“…By physically manipulating the eyes of the fish tested, the angle at which light entered the eye, rather than simply the angle of light relative to the vertical, was found to be the important parameter of the optic stimulus (von Holst and Schoen 1954). Histological analysis of the eyes of several behaviorally tested species showed that variation in the effectiveness of light correlates with the density and arrangement of rods and cones in the retina (Bogenschutz 1961). In some species the visual input has a greater influence on posture than the vestibular input (see Pfeiffer 1964).…”