Vergence -- from Eric Weisstein's World of Physics

Meyer-Arendt, J. R. Introduction to Classical and Modern Optics, 3rd ed. Englewood Cliffs, NJ: Prentice-Hall, p. 27, 1989.

Keating, M. P. "Vergence, Vision, and Geometric Optics." Amer. J. Phys. 43, 766-769, 1975.