ILLUSION. 472 ILLUSION. Fio. 7. TARiABLE iLLrsioxs OP MBECTioNB (Ziillner's FlfTuree). Id a the Tertica] Unee aad in n the diaKoual llDes Id the square do not appear parallel, tfaough thej are. working of the eye as a 'space organ'; the per- later investigators; the one by Wundt (q.v.), ctptions are illusory only in the sense that they who seeks, so far as possible, to account for the conflict with our other knowledge, i.e. with the illusions in physiological terms (fixation and verdict of the "mathematioar or 'measuring' eye. eye-movement) ; the other by Lipps (q.v.), who The most important of these 'normal' illusions offers a 'mechanical a'sthetic' principle of ex- planation. Lipps's theorj' is, in brief, that we read into the lines and figures in question forces and counterforces, strivings and resist- ances, akin to those which we ex- perience in ourselves ; so that the colunm 'seeks' or 'strives' to rise, the horizontal line to stretch itself, the circle to 'hold itself together,' etc. We shall here give Wundt's classification and explanation, on the ground that, in matters of per- ception, physiological conditions must always have the priority over |isychological. The first group of geometrical optical illusions contains (a) tlie illusions of reversible perspective. It is a familiar fact that many outline figures — a bowl, an open book, a cube, a stairway — will 'turn inside out' in the ni.'st perplexing way as one looks at them. Kven two crossed lines, drawn X-wise upon a sheet of paper, may be made to take on the appearance of a right-angled cross, seen in per- spective, with either the lower or the higher point of the line which forms the horizontal arms nearer to the obsen'er. Very little prac- tice suffices to show that the direc- tion of perspective depends here upon the point fixated and the course along which the eye moves; the point of fixation is the point nearer the ob- server, and jnovement of the eye toward a given point brings that, in its turn, to the front of the figure, (b) Next follow the illusions of extent, variable and constant. A dotted line looks longer Fio. 3. CONSTANT iLLrsios OF DiREmoN (Von ReckliDghaueen). Look at the centre of the figure with one eye, and the lines will appear to be perpendlcolar to each other and make squares. are the spatial illusions of touch and sight. The latter, which were first demonstrated by J. Op- pel (1815-94) in certain simple geometrical fig- ures, have been termed collectively, the geometri- cal optical illusions. They are exceedingly nu- merous, and explanations have been almost as than a drawn line of equal objective length, for plentiful as illusions. Two attempts at syste- the reason that it offers more halting places as matic treatment have, however, been made by the eye moves along it. Draw an outline square;
