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by Pat Murphy & Paul Doherty

Nightfall, Revisited

"Have you ever experienced Darkness, young man?" asked a psychologist in Isaac Asimov's famous story, "Nightfall."

Of course, the answer is no. These characters live on the planet of Lagash, which is illuminated by six suns. The people of that world experience the darkness of nightfall once every 2049 years. And when nightfall finally comes, the darkness and the stars ("30,000 mighty suns" in the giant cluster that surrounds the planet) drive them mad.

Now we could take "Nightfall" and get picky. We could ask, for example, why the folks never saw any of these brilliant stars until the exact moment when the eclipse hits totality. Surely some of them would be bright enough to be visible before then. We could wonder about the gravitational situation and the seasons on a planet that somehow orbits six suns.

But we won't go there. It's a great story, and there's no need to quibble with it. Instead, we're going to focus on the question that Asimov's psychologist asked: have you ever experienced Darkness?

Oh, sure---you've been in the dark. You've probably been kept in the dark at one time or another---haven't we all? But have you really experienced darkness? Have you noticed how the world changed as your eyes adjusted to dim light.

Probably not: it's easy to overlook a shift that's been happening all your life. In this column, we'll show you a few experiments that reveal how your eyes adapt to darkness and call attention to some of the peculiarities of your night vision.

The Edge of Night

In total darkness, you can't see. But the world is rarely completely dark. In a situation that most of us would call dark---on a moonless night when the only light is from the stars, for example---you can see reasonably well. But your view by night is not the same as your view by day. When the lights are dim and your eyes adapt to darkness, 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 see differently.

It takes your eyes some time to adapt to darkness. Go for a walk on a country road on a moonless night. For the first few minutes you can't see very well at all, but after five minutes in the dark, 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 will continue to adapt to the darkness. 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.

Letting in the Light

As your eyes adapt to the dark, they undergo a number of changes. You can easily observe one of these changes. All you need is a mirror and a dimly lit room with a light you can switch on.

Switch on the light and take a close look at your eyes in the mirror. In the center of each colored iris is a round black opening called the pupil, Light enters your eye through the pupil. The bigger the pupil is, the more light can enter your eye.

Now turn the light off while watching your pupils. You'll see them open wide to let in more light. Turn the light on again, and watch your pupils shrink.

The moment that you step into darkness, the muscles of the iris relax to let your pupil open wide and let in more light. At its smallest, the pupil of the human eye is just over one thousandth of a square inch in area. In a fifth of a second, the pupils can expand dramatically. If you linger in the darkness, they continue to open wider. A flash photo of a fully dark-adapted eye is truly eerie---the pupils look far too large. At its largest, the area of the pupil is up to fifty times its contracted size, letting in up to fifty times more light.

Your pupils are constantly adjusting to minor changes in lighting, expanding when a cloud covers the sun or shrinking when you walk from a shadowy hallway to a brightly lit room. If you watch the pupils of other people's eyes, you may notice that the pupils sometimes change size even when there has been no change in lighting. Under normal lighting, have a friend focus on something far away. Watch her eyes as she shifts her gaze to something nearby. Chances are, you'll see her pupils shrink. Closing down the pupil helps a person see nearby objects in better focus. When gathering light is more important than a sharp image, the pupil opens up. When there's enough light to sacrifice a little to improve focus, the pupil may close down. The older you get, the more your pupil works to compensate for your failing vision.

Psychologists say that your pupil also expands when you see something you like. We've heard that ancient Oriental jade merchants used to watch a customer's eyes when displaying items for sale. If the pupils expanded, the merchant knew that the customer really liked a particular item---and priced it high. Not knowing any ancient Oriental jade merchants, we've been unable to confirm the story, but it sounds convincing.

At Night, All Cats Are Gray

After your pupils have opened wide, your eyes continue to adapt to darkness. Deep inside your eye, certain cells become more sensitive so that they can take advantage of the dim light that's available. You can't see these changes, but you can see the results if you are patient.

To understand how your eyes adapt to darkness, you need to know why you can see at all. You see when light stimulates the light-sensitive cells in your eye. These cells are called photoreceptors---"photo" is 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 that it forms on the retina, then sends this information to your brain via the optic nerve.

Your retina contains two kinds of photoreceptors: cones and rods. Cones operate in bright light and let you see color; 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.

In the first five minutes after you step into a dark room, your eyes' sensitivity to light increases dramatically. Then the rate of change slows and levels off to a plateau. After a few minutes, sensitivity begins to increase once again. Over the next half hour or so, your eyes gradually become more and more sensitive to light.

That plateau when your eyes' sensitivity seems to have leveled off indicates a shift from one system of photoreceptors to another. During the first five minutes that you are in the dark, all the photoreceptors in your eye---both rods and cones---are gradually becoming more sensitive to light. As the cones become more sensitive, you can see dimmer colored lights. But eventually the cones reach their limit---they become as sensitive to light as they can get.

At that point, 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.

In bright light, your vision depends on the cones; in dimmer light, you use 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. But the bright pictures of 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.

If you feel like experimenting with this, find a batch of identical colored pencils, markers, or crayons. Take these and some white paper into a dark room. When your eyes have adapted to the darkness, write down, with each marker, the color that you think it is. Or draw a picture with your markers---like maybe a little house with a green lawn, a yellow sun in a blue sky, and some orange flowers in the yard.

Then turn on the lights and see how you did. Chances are, you didn't do very well. The world that you see in dim light is similar to the world of the achromat, that rare person who has no color vision at all. Knut Norby, a vision researcher who was also an achromat, wrote of his childhood experiences with crayons: " . . . I always confused the colors, breaking all the conventions and 'rules' about what were the 'correct' colors to use: I would happily color the sky light green, yellow, or pink; the grass and leaves orange or dark blue; the sun white or light blue, and so on. I was always corrected in my choice of colors by those who knew better, and, eventually, I gave up painting and coloring my drawings."

If you start paying attention to the colors you see in dim light, you may notice a strange shift. In bright light, reds and yellows often look brighter---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 during the day, they are. But on a dark night, a bright red fire engine fades to black (which is why some towns are painting fire engines bright yellow-green, a color that's bright in dim light as well as in daylight).

Back in 1825, Czechoslovakian physiologist Johannes Purkinje noticed that colors change with the 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 aren't 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, the ready rooms where pilots prepared for night missions were illuminated with red lights. Because the red light kept pilots' rods in the dark, the rods began adjusting to darkness before the lights actually went out.

The Monster in the Closet

Under red light, you can read, write, and make out the details of your surroundings. Switch off the red light and shift to rod vision alone, and the type on the page becomes illegible. In a darkened room, you have to fill in the details as best you can. The process can transform the 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. Outside the fovea, there are fewer cones and many rods.

The rods at the periphery of the retina tend to merge their signals to the brain. A single nerve fiber may carry information from as many as 600 rods. By contrast, one nerve fiber typically carries information from a single cone.

As a result, the views of the world provided by the rods and the cones are very different. The cones of your fovea give you a very detailed view of the world. Your rods provide a coarser, less detailed view than your cones, partly because they provide the 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---and the view provided by the rods as the same painting, recreated with a wide brush and shades of gray. Details that are distinguished in the first painting are lost in the second.

The distribution of rods, and cones in the retina also explains a trick that nighttime security guards 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 are trying to see. Instead, they look slightly above or below the object of interest. Try this yourself: in dim light, you'll see an object more clearly if you don't look at it directly. You can use this technique, known as "averted vision," to try to count all seven of the "seven sisters" that make up the constellation Pleiades.

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 in dim light, your cones don't function. Since the fovea lacks rods, it's virtually blind in the dark. When you look just above or below something, the images falls outside the fovea, on the periphery of the retina, where there are more rods than cones. giving you a much better view.

Slow Sight

Many of the changes in your vision that come with darkness are obvious. But the last change that we'll describe is one that you'll probably never notice under ordinary circumstances: your eyes see more slowly in dim light than they do in bright light.

The photoreceptors of your retina don't respond immediately to a flash of light. The dimmer the light, the longer the delay between the flash and the photoreceptor's response. When you put on a pair of sunglasses and dim the light that reaches your eyes, you are actually slowing your vision down by a fraction of a second. Practically, this delay doesn't affect your vision, but it can produce a dramatic optical illusion called the Pulfrich effect, named after Carl Pulfrich, the man who discovered it. We describe how you can duplicate this illusion at home below.

Beyond Nightfall

In this column, we've been talking about what happens to your vision in what most of us call darkness---that is, in very dim light. Few of us have experienced complete darkness, where there no light at all.

Paul says that the best place to experience darkness is deep in the bowels of a cave. Turn off all the lights in a cave and you are really, truly in the dark. Paul has done this on several different occasions and he reports that somehow the darkness felt "thick." He held up his hand an inch in front of his face and yet could see nothing.

The darkness of a cave, Paul says, is much more alarming than the darkness of night. Outdoors, no matter how dark the night, he knows that the sky will eventually brighten. But in the cave, he knows that light will never return naturally. Sitting in a dark cave, he says, gave him more sympathy for the characters in "Nightfall."

Pat says she doesn't need to sit in a dark cave to have sympathy for the characters in "Nightfall." As a child, she was certain that monsters lived in dark places---in caves, in the basement, under her bed. She knows now that the space below her bed is filled with dust bunnies and boxes of books. There's no room for monsters. But Pat can remember the feeling of terror quite clearly.

Fear of the dark echoes the memories of a long distant past, when only the light of a bonfire stood between our ancestors and the fierce predators that roamed in the night. Our ancient ancestors huddled by the fire; we switch on the electric lights and chase away the night.

But tonight, before you switch on the lights, take a moment to appreciate the darkness. No one likes to be kept in the dark, a condition that we equate with ignorance and confusion. But only by keeping yourself in the dark can you test the limitations of your vision and explore its abilities. We suggest you give it a try.

Note: For more about Pat Murphy's and Paul Doherty's work, check out their web sites at: and

===THE END===

Experiment #1: The Pulfrich Illusion

To create a startling optical illusion, you'll need a pair of sunglasses or a dark filter, a helper, a piece of string about three feet long, and something white that you can tie or tape to the string. (Pat usually uses a rock wrapped in typing paper.)

Use your white object and the string to make a pendulum. In a well-lit place, have your helper stand a few yards away from you and swing the pendulum back and forth, perpendicular to your line of sight.

Cover one eye with the dark filter or with one lens of the sunglasses. Keeping both eyes open, watch the pendulum. Does it still look like it's swinging back and forth, or has its path changed?

Now move the filter to the other eye. How does the path of the pendulum change?

When you covered one eye with a filter, the pendulum's path apparently changed from a back-and-forth motion to an ellipse. When you switched the filter from one eye to the other, the direction of the ellipse changed---if it was traveling clockwise, it switched to counterclockwise, and vice versa.

In dim light, your eyes respond a little bit more slowly than they do in bright light. As a result, the eye with the filter sees the pendulum where it used to be---a fraction of a second behind the position observed by the eye without the filter. Your brain combines the views of your two eyes to make a three-dimensional picture of the world. By changing the view that one eye sees, you change the picture that your brain creates.

Experiment #2: Half-Adapted Vision

To try this experiment, all you need is a weak bladder.

Before you go to bed, drink a couple of glasses of water. When you wake up in the middle of the night (as you almost certainly will), DON'T turn on the lights.

First, cover your right eye with your hand. Keep that eye firmly covered and in the dark as you turn on the bathroom light and go about your business. (Yes, we know this can be a bit of a challenge. No one said being a scientist was easy.)

When you return to your dark bedroom, notice what you can see. Now uncover your right eye and cover your left. What can you see now?

Your right eye is adapted to darkness; your left eye lost its adaptation to darkness when you flicked on the bathroom light. So your right eye can see more in your dark bedroom than your left.

Spelunkers use this trick. Before exploring a cave, they wear a patch over one eye, adapting that eye to darkness. When they enter the cave, they take off the patch.

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