Amongst the many illusions discovered by Ehrenstein, his brightness illusion is perhaps the most prominent one. In its original version, it is evoked by a pattern that consists of four radial black or white lines positioned at right angles to one another (Fig. 1). The lines do not intersect, but originate at a given distance away from the theoretical point of intersection. In the resulting gap, situated between the proximal end points of the radial lines, an illusory patch can be seen which appears to be brighter or darker than the surrounding background. This patch is delineated by an illusory border and, to most observers, looks circular.