THE INFLUENCE OF MENTAL PROCESSES VISUAL PERCEPTIONS. Our estimate of the size, form, distance, and direction of objects is based largely upon previous experience, and is the re sult of the physiological process of vision aided by former judgments which have been stored up in the brain. Such deductions are liable to error, either on account of error in judgment, or because of faulty vision. Vertical lines, for instance, ap pear longer than horizontal ones because Nve re quire a more powerful eontraction of the relatively weak upper straight muscle of the eye concerned in the required movement than is required of the comparatively strong muscles which move the eye laterally; we judge the upward motion of the eye to be greater because for an equal amount of motion we require a greater muscle impulse. A familiar example of this tendency to overestimate the vertical dimension in the common error of supposing the height of the crown of a silk hat to he greater than its breadth, an illusion which disappears when the hat is laid upon its side.
A striking example of illusion of space per ception is afforded by Zi;l1ner's lines, illustrated in Fig. 5. These lines appear to diverge and con verge alternately, although really parallel to each other, owing to their apparent direction being changed to make them appear more nearly perpendicular to the oblique lines crossing them; this tendency for the eye to make acute and ob tuse angles approach right angles, is the result of our knowledge that images of familiar sur rounding objects of square shape are often seen obliquely. and then form acute and obtuse angles
upon the retina ; we thus acquire the habit of mentally correcting such visual impressions. tee Consult: Von Helmholtz, Handbuch der physioloyischen Optik (Leipzig, 1896) ; Le Conte. Sight (New York. 1897) ; Foster. Textbook of Physiology (ih., 1900) ; Kirkes, Handbook of Physiology (lb.. 1902) ; Schaefer, Text book of Physiology (Edinburgh and New York, 1900) ; American Textbook of Physiology (Phila delphia. 1902) ; Graefe-Saemisch. Handbuch der peso m m ten A u (ye nh cilk u nde ( Leipzig, 18i)3); Norris and Oliver, Systems of Diseases of the Eye (Philadelphia. 1900) : May. Manual of Dis eases of the Eye (New York, 1904) ; Baldwin, Dictionary of Philosophy and Psychology (New York and London, 1901) ; Calkins, Introduction to Psychology (New York, 1901).