distances, the movement is made more difficult by reason of the
frequent stoppages. The fact that the space which is filled with only
one point in the middle is underestimated, is explained by Wundt on
the theory that the eye has here the tendency to fix on the middle
point and to estimate the distance by taking in the whole space at
once without moving from this middle point. A different explanation
for this illusion is offered by Helmholtz.[16] He makes use of the
æsthetic factor of contrasts. Wundt insists that the fact that this
illusion is still present when there are no actual eye movements does
not demonstrate that the illusion is not to be referred to a motor
origin. He says, "If a phenomenon is perceived with the moving eye
only, the influence of movement on it is undoubtedly true. But an
inference cannot be drawn in the opposite direction, that movement is
without influence on the phenomenon that persists when there is no
movement."[17]

[15] Wundt., W., 'Physiolog. Psych.,' 4te Aufl., Leipzig, 1893,
Bd. II., S. 144.

[16] v. Helmholtz, H., 'Handbuch d. Physiol. Optik,' 2te Aufl.,
Hamburg u. Leipzig, 1896, S. 705.

[17] Wundt, W., _op. citat._, S. 139.

Satisfactorily as the movement hypothesis explains this and other
optical illusions, it yet falls short of furnishing an entirely
adequate explanation. It seems to me certain that several causes exist
to produce this illusion, and also the illusion that is often
associated with it, the well-known Müller-Lyer illusion. But in what
degree each is present has not yet been determined by any of the