354 ADAPTATIONS TO SPACE AND MOTION 



are responsive to any change. Another, and minor, advantage of the 

 periphery which depends upon the morphology of the eye (actually, 

 upon one of its so-called imperfections) is the 'barrel distortion' of the 

 peripheral field: as a circular image swings steadily outwards along a 

 meridian of the retina into the far periphery, it becomes elUptical with 

 its long axis meridionally oriented. If two such images move together 

 into the periphery, hailing from two objects whose separation in space 

 remains constant, the distance between the images (hence, the disparity 

 between their apparent relative speeds) incre^es along with the dis- 

 tortion. We may not be conscious of any peripheral aberration of shapes 

 under ordinary conditions; but nevertheless peripheral movements, in 

 meridional directions at least, are optically exaggerated to a not unim- 

 portant degree by this increased speed of the sweep of the image over 

 the retina. In some animals' eyes, where the retina is broadened in the 

 horizontal meridian by an ellipticity of the eyeball (horse, swift fishes) , 

 this factor may be quite important. 



Many more factors, however, operate to the disadvantage of the 

 periphery in movement-detection. Foremost of these is the rapid fall of 

 visual acuity from center to periphery, which is an expression of the 

 increasing size, meridionally outward, of the circles of innervation. The 

 acuity of displacement-discrimination also falls from center to periph- 

 ery, though not as rapidly as does the resolving power. In light-adapt- 

 ation, when central (i.e., cone) vision is at its best, the central fusion- 

 frequency is higher than in dark-adaptation; and, of course, visual acuity 

 also rises with intensity. In dark-adaptation, where the rods are under 

 optimal conditions, acuity is low; but it so happens that the peripheral 

 critical frequency is higher than it is in light-adaptation. In a duplex 

 retina under any given adaptation-condition, the central acuity and 

 critical frequency are ordinarily higher than the corresponding peripheral 

 values. The higher the illumination, the farther peripherally a movement 

 of a given speed is appreciated, due to the cones coming into play. 

 When the periphery is dark-adapted, however, it may still record flicker 

 when this has disappeared for a partly light-adapted center. One can 

 experience this at the movies, where the screen is flickerless but a hand 

 moved across the lap (while the screen is fixated) is seen intermittently. 



Similarly, rod-rich retinae (cat, owl) have been found to have lower 

 critical frequencies than cone-rich ones (pigeon) in the same condition of 

 adaptation. We cannot compare with perfect fairness the light-adapted 

 central vision and dark-adapted peripheral vision of a duplex retina with, 



