The ability of an observer to maintain steady fixation on a test object is limited by involuntary eye tremor. Such tremor is characteristic of balanced muscular systems such as those serving to rotate the eye. Rotations of the eye cause proportional displacements of the retinal image of a stationary test object. Data are given for typical excursions of the retinal image during various exposure intervals from 0.01 to 1 sec in duration. The data show that under good experimental conditions the retinal image is virtually stationary for exposures up to 0.01 sec in duration. Exposures of 0.1 sec entail an average displacement of 25 sec of arc (a visual angle corresponding to the diameter of a single foveal cone). Exposures as long as 1 sec permit an average excursion of about 3 min of arc.
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