SPATIAL PERCEPTION. 



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the lenses farther apart than normal, that blue stood out, while with 

 the head inclined to the opposite side red was in relief. 



The explanation originally given by Donders was based on the sup- 

 position that all individuals see red in front of blue. A greater effort 

 of accommodation is necessary to focus red distinctly, and Donders sup- 

 posed that this greater effort gives rise to the idea of greater nearness. 

 The correct explanation was given by Einthoven, who referred the 

 appearance primarily to chromatic aberration, acting together with 

 the deviation between the optic axis and the line of vision. In an 

 eye accommodated for blue, the red rays will form a diffusion circle 

 round the focus for blue, and owing to the deviation of the optic and 

 visual axes, as in Fig. 415, the focus for blue will not be in the centre, 

 x, of the diffusion circles, but towards its nasal side, i.e. with a median 

 pupil. If the pupil is temporal, this eccentricity will be increased ; if 



Fig. 415. 



slightly nasal, it will be compensated for; while, with marked nasal 

 pupil, the focus for blue will be on the temporal side of the diffusion 

 circle. In an eye accommodated for red, as shown in Fig. 416, the focus 

 for blue will have the same relation to that for red as in the eye accom- 

 modated for blue. Assuming that red is localised in the centre of 

 its diffusion circle, the foci for red and blue in one eye will be on 

 different points, while in binocular vision the red foci will be on 

 disparate points. With median pupils the red focus will be on the 

 temporal side of blue in each eye, and the disparation will be crossed ; 

 consequently, red will appear the nearer. With temporal pupils the dis- 

 paration will be greater, and red will appear in stronger relief. With 

 marked nasal pupils, on the other hand, the red focus will be on the nasal 

 side of the blue focus in each eye ; consequently, the disparation will be 

 uncrossed, and red will appear further away than blue, i.e. blue will 

 appear in relief. 



The less marked appearance of relief which occurs in monocular 



