The perception of curvature.

Threshold of curvature were determined for two chord length fat three orientations-vertical, horizontal and oblique. The radius of curvature at threshold was smaller for the shorter lines. Performance was about the same horizontal and vertical lines but not as good for oblique lines. The sagitta or "off-straightness" of the threshold curves varied between 2 and 4 sec. of arc at the eye. The problem of the appropriate measure of acuity is discussed.