The Sander illusion as a function of relative space and component lines

Magnitude of the Sander illusion was measured as a function of two variables: (1) the orientation of the line normally separating the two smaller parallelograms and (2) the presence or absence of the horizontal lines and of the diagonal test lines. The results showed that, as the angle of the dividing line varied so as to shift the relative sizes of the two parts of the figure toward equality, the illusion effect decreased, approaching zero when the two areas were equal. The illusory effect was enhanced by the removal of the two test lines. Results were discussed in relation to the problem of assimilation vs contrast effects.