In ordinary use the eyes are never entirely at rest but are continually moving. The visual axes are constantly changing so that images of objects in the outside world are brought to lie on the macular region of each eye. In this way objects in the external world are seen in much the same manner as one feels the character of a surface by palpating it with a finger. By moving the finger over the surface the sensory endings of touch are stimulated to send up their message to the brain.
In order to discriminate the finer details of an object, on the other hand, it has often been assumed that the image of an object must rest on the fovea, permitting the parts of the image to stimulate a definite pattern of cones. The term fixation is used in this connection to signify that under these conditions the
ADLER FH, FLIEGELMAN M. INFLUENCE OF FIXATION ON THE VISUAL ACUITY. Arch Ophthalmol. 1934;12(4):475–483. doi:10.1001/archopht.1934.00830170013002
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