 The debate between Helmholtz and Hering focused on several specific cases, in particular, how to account for the so-called “horopter” problem. The horopter is the set of points that the eye perceives as equidistant from the perceiving subject when the eye is focusing on a single point. The eye perceives points on the horopter as if they were lying on a straight line at a fixed distance from the eye. However, in reality the horopter line is a curve, that is, in reality the points are not equally distant from the eye. 