The stereoscopic acuity of five subjects was investigated in two consecutive and related studies. In the first one a modified horopter apparatus was used and the stereoscopic acuity was determined, at a mean retinal locus of 4° to the left of the fovea, at the 333- and 1000-mm observation distances. Two types of fixations were used, and the exposure time was kept above 1 sec. The results indicated a significant variation of the stereoscopic acuity with observation distance. In the second investigation a haploscopic apparatus was used to study the stereoscopic acuity at a mean retinal locus of 4° and to the left of the fovea, at 500, 750, 1000, 1500, and 2000 mm for the 4° and at 500, 750, and 1000 mm for the locations. The duration of the stimulus was accurately controlled by a mirror-shutter. The exposure times used were 0.05, 0.10, 1.00, and 2.00 sec. Fixation was central and all known physical variables were kept constant. The results again indicated a significant variation of the stereoscopic acuity with the observation distance. The possible implications of this finding are discussed.
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