All indicate that the movement is to us
something different from merely seeing an object first at one and
afterward at another place. We can easily find the analogy in other
senses. If we touch our forehead or the back of our hand with two blunt
compass points so that the two points are about a third of an inch
distant from each other, we do not discriminate the two points as two,
but we perceive the impression as that of one point. We cannot
discriminate the one pressure point from the other. But if we move the
point of a pencil to and fro from one point to the other we perceive
distinctly the movement in spite of the fact that it is a movement
between two end points which could not be discriminated. It is wholly
characteristic that the experimenter in every field of sensations,
visual or acoustical or tactual, often finds himself before the
experience of having noticed a movement while he is unable to say in
which direction the movement occurred.
We are familiar with the illusions in which we believe that we see
something which only our imagination supplies. If an unfamiliar printed
word is exposed to our eye for the twentieth part of a second, we
readily substitute a familiar word with similar letters. Everybody knows
how difficult it is to read proofs. We overlook the misprints, that is,
we replace the wrong letters which are actually in our field of vision
by imaginary right letters which correspond to our expectations.
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