英语时差：鹰的眼睛（上）

An eagle circling three hundred feet in the air looks to us like a dot in the sky. But even from that height, the eagle is scanning the ground for prey. To help them see small objects from a distance, birds eyes produce a telephoto effect by combining two lenses in much the same way that Galileo combined glass lenses to build some of the earliest telescopes. Light entering our eyes passes through a tiny lens in the front of the eye and is then projected, like a slide in a slide projector, on the retina in the back of the eye. In humans, the surface of the retina is smooth, like a movie screen and so the objects at the edge of our vision appear the same size as objects at the center. In a bird’s eye, a small pit on the surface of the retina acts as a second lens to enlarge a portion of the image.