are shifted, so as to avoid the blur that would
occur if the picture were itself moved before the eyes. But the series
of snap-shots has so much in common with the visual stimulus got from
an actually present moving object that we make the same perceptive
response. {455} The same illusion in a rudimentary form can be
produced by holding the forefinger upright three or four inches in
front of the nose, and looking at it while winking first the one eye
and then the other. Looked at with the right eye alone it appears to
be more to one side and looked at with the left eye alone it appears
to be more to the other side; and when the one eye is closed and the
other simultaneously opened, the finger seems actually to move from
one position to the other.

[Illustration: Fig. 69.--The pan illusion. The two pan-shaped outlines
are practically identical, but it is hard to compare the corresponding
sides--hard to isolate from the total figure just the elements that
you need to compare.]

4. Illusions due to imperfect isolation of the fact to be perceived.

Here belong, probably, most of the illusions produced in the
psychological laboratory by odd combinations of lines, etc. A figure
is so drawn as to make it difficult to isolate the fact to be
observed, and when the observer attempts to perceive it, he falls into
error. He thinks he is perceiving one fact, when he is perceiving
another. The best example is the Mueller-Lyer figure, in which two
equal lines are embellished with extra lines at their ends; you are
supposed to perceive the lengths of the two main lines, but you are
very apt to take the whole figure in the rough and perceive the
distances between its chief parts. You do not succeed in isolating the
precise fact you wish to observe.

{456}

The Mueller-Lyer Illusion

The most familiar form of this striking illusion is made with arrow
heads, thus

[Illustration: Inward and outward arrowheads on two equal length lines.]

In attempting to compare the two horizontal lines one is confused so
as to regard the line with outward-extending obliques longer than that
with inward-extending obliques, though, measured from point to point,
they are equal. The same illusion occurs in a variety of similar
figures, such as

[Illustration: Inward and outward arrowheads.]

where the main lines are not drawn, but the distances from point to
point are to be compared; or such as

[Illustration: Inward and outward