The new international encyclopaedia . suAL Perceptions. Our estimate of the size,form, distance, and direction of objects is basedlargely upon previous experience, and is the re-sult of the physiological process of vision aidedby former judgments which have been stored upin the brain. Such deductions are liable to error,either on account of error in judgment, or becauseof faulty vision. Vertical lines, for instance, ap-jiear longer than horizontal ones because we re-quire a more powerful contraction of the relativelyweak upper straight muscle of the eye concernedin the required movement than i
The new international encyclopaedia . suAL Perceptions. Our estimate of the size,form, distance, and direction of objects is basedlargely upon previous experience, and is the re-sult of the physiological process of vision aidedby former judgments which have been stored upin the brain. Such deductions are liable to error,either on account of error in judgment, or becauseof faulty vision. Vertical lines, for instance, ap-jiear longer than horizontal ones because we re-quire a more powerful contraction of the relativelyweak upper straight muscle of the eye concernedin the required movement than is required ofthe comparatively strong muscles which movethe eye laterally; we judge the upward motionof the eye to be greater because for an equalamount of motion we require a greater muscleimpulse. A familiar example of this tendencyto overestimate the vertical dimension is theconunon error of supposing the height of thecrown of a silk hat to be greater than itsbreadth, an illusion which disappears when thehat is laid upon its Fio. 5, Zollner8 Lines. Anexample of Illusion of apaceperception. A striking example of illusion of space per-ception is afforded by ZJiUncrs lines, illustratedin Fig. .5. These lines appear to diverge and con-verge alternately, although really parallel toeach other, owing to their ap[)arenf directionbeing changed to make them appear more nearlyperpendicular to the oblique lines crossing them; this tendency for tlie eye to make acvite and ob-tuse angles approach right angles, is the resultof our knowledge that images of familiar sur-rounding objects of square shape are often seenobliquely, and then form acute and obtuse anglesupon the retina; we thus acquire the habit ofmentally correcting such visual impressions. SeeIllusion. Consult: Von Helmholtz, Handbuch derphpsiologisclien Optik (Leipzig, 1890) ; Le Conte,Sight (New York, 1807) ; Foster, Textbookof Physiology (ib., 1900) ; Kirkes, Handbookof Physiology (ib., 1902); Schaefer, Text-book of Phy
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