Opening our eyes to some amazing facts about vision
WHAT is your eyesight like? Pretty good? Great - but did you know that some animals have much better eyesight than we do? Eagle-eyed Birds of prey have the keenest vision of any animal. Their big eyes let in the maximum amount of light and contain two foveas, one for sideways vision and the other for seeing straight ahead. Each spot contains three times as many cones as that of human eyes! Eyes that shine Have you ever seen a cat's eyes shining in the dark? This reflection of light from an animal's eyes is called ''eyeshine'' and is an adaption to help nocturnal animals see. Behind the retina is a reflecting ''mirror'', called the tapetum. Animals with a tapetum see twice as well in the dark as those without one.
All-round vision A chameleon is the only animal which can look both backwards and forwards, or upwards and downwards, at the same time. Each of its eyes can swivel about independently in almost any direction. Only a fly that is perched on the top of a chameleon's head can possibly escape being detected by its all-seeing eyes! Seeing (ultraviolet) stars Humans have good colour vision, but they cannot see beyond the ends of the visible spectrum - infrared and ultraviolet (UV) light. Although bees, flies and other insects cannot properly see the reds and yellows we see, their eyes are extra sensitive to ultraviolet light so a white flower is a beautiful shade of blue to them! Their multi-faceted compound eyes cannot focus (they ''see'' hundreds of separate tiny images) so they use this UV vision to guide them to the nectar and pollen.
Build-up bifocals A small fish in Central America has a horizontal division across its pupils which effectively gives it four eyes, the lower pair for seeing underwater and the upper pair for seeing above - at the same time!