ILLUSION. 470 ILLUSION. may take the form of subjective enhancement of the intensity of stimulus, as when n penile kniK-king at the <hior is taken for the growl of thunder; it nuiy involve a qualitative alteration of the stimulus, as when the monotonous sighing of the wind is heard as anpelir niu-iio; or it may consist in a fantastic modelinj,' and grouping of the forms of perceived objects, as when a dimly A V V A U n A^ V A (—^') — c > — <^-^ Flo. 2. PCHnclEIlKUS BEVEBIIIILK 8TA1R KHiCRE. The liKiire tluotuat<-« ii» .tou I'luk ut It. Ifiit ttrst .vou Beem to be looking ot the top of the trfiida, the aiipenrume Boon cbanges, 80 tbatyoueeem to see the underside of the stairs. .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 faces in the fire and monsters in the clouds 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 <-> > < o < ^^ . Fio. 3. TABiiBLE iLLCsioss OP EXTENT (frnm Sanford, Coarse Id Experimental faycbulog^). In A the space covered b.T the 8.dotH at the left seems greaterthan that between the 8 and the one at tiie extreme ri^ht. In B the qnadrantw witti inscribed radii appear greater than those that are empty. The figures C and f) (Helmholtz's Squares) have a slightl.v oblong appearance. Figure £ Is a variant of ., the Illusion being the reverse of that In A. sense-organs. So far from being abnormal, this of circularity will form of illusion is both natural and necessary, the circle seems We speak of the perceptions as illusory' simply points where its circumference is touched by Fio. 4. THEiRROw-oEAn Asn featheb ii.i.rsiox (from Sanford, Course in Kxperimcatnt I'sycbology}, In the first row, the distances between the polnt« of tliff aUKles are equal, and the 1st, M. Ith, Cth, 8th, and 10th to 11th nnd I'Jth vertical lines are all equal In length to the distance between the points of the arrows, though they do not appear so. In the second row of flgun-s the top and bottom lines are all equal ; in the third and fourtli rows the horizontnl. In the fltth row the vertical, and in the sixth and seventh rows the horizontnl lines are ail equal in length. In the last row the parts of the horizontal lines marked off by the point* of the angles are all equal. because they are of a char- • acter different from that which our previous knowl- edge of the stimuli, or our experience under other con- 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 mat- ter of fact, the perfect .square always looks like a rectangle whose vertical dimension is somewhat greater than the horizontal. Again, we are extremely familiar with the appear- ance of the circle, which we have seen in all sorts of figures and under the most varied comlitions. When, then, we inscribe a square in a given circle, we do not expect that the appearance be affected. In actual fact, to l)e pinched in. at the
