The influence of discontinuous side on regularity of shape.

When a side of an equilateral triangle is interrupted by a spatial gap, an apparent irregularity of the shape is perceived. In order to explain this optical-geometrical illusion we carried out three experiments. The task of participants was to evaluate the regularity of the figures by two different methods of measurement. In experiment 1 we measured the influence of position and relative size of the discontinuity on the regularity of equilateral triangles. Experiment 2 was designed to evaluate the effect of the discontinuity on the apparent length of the side, in terms of the Oppel ^Kundt illusion, comparing figures with and without discontinuity. Experiment 3 veri- fied the influence of the discontinuity on the apparent inclination in depth of the adjacent side. The results show that position and size of the discontinuity are functionally related to the magni- tude of the illusion. Moreover, this illusion is not traceable back to the Oppel ^Kundt illusion, but we conclude that the apparent inclination in depth of a side is a crucial factor in providing a comprehensive account of the illusion.