To imitate this lens, you can use a slide projector and what’s called a negative lens — the type used in glasses for people with myopia, or near-sightedness. First, cut a hole the size of the lens in a piece of cardboard. Place the lens in the hole and hold it in front of the projector near the screen. You should see at the center of the screen an enlarged image of one portion of the picture from the slide. Alone, the negative lens won’t enlarge an image, but combined with the lens of the projector, you get the same sort of telephoto system that Galileo used in his telescopes three centuries ago — and the same sort of system that birds have been using for millions of years to spot their prey from the air.