the studies in the optical illusions are generally thought to yield
more important results for psychology than corresponding studies in
the field of touch. Then, too, the optical studies are more attractive
by reason of the comparative ease and certainty with which the
statistics are gathered there. An optical illusion is discovered in a
single instance of the phenomenon. We are aware of the illusion almost
immediately. But in the case of most of the illusions of touch, a
large number of experiments is often necessary in order to reveal any
approximately constant error in the judgments. Nevertheless, it seems
to me that the factors that influence our judgments of visual space,
though their effects are nearly always immediately apparent, are of no
more vital significance for the final explanation of the origin of our
notion of space than the disturbing factors in our estimations of
tactual space whose effects are not so open to direct observation.

The present investigation has for its main object a critical
examination of the tactual illusions that correspond to some of the
well-known optical illusions, in the hope of segregating some of the
various disturbing factors that enter into our very complex judgments
of tactual space. The investigation has unavoidably extended into a
number of near-lying problems in the psychology of touch, but the
final object of my paper will be to offer a more decisive answer than
has hitherto been given to the question, _Are the optical illusions
also tactual illusions, or are they reversed for touch?_

Those who have given their attention to illusions of sight and touch
are rather unequally divided in their views as to whether the
geometrical optical illusions undergo a reversal in the field of
touch, the majority inclining to the belief that they are reversed.
And yet there are not wanting warm adherents of the opposite view. A