ILLUSION (from Lat. illusio, mockery, from illudere, to mock, from in, in + ludere, to play). In general, a distorted or abnormal perception (q.v.). The word is used in English psychology holy legion was to be led on, as it was imag ined, to the benefaction of mankind. But, from this cause, the dissolution of the Order soon ensued. \Veishaupt and Knigge, its two leaders, quarreled with one another. The Order be gan to be openly denounced as dangerous, and in two principal senses: (1) As the counterpart of hallucination (q.v.). "an assimilation of an hallucinatory character" (\\'undt), a distortion or misinterpretation of the contents of percep tion, due to an abnormal irritability of the sen sory centres of the brain-cortex. The distortion may take the form of subjeetive enhancement of the intemsiiy of stimulus. as when a gentle knoeking at the door i. taken for the grow I of thunder; it May involve a qualitative alteration the stimulus, a. when the monotonous sighing of the wind is heard as angelic music; or it may consist in a fantastic modeling and grouping of the forms of perceived objects, as when a dimly seen stone or tree.stump is regarded as a ghost. :sane persons of imaginative disposition are lia ble to illusion, more especially at times of mental stress or overwork; and. indeed, the readiness with which we can find fates in the lire and monsters in the cloud• shows how short is the path from normal to abnormal assimila tion. We reach the stage of unquestioned abnor mality with patients who interpret the half heard conversation of passers-by as threats di rected against their life. and see on the face of every stranger an expression of disgust or con tempt or menace.
(2) In its second meaning, illusion is a per version of the contents of perception due to structural or functional peculiarities of the sense-organs. So far from being abnormal. this form of illusion is both natural and nece.sary. We speak of the perceptions as 'illusory' simply because they are of a acter different from that which our previous edge of the stimuli, or our experience under other ditions, would lead us to expect. For instance. we know that all four sides of a square are equal. We expect, therefore, that a square will look as high as it is broad. As a
ter of fact, the perfect square always looks like a rectangle \rhos° vertical dimension is somewhat greater than the horizontal. Again, we are extremely familiar NVith the amp of the circle. which we have seen in all sorts of figures and under the most varied When, then. we inscribe a square in a given circle, we do not expect that the appearance of circularity will he affected. In actual fact, the circle seems to he pinched in, at the points where its circumference is touched by the angles of the square, so that we perceive not a circle at all, hut four arcs apparently be longing to circles of less radius than the circle which should include the square. No one whose vision is normal can escape these illusions; their occurrence is, as we shall see, intrinsic to the working of the eye as a `space organ'; the per ceptions are illusory only in the sense that they conflict with our other knowledge, i.e. with the verdict of the 'mathematical' or 'measuring' eye.
The most important of these 'normal' illusions 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 plentiful as illusions. Two attempts at syste matic treatment have, however, been made by later investigators; the one by Wundt (q.v.), who seeks, so far as possible, to account for the illusions in physiological terms (fixation and eye-movement) ; the other by Lippe who offers a 'mechanical esthetic' principle of ex .A.,1 pion An, "I 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 column '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 psychological.