from a point at some little distance with one eye, you easily suppose
that it stands in front of the side pieces. The explanation of the
illusion is that this particular arrangement powerfully suggests that
the outline of the whole figure, of which the two side pieces are parts,
is broken by an intervening object. Owing to the force of these and
other suggestions, it is easy for the spectator, when attending to the
background of a landscape painting, to give himself up for a moment to
the pleasant delusion that he is looking at an actual receding scene.

In connection with pictorial delusion, I may refer to the well-known
fact, that the eye in a portrait seems to follow the spectator, or that
a gun, with its muzzle pointing straight outwards, appears to turn as
the spectator moves.[41] These tricks of art have puzzled many people,
yet their effect is easily understood, and has been very clearly
explained by Sir D. Brewster, in the work already referred to (letter
v.). They depend on the fact that a painting, being a flat projection
only and not a solid, continues to present the front view of an object
which it represents wherever the spectator happens to stand. Were the
eye in the portrait a real eye, a side movement of the spectator would,
it is evident, cause him to see less of the pupil and more of the side
of the eyeball, and he would only continue to see the full pupil when
the eye followed him. We regard the eye in the picture as a real eye
having relief, and judge accordingly.

We may fall into similar illusions respecting distance in auditory
perception. A change of wind, an unusual stillness in the air, is quite
sufficient to produce the sense that sounding objects are nearer than
they actually are. The art of the ventriloquist manifestly aims at
producing this kind of illusion. By imitating the dull effect of a
distant voice, he is able to excite in the minds of his audience a