The ratio of the apparent size of two stereoscopic targets and the vergence of subjects viewing these targets were measured. When the two targets have small disparities within Panum's fusional zone, the ratio of apparent size conformed to the ratio of the geometrically calculated size. On the other hand, when the two targets have large disparities, subjects could not fuse two targets simultaneously, and the ratio of apparent size did not conform to the ratio of the geometrically calculated size. This can be explained by the changes of retinal disparity that depend on the vergence displacement.
