Data from one of the participants in our study to conventional displays of the Delboeuf illusion.

<p>The inner circle (<i>c</i>) had a fixed diameter of 10 mm. The outer, inducing circle had a range of values from 20 mm (relative inducer size of 2) to 120 mm (relative inducer size of 12) in 10-mm steps. The participant adjusted the size of a circle (<i>s</i>; not shown) in the lower left of a computer-monitor screen to match the apparent size of <i>c</i>. Three such stimuli are illustrated, spatially to scale, in grey–one in which the relative inducer size is 2, one in which the relative inducer size is 7, and one in which the relative inducer size is 12. The small plot circles are the mean differences between the adjusted size of <i>s</i> and its true size (10 mm) for each value of relative inducer size from three trials each (the participant received the trials in a completely random order); the vertical bars are standard errors. The red line is the statistically significant, best-fitting, positively sloping, linear function.