1. For the horizontal horopter this is called the Hering-Hillebrand deviation. 2. An empirical horopter can be defined following different criteria. 3. He derived, almost simultaneously with Helmholtz, the theoretical shape of the horopter . 4. This line is called the empirical horizontal horopter . 5. The horopter can be measured empirically in which it is defined using some criterion. 6. Their descriptions identified two components for the horopter for symmetrical fixation closer than infinity. 7. Subsequently Hering empirically estimated the shape horopter . 8. This is known as the horizontal geometrical horopter , or as the Vieth M�ller circle. 9. At short fixation distances, the empirical horopter is a concave parabola flatter that a circle. 10. Finally for fixation distances farther than the abathic distance the empirical horopter is a convex parabola.
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