Three elemental illusions determine the Zollner illusion

We have discovered an apparent contraction illusion of acute angles in a sp ecial form of the Zollner figure at the intersecting angles between 36 degr ees and 83 degrees (i.e., a reversal of the Zollner illusion). The necessar y condition for this illusion is that inducing lines are long enough and th e induced line (test line) is single. When an illusory line is used as the induced line, the magnitude of contraction increases. Short inducing lines give no illusion or a slight expansion of acute angles at the intersecting angle of 45 degrees. We have ascertained that the source of this expansion is the narrow region in the vicinity of the induced line, whereas the sourc e of the contraction is much broader regions. Furthermore, we have discover ed another expansion mechanism, which is generated by the symmetrical confi guration of the standard Zollner figure.