Domestic cats evolved to do much of their hunting at night. Nowadays, that may mean locating the bowl of cat chow in a dark kitchen (and your cat could as easily do that by smell). But in a power failure, while you are still groping for candles, your cat might be strolling through the living room--without crashing into the coffee table.

In your eyes or your cat's, the pupil reacts to changing light by changing size. The pupil gets bigger to let more light in, tinier in bright sunlight. Behind the pupil, a rubbery membrane called the lens focuses the light as it passes through. Continuing on through the eye's inner chamber, the light strikes a screen called the retina. The retina's nerve cells, called rods and cones, send signals to the brain through the optic nerve, and the brain registers an image.

The cat eye difference? Cats have a special layer of cells at the back of their retinas, called the tapetum lucidum (Latin for "bright carpet"). This shiny layer of cells, acting like a mirror, reflects light back to the retina's cells.

So in near darkness, a cat's eyes collect what light there is and give the retina a second chance to absorb every photon. And domestic cats aren't the only ones with this light-enhancing device. Big cats like tigers and lions, woodland deer, ocean-dwelling whales and even your family dog all come equipped with the "bright carpet" feature.

Cats can't see in absolute darkness, however. Shut up in a windowless, pitch-black room, a cat finds its cautious way by sniffing everything around it and listening carefully. Most importantly, a cat makes use of its two dozen or so long whiskers to get a feel for the room, as they brush against unseen objects in the dark.

Because a cat's eyes are well-suited to dimness, you might guess that in bright sunlight, a cat might find it difficult to see--like a person emerging into sunlight, eyes dilated from an eye exam.

When our human eyes are behaving normally, the pupils react to bright light by shrinking down to two tiny holes. Then, if we also begin to close our eyelids against the glare, we soon cut off all light from entering the shrunk-down pupils.

But cat-eye pupils are vertical slits, which simply get narrower in bright light. The neat trick: Cats can lower or raise their eyelids to hide more or less of the slit, just like a window shade. This gives a cat more precise control than nearly any other animal over the amount of light entering his eyes.

Scientists estimate that cats can see clearly in one-sixth the amount of light we humans would need. How would a scene that is dark to you appear to your cat? To find out, visit the website www.lam.mus.ca.us/cats/C10a.