Visual acuity was measured in terms of the reciprocal of the angle in minutes subtended by the individual lines in a parallel-line test object at the limit of perception of the lines when the observer was allowed to see the target for less than one millisecond. Visual acuity, measured with the lines passing either vertically or horizontally through the visual field, was found to be approximately 20 percent higher than when measured with the lines passing diagonally through the visual field at angles of 45 and 135 degrees with the horizontal. These results indicate that the variation in the visibility of a parallel-line test object as a function of its orientation is not produced by preferential directions for eye movements. The variation of visual acuity with test-object orientation was also measured as a function of pupil diameter and fixation point, but these data do not give any conclusive evidence as to the factors producing the variations.
G. C. HIGGINS and K. STULTZ, "Variation of Visual Acuity with Various Test-Object Orientations and Viewing Conditions," J. Opt. Soc. Am. 40, 135-137 (1950)