Science fiction and fantasy is all about exploring other worlds. In fantasy, characters are always stumbling into secret worlds—they step through a mirror, walk through the back of a wardrobe, make their way to Platform 9 3/4 at London's King's Cross Station. In science fiction, space travel is usually involved.

But there's a place that most of us visit quite frequently—without being aware that we've crossed a border into a strange world. This is a world where there are no colors—just black and white and shades of gray. In this world, the normal rules of vision don't apply: You see best if you avert your eyes, looking slightly to one side of what you want to look at. You can make out general shapes, but the details are left to your imagination. A chair piled with clothes becomes a lurking monster; a tree branch becomes a reaching hand.

This strange world is just the other side of sunset—the dimly lit world of twilight and night. You may not have noticed the change in your vision when your eyes adjusted to dim light. It's easy to overlook a shift that's been happening all your life. But if you pay attention, you'll realize that the world looks very different at night.

In this column, we'll show you a few experiments that let you explore this strange world, discovering the unknown in your daily life.

You can't see in total darkness, of course. But the world is rarely completely dark. On a clear, moonless night, stars provide enough light to let you see reasonably well. But though you can see, your vision changes—dramatically and fundamentally. It's not just that you can't see as well, though that's what most of us notice. You actually see differently.

It takes your eyes some time to adapt to darkness. Go for a walk on a country road some moonless night. You'll find that for the first few minutes, you can't see very well at all. After five minutes, your eyes begin to adapt and you can see better. The sky looks lighter than the trees; you can make out vague shapes, see the path in front of you. Over the next half hour or so, your eyes continue to adapt to the darkness.

As your eyes adapt to the dark, they undergo a number of changes—some quick, and some slow. Together, these changes make your eyes much more sensitive to light. After a prolonged period in the dark, you may be able to see well enough to spot a light as dim as a candle ten miles away. When fully adapted to darkness, your eyes may be up to one hundred thousand times more sensitive to light than they are on a sunny afternoon.

You can easily observe one way in which your eyes adjust to darkness. Next time you look in a mirror, examine your eyes closely, paying particular attention to the round black spot in the center of each colored iris. That's the pupil, the opening through which light enters your eye.

The pupil of your eye responds immediately to changes in light. To observe this change, all you need is a mirror and a dimly lit room with a bright light you can switch on.

Stand in the dimly lit room and stare at your pupils in the mirror. Switch on the light, and watch your pupils shrink. Turn the light off, and watch your pupils open up to let in more light. The bigger the pupil is, the more light can enter your eye.

At its smallest, the pupil of the human eye is just over one thousandth of a square inch in area. At its largest, the area of the pupil is up to fifty times that size and lets in up to fifty times more light.

In just one-fifth of a second, your pupils can expand from their smallest size to their largest. They are constantly adjusting to minor changes in lighting—expanding when a cloud passes over the sun or shrinking when you walk from a shadowy hallway to a brightly lit room.

The expansion of your pupils is the first thing that happens as your eyes adapt to darkness, but it's not enough to explain the dramatic increase in your eyes' sensitivity over time. Other changes take place deep inside your eye to make it more sensitive to the light that the pupil lets in.

In darkness, certain cells deep inside your eyes become more sensitive to light so that they can take advantage of the dim light that's available. You can't see these changes by looking in a mirror, but there's an easy way to compare your dark-adapted vision to your vision in bright light. Just drink a big glass of water right before you go to bed. (Hey, it's good to remain hydrated!) You will, of course, wake up in the middle of the night with an urgent need to pee.

When you wake up, your first instinct will be to switch on the light. Don't do it! (At least not yet.)

Close one eye or put a hand over it. If you feel piratical, you can put a patch over that eye before you go to bed. Whatever you choose to do, protect one eye from exposure to light.

Once you've done that, you can switch on the light and go about your business. Finally, relieved and happy, switch off the light. In a dark room, uncover (or open) the eye that you protected from the light. And look around the room with both eyes open.

You have one eye that's adapted to the dark—and one that isn't. The dark adapted eye can see much more than your other eye, but you may find this a little disconcerting. (Pat always finds herself blinking, trying to get the non-dark-adapted eye to see as clearly as the dark-adapted eye.)

To understand why the dark-adapted eye can see better, you need to know why you can see at all. Light bounces off objects in the world and shines through the pupil of your eye. You see the world when this light stimulates the light-sensitive cells in your eye.

These cells are called photoreceptors—pboto being the Greek word for "light." The photoreceptors of your eye are part of the retina, a layer of cells at the back of your eyeball. The photoreceptors detect light and the patterns it forms on the retina, then send this information to your brain via the optic nerve. And your brain uses that information to create an image of the world.

Your retina contains two kinds of photoreceptors: cones and rods. Cones operate in daylight and let you see colors; rods are more sensitive to dim light than cones, but can't distinguish color. Each of your eyes contains about 6.5 million cones and 125 million rods.

During the first five minutes that you are in the dark, all the photoreceptors in your eye—both rods and cones—gradually become more sensitive to light. As the cones become more sensitive, you can see dimmer colored lights.

After about five minutes, the cones are as sensitive as they are going to get. That's when the rods take over. The rods are much more sensitive to light than the cones. And the longer you keep them in the dark, the more sensitive they become. After a little more than five minutes in the dark, your eyes switch from relying mainly on the cones to relying on the rods.

When you are depending on the rods for your view of the world, your color vision goes away. The rods register the pattern of light on the retina—they can distinguish bright light from dim light—but they don't distinguish one color from another.

Next time you're in a darkened movie theater, take a look at your clothing. To your dark-adjusted eyes, a yellow shirt may appear to be pale gray, red will look black, and other colors will appear as various shades of gray. Your rods can't see color.

But here's a surprise. In a dark movie theater, the bright pictures on the movie screen and the glowing EXIT signs will still appear in color. Even when you are seeing the world with your rods, your cones are there. If there's a light that's bright enough to stimulate them, you'll see that bright light in color, even though you see the rest of your surroundings in shades of gray.

The limitations of your rods explain why all cats are gray at night. To your eyes' rods, that orange tabby is just as colorless as a gray one. (Unless, of course, you're watching the orange tabby on a brightly lit screen—in which case we suggest you step away from the cat videos and read a good book.)

Here's another experiment to try. Cut out some squares of colored paper. Use a variety of shades—light blue, dark blue, yellow, green, red, white. Put all the squares in an envelope.

Take your envelope of colored squares into a dark room and give your eyes some time to adjust to the dark. (We suggest listening to a podcast or audio book. Stay away from the cat videos if you want your eyes to adjust fully.)

Then take a look at your colored squares. You might want to write the color on each square. Note which squares look brightest.

You'll find sorting the colors difficult, and you may notice a strange shift. In bright light, reds and yellows often look brighter and more intense than blues. But in dim light, blues often look brighter than reds. Fire engines are painted red so that they'll be bright and easy to see. And they are—during the day. But on a dark night, a bright red fire engine fades to black (which is why some towns paint fire engines bright yellow-green, a color that's bright in dim light as well as in daylight).

Back in 1825, Czech physiologist Johann Evangelist Purkinje noticed that colors changed with shifts in light. He observed that two painted posts, one red and one blue, were equally bright when he saw them at noon. At dawn, however, the blue post looked brighter than the red one.

Dubbed the Purkinje shift, this subtle perceptual change happens when your eyes shift from relying primarily on cones to relying primarily on rods. Rods only detect whether a light is bright or dim. But they are not equally sensitive to all colors of light. Viewing a red light and a blue light of equal brightness, the rods will see the blue light as brighter. They'll barely detect the red light at all.

Night pilots and astronomers have turned the rods' low sensitivity to red light to their advantage. During World War II, pilots preparing for night missions wore red goggles in the ready room and in the cockpit when it was illuminated. Because the red light kept the rods in the dark, the rods began adjusting to darkness before the lights actually went out. In astronomical observatories, illumination is provided by red light so that the astronomers' night vision will not be disrupted.

In a darkened room, you have to fill in the details as best you can. The process can transform the shadowy clothes in the closet into the monsters of childhood nightmares. Blame those night monsters on an overactive imagination—and, of course, the rods in your retina.

Rods and cones are not evenly distributed across your retina. Near the center of the retina there's a small region called the fovea, where the cones are packed tightly together and there are no rods at all. Each cone is connected to a nerve fiber that carries its signal to your brain. As a result, the fovea gives you a very detailed view of the world.

Outside the fovea, there are fewer cones and many rods. Unlike the cones, the rods at the periphery of the retina tend to merge their signals. A single nerve fiber may carry information from as many as six hundred rods. Your rods provide a coarser, less detailed view than your cones, partly because they provide your brain with less information about the light they detect. You can think of the view provided by the cones as a pointillist painting created with a fine brush and bright colors. The view provided by the rods is the same painting, recreated with a wide brush and shades of gray. Details that are distinguished in the first view are lost in the second.

The distribution of rods and cones in the retina explains a trick that night watchmen and astronomers use. To spot an intruder in a dark warehouse or a dim star in the night sky, they never stare directly at what they're trying to see. Instead, they look slightly above or below the object of interest. Try this trick yourself when you are trying to see something in dim light.

When you stare at something, you are focusing its image on the fovea. In daylight, that's great: The densely packed cones in the fovea give you a very detailed, colored view of the world. But when your cones aren't functioning, staring directly at something you want to see is a rookie mistake. Since the fovea lacks rods, it's virtually blind in the dark. When you look just above or below something, the image falls outside the fovea, on the periphery of the retina, where there are more rods than cones, giving you a much better view.

Human eyes are built for daylight—for sunshine and rainbows and cloudy afternoons. We are diurnal animals—generally awake and active during daylight hours but resting at night. We grow uneasy when the light begins to fade and the visual sense that we trust above all others begins to change.

The limitations of your night vision reflect the history of our kind. The fear of the dark that plagues many a small child echoes the memories of the distant past, when only the light of a bonfire stood between ancestral humans and fierce predators. Our ancient ancestors huddled by the fire; these days, most of us just switch on the lights and chase away the night.

Our language reflects our visual prejudices: No one likes to be kept in the dark, a condition that we equate with ignorance and confusion. But darkness offers its own sort of wisdom, its own opportunities to explore and learn. Only by keeping yourself in the dark can you test the limitations of your vision—and explore its abilities. Don't chase back the night; explore it. After a visit to the night world, you may find yourself dazzled by the sun, blinking in the daylight, and eagerly awaiting the return of the darkness, as fear of darkness gives way to fascination for a different world.
 

Paul Doherty works at The Exploratorium, San Francisco's museum of science, art, and human perception—where science and science fiction meet. For more on Paul's work and his latest adventures, visit www.exo.net/~pauld. Paul's latest book is And Then You're Dead, the science behind the most interesting ways to die.

Pat Murphy is a science educator, a science fiction writer, and occasionally a troublemaker. She works at Mystery Science, developing hands-on lessons for elementary school. You can learn more about what she's up to at www.brazenhussies.net/Murphy.
 

We are saddened to report that, as this issue was being compiled in August, long-time F&SF contributor Paul Doherty passed away from complications due to cancer. Pat Murphy writes in remembrance: "As Senior Scientist at the Exploratorium for thirty-one years, Paul shared his enthusiasm for and deep understanding of natural phenomena with thousands of people. An amazing educator, he was given the Faraday Science Communicator award by the National Science Teachers Association and, in 1996, was chosen as the Best Science Demonstrator at the World Congress of Museums in Helsinki. He appeared on Late Night with David Letterman more than once, demonstrating strange science experiments, and shared his knowledge in numerous books and articles for children and adults. He didn't limit his teaching to science—I once accompanied him on a rock climbing expedition to Pinnacles National Monument where he taught me to deal with a challenging overhang. (This involved terror and swearing on my part, but all worked out well.) Most recently, he ventured into science fiction writing, collaborating with me on a story titled 'Cold Comfort.' It was a joy to collaborate with him. His enthusiasm for all things was wonderful and contagious."