On Constancy of Visual Speed by Hans Wallach

When investigating the constancy of visual speed J. F. Brown 2 discovered what he called the transposition principle of velocity. In his account the constancy of visual speed and the principle of transposition occur as unrelated facts. This paper attempts to show that constancy of visual speed can be understood as a consequence of the transposition principle. When objects which move with the same objective velocity are presented to the resting eye at different distances one perceives them as moving with approximately equal speed, although the displacements of their retinal images per unit of time vary in inverse proportion to the distance. This is what we call the constancy of visual speed. Its formal similarity to the constancy of size is obvious. Two identical objects presented at different but moderate distances from the eye have almost equal phenomenal sizes, although the linear extensions of the corresponding images are inversely proportional to the distances at which the two objects are presented to the eye. It seems plausible to assume that constancy of visual speed is simply a consequence of size constancy. One might argue that visual speed depends not on the length through which an image passes on the retina per unit of time but on the visual extension through which the object moves. Since the latter extension remains approximately constant even if its objective size is projected from different distances and therefore with varying retinal size, the constancy of visual speed seems to follow without any further assumptions. 1 The writer wishes to express his appreciation to Professor Wolfgang Kohler and also to Dr. Richard S. Crutchfield for their aid in preparation of this paper.