The eye of the chameleon
ARISTOTLE NOTED THE CHAMELEON’S remarkable ability to change its color some 2,3 00 years ago. Equally intriguing, though of less ancient interest, are the lizard’s eyes. Frank Schaeffel, a neurobiologist at the University Eye Hospital in Tubingen, Germany, found that chameleon eyes are unique among vertebrates: they contain lenses that cause light to diverge rather than focus it. And apparently that explains how a chameleon can judge the distance to an insect so accurately, dispatching it with a single deadly flick of the tongue.
Vertebrates, including humans, focus on objects at various distances by using muscles in their eyes to change the curvature of their convex lenses–a process called accommodation. For the most part, though, vertebrates don’t control the curvature of their lenses with enough precision to judge distances by this means alone. Rather, they rely primarily on depth perception, made possible by the different perspectives of their two eyes.
Chameleon eyes, Schaeffel and his colleague Matthias Ott found, are different. The researchers used infrared light from an instrument called a retinoscope to probe a chameleon’s eyes as it shot out its tongue to catch crickets at various distances. After the infared light bounced off the chameleon’s retina, it passed back out of its eye–through the lens, and out the cornea, the transparent membrane covering the eye. The lens and cornea bent the infrared light going out, just as they bend visible light coming. By precisely measuring the focal point of the infrared beams, Schaeffel and Ott could determine just how well the chameleon was focusing on the cricket by accommodation. It turned out that the chameleon could adjust its focus to a much finer degree than other vertebrates do–so fine that the amount of stretching or relaxing its lens muscles were doing contained all the information it needed to judge the distance to the cricket.
How does the chameleon do it? Precise focusing, says Schaeffel, depends on the size of the image formed as light converges on the retina: the larger the image, die easier it is to detect and correct fine shades of focus. When Schaeffel and Ott dissected out a chameleon eye and projected an image into it (an infrared square), they found that the image produced on the retina was 15 percent larger than the retinal image produced by a chicken eye of the same size.
According to Schaeffel, the chameleon’s secret for maximizing image size is its unique lens. In the standard vertebrate eye, the lens and the cornea collaborate to focus light. But the chameleon’s cornea, Schaeffel discovered, overfocuses the image. “Just using the cornea, the image would be 1 millimeter in front of the retina,” he says. The chameleon’s lens corrects for this: instead of focusing light, it actually makes it diverge just enough to cast a magnified image on the retina. Some telescopes work on this same principle. “But to our knowledge,” says Schaeffel, “such a lens has not been described before in vertebrates.”
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