Fig. 13 A. Projection of vertical meridian, showing that if the meridians are aligned with each pupil they must project in two planes that meet in a straight line, rather than any kind of curve. In addition, if the vertical meridians are slanted relative to each other, the line in which they meet will be slanted in the third dimension toward and away from the observer. B. The angle of tilt of the vertical horopter line depends jointly on the tilt of the vertical meridians and the fixation distance. The average midline tilt is 2 degrees, which means the vertical horopter line will pass approximately through the feet. With fixation at infinity, the horopter line will become horizontal, lying in the ground plane. (von Helmholtz H: Handbuch der Physiologische Optik. Hamburg: Voss, 1866)