ABSTRACT
The illusions referred to as primary correspond to most of the optico-geometric illusions. Among them are: the illusions of rectangles (the over-estimation of the longer side); the T figure (to the extent that it introduces semi-rectangles, the over-estimation of the vertical as such being actually 'secondary'); the over-estimation of a segment of a straight line either when extended by a smaller segment or when inserted between two smaller segments; the Delbœuf illusions (of concentric circles); the illusions of angles (the over-estimation of acute and the under-estimation of obtuse angles, and the illusions of the arms of angles); the illusions of lozenges (the under-estimation of the longer diagonal); the Müller-Lyer illusions; illusions of parallelograms; of curvature, etc., and finally, marking the transition to secondary illusions, the Oppel-Kundt illusion of divided spaces.
