An experimental and theoretical appraisal of the perspective and size-constancy theories of illusions

The perspective and size-constancy theories of illusions are described. They both imply that illusory distortions arise as a consequence of mistaken perceptual impressions of depth. Predictions from these theories are tested in six experiments and several ancillary studies. They show that although differences in apparent distance may be sufficient for the appearance of illusions their presence is not a necessary condition. Detailed theoretical implications of these results are considered in discussion.