The Philosophical Review/Volume 1/Summary: Greeff - Untersuchung über binokulares Sehen mit Anwendung des Heringschen Fadenversuchs
In Hering's experiment, perception of depth was tested by the subjects determining whether a ball that fell across the plane of a vision, passed before or behind the point of fixation. With an improved form of Hering's apparatus the author found: 1st. Convergence of optic axes is not essential to perception of depth; the position of the line of fall of the ball, before or behind the point of fixation, was still determined, when, by means of lenses, the optic axes were made parallel or convergent. 2d. At distances too great for convergence and accommodation to come into play, binocular vision is retained so long as the ratio of the distance between the lines of fall, before and behind the point of fixation, to the distance of the point from the eye of the observer, is not less than 2: 100. This corresponds to a projection of about 0.002 mm. on the retina. 3d. When clearness of vision in one eye is impaired, whether artificially or by disease, binocular vision is possible so long as the weak eye receives an impression of the object, however dim or indistinct the object may appear. 4th. The author found no case among children, in which, after operation for strabismus, perfect binocular vision was gained; the patients perceived the object through each eye, but could not join the percepts into the impressions of depth. It seems, accordingly, that the coalescence of the retinal pictures is a purely mental act, which can only be learned in earliest youth.