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"Just behind the iris of the eye is a second sphincter, the ciliary muscle, which rings the lens and is attached to it by tiny strands called the zonules of Zinn. While the iris is opening or closing its aperture in response to bright and dark, the ciliary is contracting or relaxing to accommodate focus. When it is relaxed, the zonules are taut, and the lens is pulled thin to focus on distant things. When it contracts, the zonules go slack, and the lens rebounds into a thicker onion, a shorter focal point, to see things close at hand. ("Lens" comes from the word lentil.) When I hold up my finger in front of my nose and look at it, and then shift my attention to the keyhole beyond it in the door twenty feet away, or when I stoop to look at the keyhole in the gate of #3 Via di Santa Sabina in Rome and then shift to the cupola of St. Peter's it frames two miles away, my eye knows how to make the transfer. How does it know?"