Contrast and Confluxion as Components in Geometric Illusions

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Abstract
Two experiments on illusions were planned to test the predictive power of explanations based on size contrast and confluxion. The predictions turned out to be correct. A modification of the Müller-Lyer figure and an illusion of divided distance were used as stimulus figures. In the latter the dividing distances were underestimated. It proved to be methodologically necessary to measure contrast effects as differences between results of two experimental procedures, because small intervals as such were over-estimated. It was not possible to explain all the results by means of the constancy theory in the form suggested by Gregory. Neither contrast nor constancy alone is sufficient to explain the geometric illusions. The development of an adequate theory by combining these two explanations seems possible.