Fig. 4. A cross is marked on the cornea for reference. In the primary position and in secondary positions (up, down, right, left), the cross coincides with the reference vertical and horizontal. (These reference lines work on the same principles as longitude and latitude.) All other gaze positions are tertiary; the cross is tilted relative to the vertical meridian existing in that position. The amount of this tilt or torsion around the visual y axis is angle T. For gaze obliquely, the amount of torsion increases as the eye goes farther from the primary position: T = 1 degree for 10-degree gaze out and 10-degree up; T = 3 degrees for 20-degree gaze; T = 8 degrees for 30-degree gaze; T = 15 degrees for 40-degree gaze. The cornea does not tilt relative to the oblique line connecting the primary position to the new gaze position; and A remains constant, in accord with Listing's law, because the objective vertical meridian is constantly changing.