History of Three-Dimensional Eye Movement Research
To continue in an approximate chronological order, Listing’s law should be introduced. It states that with the head erect and not moving, all eye positions can be characterized by a fixed axis rotation from a reference position, and that all such rotation axes lie in a plane. The standard deviation of eye positions from the plane, commonly called ‘thickness’, is smaller than 1.0 degree. Listing himself did not have the technical possibilities to make measurements with such a precision, and it is even unclear whether he did any measurements at all. Parenthetically, it might be added that Fick gave a careful overview of the literature on eye muscles, and was disappointed about the lack of quantitative measurements. The standard deviation of eye positions from the plane, commonly called ‘thickness’, is smaller than 1.0 degree.