Abstract: To study the influence of the direction of convergence on the judgement of the visual vertical, Ss were required to gaze straight forward binocularly at a vertical line, and also to gaze askew, starting from this primary position (0°), to the extent of 60° on either side of the body. Binocular observations were compared with monocular observations. The results showed that: (1) the amount of error in vertical judgement is proportional to the increase of the degree of looking askew (0—60°); (2) under both conditions, when looking askew or looking straight forward, the error in vertical judgement with one eye is always much more marked than that with both eyes. On the basis of these results, the author holds that the normal convergence of two eyes and their coordinate movements are necessary conditions for the visual vertical orientation.

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