feeling of
convergence take no part, we have to interpret solidity, and relations
of nearer and further, by such signs as linear perspective and cast
shadow. On the other hand, it is only in the artificial life of indoors,
on our picture-covered walls, that we experience such effects without
discovering corresponding realities. Hence a deeply organized habit of
taking these impressions as answering to the solid and not to the flat.
If our experience had been quite different; if, for example, we had
been brought up in an empty room, amid painted walls, and had been
excluded from the sight of the world of receding objects outside, we
might easily have formed an exactly opposite habit of taking the actual
mountains, trees, etc., of the distant scene to be pictures laid on a
flat surface.

[Illustration: FIG. 4.]

It follows from this that, with respect to the distant parts of a scene,
pictorial art possesses the means of perfect imitation; and here we see
that a complete illusory effect is obtainable. I need but to refer to
the well-known devices of linear and aerial perspective, by which this
result is secured.[40] The value of these means of producing illusion at
the command of the painter, may be illustrated by the following fact,
which I borrow from Helmholtz. If you place two pieces of cardboard
which correspond to portions of one form at the sides and in front of a
third piece, in the way represented above, so as just to allow the eye
to follow the contour of this last, and then look at this arrangement