Fig. 18 A. The empirical fusion horopter for symmetric fixation in the visual plane. Note the spread away from the geometric horopter (dashed line) due to the Hering-Hillebrand deviation and the vertical tilt caused by the Volkmann-Helmholtz shear of the vertical meridians. B. The generalized empirical fusion horopter for any other fixation point. Note that asymmetric fixation produces a dramatic thinning of the fused region near the fixation point.