SENSITIVITY TO BRIGHTNESS IS GREATER IN THE PERIPHERAL REGIONS OF THE RETINA THAN IN THE CENTER. THIS SENSITIVITY IS A MAXIMUM AT A CERTAIN DISTANCE FROM THE CENTER, WHICH DEPENDS ON THE DIRECTION, AND THEN SLOWLY DECLINES FURTHER OUT. THE PERIPHERAL RETINA IS MORE SENSITIVE THAN THE CENTER TO RAPID MOTION. IN ORDER TO MAKE THE ALTERNATING SECTORS OF A ROTATING DISC BLEND INTO ONE ANOTHER, A HIGHER RATE OF ROTATION IS REQUIRED IN INDIRECT VISION THAN IN DIRECT VISION. THESE PROPERTIES OF THE EYE SEEM USEFUL FOR VISION, AND OFFER SUBSTANTIAL ADVANTAGES WITH RESPECT TO PERCEPTION OF OBJECTS UPON WHICH THE EYE IS NOT FIXED AND OF MOTIONS OCCURRING AT THE BOUNDARIES OF THE FIELD OF VISION. IT IS VERY LIKELY THAT THE OUTER SEGMENTS OF THE RODS, ACTING AS CATOPTRIC INSTRUMENTS, BRING ABOUT THIS INCREASED SENSITIVITY OF THE RETINAL PERIPHERY, WHICH WOULD ALSO EXPLAIN THE DIFFERENT DISTRIBUTION OF RODS AND CONES IN THE HUMAN RETINA.

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