The Law of Equal Retinal Position with Equal Fixation Position

Abstract

From the rule of eye movement described above it follows that every specific position of the fixation point in bifixation space corresponds to a quite specific state of contraction of specific eye muscle groups, which always occurs when the fixation of the double eye comes to Lie on the same point of the fixation space. So, if the gaze rests on a point A to the right and above in the far distance, for instance, a given shortening of the raisers and right-turners will be necessary to hold fixation there. This shortening will always occur to exactly the same degree whenever fixation returns to point A with unchanged head position after any deviation. At the same time, all other eye muscles will always show the same conditions of passive tension as long as their elastic forces are not somehow changed. Thus all other forces working on the eyeball will also be the same and will work in the same directions. It follows that not only the line of sight but also every other point of the eyeball and the retina (which is of special interest to us here) will always have the same position. Briefly, the cyclorotary orientation of the eyes will be a single valued function of the fixation position.