904 



\CIENCE. 



[N. S. Vol. IX. No. 235. 



always sees things at a diiferent angle — in 

 other words, it sees a diiferent thing. Sup- 

 pose that for the normal eye A and the 

 myopic eye B the ranges of distinct vision 

 be respectively a b c and b c, and that 

 there be viewed an object consisting of a set 

 of plane surfaces at right angles to the line 

 of vision of the normal eye and a second set 

 coincident with it. The normal eye will 

 habitually see only the set of plane surfaces 

 at right angles to its axis of vision, and at 

 successively greater distances from its point 

 of view ; while the myopic eye, observing 

 the same object, will not only regard these 

 planes at a different angle, but will see also 

 the surfaces connecting the extremities of 

 the first mentioned planes. In other words, 

 the two eyes will have before them different 

 sets of visual elements. The same principle 

 applies in detail to all objects of distinct 

 vision ; therefore, as the point of view 

 changes to a ntw focal distance from nor- 

 mal to myopic, or the reverse, the constitu- 

 ents of the visual field are altered and an 

 accent of strangeness and unfamiliarity is 

 given to its objects. This matter of focal 

 distance becomes of distinct importance in 

 photography, where the space relations of 

 camera and object must be as nearly as pos- 

 sible those under which the picture will af- 

 terward be viewed ; otherwise a distortion 

 of perspective appears which materially 

 interferes with the truth of the represen- 

 tation. 



/. There is a final group of changes in 

 visual perception to be considered in con- 

 nection with concomitant motor adjust- 

 ments. These consist, in the first place, of 

 deflections and curvatures of right lines 

 when viewed through the marginal areas of 

 the glasses, which are obviously due to the 

 non-homogeneous refractive qualities of the 

 lens. They are identical with the distor- 

 tion of vertical lines upon the sides of the 

 visual field in a photograph the focal dis- 

 tance of which is short in relation to the 



length of these lines. The divisions of the 

 sidewalk, the rails of the car tracks, and all 

 lines whose direction lies at right angles to 

 that of vision, are thus warped from the 

 rectilinear. The same is true of house- 

 walls and trees, and of all vertical lines at 

 the sides of the visual field. When coming 

 down a flight of stairs the steps curve for- 

 ward at the sides, making them appear a 

 semicircular, hollow flight. 



The result of these changes is a confusion 

 of the relations between visual perception 

 and motor-adjustment. The familiar visual 

 cues by which the latter is habitually gov- 

 erned have been destroyed, and movements 

 are awkward and mal-adjusted. It is im- 

 possible to walk down a familiar flight of 

 steps without stumbling repeatedly'. The 

 illusionary reduction in visual size and fore- 

 shortening of perspective work disastrously 

 here, and result in a short, mincing step 

 which brings the foot constantly into colli- 

 sion with the step from which it is descend- 

 ing, instead of allowing it to clear for the 

 next. There is an absolute contradiction 

 between visual measurement and motor ad- 

 justment. The only way to secure such 

 adjustment and reach the bottom in safety 

 is to look quite away from the steps and to 

 trust wholly to joint and limb perception. 

 Thus the connections of muscular memory 

 become the controlling cues, uncontradicted 

 by present visual impressions, and the 

 descent grows at once secure and rapid. 



Secondly, the shortening of perspective is 

 not uniform for all areas of the lens, but 

 increases continuously from the margin 

 towai'ds the center. The effect of this 

 appears in a curious optical illusion and a 

 second form of mal-adjustment of motor re- 

 action in consequence of it. The ground in 

 front, as one walks, appears constantly to rise 

 in a sharp curve, as if a steep hill were being 

 mounted, and the foot is raised to meet the 

 imaginary elevation, only to be brought 

 down again with a shock to the original 



