Variation of convergence limits with change in direction of gaze.

Until recently, convergence limits have been measured only in the primary position of gaze. A new instrument permits such measurements in all directions of gaze and at various angular distances from the primary straight-ahead position. Aberration and distortion-free stimuli based on Heine's principle were incorporated in the apparatus. For each of the nine subjects participating, convergence limits were obtained in the primary straight-ahead position, secondary, and tertiary positions and in varying degrees of obliquity of these directions of gaze. The convergence limits were measured in 10 degrees steps away from the straight-ahead position of gaze in 18 different meridians. It was found that convergence limits vary markedly with the direction of gaze, the maximal vergence range usually being found between 10 to 20 degrees below the primary horizontal plane in the lower right-half field, possibly because subjects spend most of their time looking in this direction.