able to discriminate the poorly illuminated red car from its surrounding neighbors. 5. Adaptation. When stimulated by light, rods rapidly lose their sensitivity, and it takes a long time for them to regain it (up to a half hour) once they are re- turned to the darkness that is characteristic of the rods' "optimal viewing envi- TABLE 4.2 Some Measures of Acuity Minimum separable acuity Vernier acuity Landolt ring Snellen acuity General measurement of smallest detail detectable Are two parallel lines aligned? Is the gap in a ring detectable? Measurement of detail resolved at 20 feet, relative to the distance at which a normal observer can resolve the same detail (e.g., 20/30) , , I
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