The Effect of Oculomotor Adjustments on Apparent Size

Abstract

An object of fixed angular size looks smaller when the eyes are adjusted for near vision than when they are adjusted for far vision. This fact was reported first by Wheatstone, the inventor of the stereoscope.1 Wheatstone's qualitative observations were confirmed by several later investigators,2 and quantitative studies of the phenomenon have been made by Adams, Frank, Hermans, and Swenson.3 The research lXeported below deals mainly with two questions to which the previous investigators have not given clear answers. The first is whether the changes in apparent size are related to changes in apparent distance in the manner postulated by several current theories of 'size-constancy.'4 The second is which of the oculomotor adjustments, or possibly which combination of adjustments, controls the changes in apparent size. The principal oculomotor adjustments that are made in response to a change in viewing distance are (1) changes in the angle of convergence of the visual axes, (2) changes in the power of the lens (accommodation), and (3) changes in the diameter of the pupil. Convergence and accommodation are so linked that a change in one produces a change in the other, and a change in either is usually accompanied by a change in the diameter of the pupil.