One of the most interesting illusions, not being visual, can {459}
only be described and not demonstrated here.

[Illustration: Fig. 72.--By aid of this simple figure, the
Poggendorf and barber-pole illusions can be seen to be instances of
the Mueller-Lyer illusion, Try to bisect the horizontal line in this
figure. The oblique line at the right tends to displace the
right-hand end of the horizontal to the right, while the oblique at
the left tends to displace the left-hand end of the horizontal also
to the right. Similar displacements account for the Poggendorf and
barber-pole illusions.]

[Illustration: Fig. 73.--The Zoellner illusion. The long lines are
really parallel. The illusion is increased by holding the figure so
that these main lines shall be neither vertical nor horizontal. It
is more difficult to "deceive the eye" in regard to the direction of
vertical and horizontal lines, than in regard to the direction of
oblique lines. This illusion must be related in some way to the
Mueller-Lyer and Poggendorf illusions, since the elements employed in
constructing the three figures are so much the same.

If you treat this figure according to the directions given for Fig.
67, and sight along the obliques, you get an illusion of
perspective.]

It is called the "size-weight illusion", and may be said to be based
on the old catch, "Which is heavier, a pound of lead or a pound of
feathers?" Of course, we shrewdly answer, a pound's {460} a pound. But
lift them and notice how they feel! The pound of lead feels very much
heavier. To reduce this illusion to a laboratory experiment, you take
two round wooden pill-boxes, one several times as large as the other,
and load them so that they both weigh the same; then ask some one to
lift them and tell which is the heavier. He will have no doubt at all
that the smaller box is the heavier; it may seem two or three times as
heavy. Young children, however, get the opposite illusion,
assimilating the weight to the visual appearance; but older persons
switch over to the contrast effect, and perceive in opposition to the
visual appearance. What seems to happen in the older person is a motor
adjustment for the apparent weights, as indicated by their visual
appearance, with the result that the weight of larger size is lifted
more strongly than the weight of smaller size; so that the big one
comes up easily and seems light, the little one slowl