Disparity limits of stereopsis.

THE OLDER visual physiologists, in particular Volkmann, von Helmholtz, 1 Hering, 2 and von Kries and Auerbach, 3 knew that stereoscopic depth perception exists even when the disparities between images in the two eyes are large enough that those images appear double. It is a common experience for observers using the Hering "drop test" to report that the falling beads appear unmistakably nearer or farther than the fixation point even when the beads are seen as double images. Yet von Helmholtz stated that if the disparity was too large then indeed would stereopsis cease. Hillebrand, 4 however, in his book published in 1929 declared that stereopsis occurs only so long as the disparate images in the two eyes are fused. Also, Matsuda 5 declared that for vision in low illuminations the limit of the perception of double images coincides with the limit of stereoscopic depth perception. Tschermak 6 took a