Mechanical Prerequisites

Abstract

For simplicity, the eyeball can be considered as a sphere which can be rotated about an immovable middle point. To determine the position of the sphere it is adequate to give the positions of three* points on it which do not lie on the same diameter, or equivalently, to give the positions of two diameters. In the eye it is best to use first the line of sight for this, which one can assume to go through the point of rotation, and second the diameter which is vertical when the line of sight is directed horizontally straight ahead with an upright head. If we know the positions of these two diameters relative to the orbits or to a coordinate system of the head with any eye position, we have determined the position of the eyeball unambiguously in its socket. It is apparent that knowledge of the position of only one diameter, for instance the line of sight alone, is inadequate, because with the same position of the line of sight the rest of the eye can be thought of in many varied positions, which it would run through one after another if it were rotated around the undisturbed line of sight as an axis.