June 30, 1899.] 



SCIENCE. 



903 



opportunity to observe if in the case of 

 concave surfaces there is an analogous ex- 

 aggeration of hollowness or depth. 



c. The use of stronger ghvsses produced 

 an apparent dimunition in the perspective 

 rehitions of objects within the visual field, 

 VFhich at times reached almost the vanish- 

 ing point. Men and women on the street 

 were silhouetted against the background of 

 trees and houses, or moved like shadows 

 over a screen. A similar reduction in per- 

 spective can be produced by piercing a bit 

 of cardboard with a small hole, and viewing 

 a. group of objects in the middle distance 

 through it, while the cardboard is held 

 close to the eye. The fineness and cer- 

 tainty of distance perception depend gi-eatly 

 upon the continuity of the visual field from 

 the feet of the observer to the object viewed, 

 and in the last mentioned case the obscura- 

 tion of this sense is due to the interruption 

 of these conditions. In the case of my- 

 opic glasses the illusion is due, in part at 

 least, to an underestimation of the distance 

 of the objects, resulting from their abnor- 

 mal definition as seen through the stronger 

 glasses. In any series of uniformly spaced 

 objects the apparent size and the visual dis- 

 tance between any two adjacent members 

 decreases as their absolute distance from 

 the eye increases. In all normal cases this 

 decrease is correctly interpreted through the 

 coordinated perception of increased dis- 

 tance. If, however, an illusion of increased 

 nearness to the observer arises from any 

 cause, not only do the objects themselves 

 appear smaller, but the relative distances 

 between them are likewise reduced, and the 

 perspective of the field of individualized ob- 

 jects thereby diminished. 



d. The faces of persons in the middle 

 distance — that is, towards the farther limit 

 of distinct vision for the character of the 

 facial lines and expression — appeared to 

 hang in the air near by when first caught 

 sight of. Here the distance of the object 



appears to have been estimated correctly by 

 the use of various familiar criteria, chiefly 

 the multiplicity of objects between the ob- 

 server and the person seen. When, how- 

 ever, the eye first rested upon the face of 

 the person in question these cues fell into 

 the background and the abnormal definition 

 of the face became the dominant factor of 

 the experience, a definition possible to the 

 unaided myopic eye only within a much 

 narrower range of vision ; and the shock of 

 contradiction between the felt distance of 

 the object and its observed distinctness re- 

 sulted in a dissociation of the face image 

 from that of the rest of the body, the latter 

 maintaining its estimated distance, the for- 

 mer approaching to that corresponding 

 habitually with the observed definition. 

 The illusion maintained itself only during 

 a few moments while the attention was 

 strongly centered on the face. 



e. This focussing of attention upon the 

 face had itself an abnormal element in it. 

 The faces of persons at a distance appeared 

 mask-like and grotesque ; the eyes stared, 

 the light and shadow fell unnaturally, the 

 lines and expression were distorted. Sub- 

 jectively this change was manifested chiefly 

 as an alteration in the affective overtone of 

 the object, but one which itself is derived 

 from a change in the character of the percep- 

 tion. The magnitude of the visual angle 

 which any object subtends varies with its 

 distance from the observer. As this distance 

 changes, the mechanism of the ej'e must be 

 adjusted to keep the object in the focus of dis- 

 tinct vision. Up to a certain point this is 

 possible, but beyond that limit accommoda- 

 tion of the eye must be replaced by approach 

 of the point of view toward its object. 

 The latter form of adjustment is habitual 

 with the mj^opic eye as compared with the 

 normal. In consequence the angle which 

 the object of distinct vision subtends in 

 the case of the mj^opic ej'C is habitually 

 greater than in that of the normal eye. It 



