Page:The New International Encyclopædia 1st ed. v. 10.djvu/548

* ILLUSION. 474 ILLUSION. Fig. 12. MIXED iLLi'HioNs (from Titchener. Experimental Peycbology). In I compare the flirt? of lert-hunt) t-irele with liiiuT circle in iiiiiliilc flffure. alHO ouU?r clt^-le of middle flffur^ with right-hand circle. In 2 the half of the altitude of an eipiitateral triangle that is nearer the vertex lookH (*hort«?r than the other half. In :l the shorter parallel lines seem farther apart than the longer parallelH ilo (Wundtl. In 4 "the dots are reallv at the level of the lower line, bnt seem a little too high, espeeiall.v when the figure Is held so as to make the lines ohlhiuo" (Sauford). In 6 the middle sections of the five broken lines are all parallel, though they appear to be otherwise. and two narrow rectangles are drawn, of equal lieight and separated by equal spaces, the space between the larger rectangles will look smaller than the space between the narrower; the inter- spaces are differently estimated by 'contrast' with the adjacent figures. (See Contrast.) (e) Finally, we may, of course, have figures in which several of the above-mentioned motives, physiological and psychological, cooperate to l)roduce the illusion. We may further include under the general heading of optical illusions: (1) the physical illusions produced hy mirrors, lenses, prisms, etc. (see Mirage; Refi.kction ; Refbactiox) ; (2) the physiological illusions of color (see ArrER-lMAGE; Contrast); (.3) illusions of movement, such as, e.g. the apparent movement of the moon vhen viewed through drifting I'louds; and (4) illusions of distance. Under the latter categorj' falls the apparent enlargement of sun and niix)n when seen near the horizon. The distance between the observing eye and the hori- zon seems to be longer (owing to the haziness of the air, the number of intervening objects, etc.) than the distance between the eye and the zenith. But if two objects subtend the same visual anple (give the same retinal image) at .different distances, the one which is farther off must be largi-r than the other. Touch, no less than sight, is subject to spatial illusion. (a) If two compass points are set down upon the skin, first on an area of greater and then on an area of less sensitivity, the dis- tance between them will seem to be greater in the former case than in the latter. Give the points, e.g. a separation that enables them to be clearly perceived as distinct impressions when they are set crosswise upon the skin of the upper arm. Xow draw the compasses down the inner surface of the arm. to forearm, wrist, and finger- tips. The two parallel lines which they describe will appear to converge and diverge upon the regions of less and greater sensitivity. Or set the points down, vertically, upon the upper and lower lips, and draw the compasses outward, toward the ear. The points seem to converge as they travel over the skin, (b) Cross the tip of the second over that of the third finger, and place a marble or other small object between the cros.sed tips. Since the two surfaces now stimti- lated are never affected, in ordinary life, hy ona and the same object, but only by two different