Anyone who has known a true country darkness, where night falls like a black curtain, can appreciate the allure of a city's illumination. Cities attract us in part because they help us defy the dark.

In cities we have not only made great leaps into enlightenment in the realms of art, science, education, religion and governance, but we have also extended time by turning on the lights. Doing so, we come alive instead of going to sleep with nightfall.Cities of myth and imagination, like Oz with its emeralds, Timbuktu with its gold-paved streets, or Machu Picchu with its treasure of silver, copper and gold, often suggest unnatural glitter. It's no surprise that that queen of cities, Paris, is the City of Light.

New York, Rio, Cairo, Hong Kong, Tokyo - we picture them best with their lights on. It is then, too, they are most seductive, and most dangerous.

Where a million lights come on together, we acquire a kind of night vision, allowing us to see differently than we do in daylight.

There is also a strangely revealing quality about cities at night. Fly over a vast, spread-out city, Houston, say, or Los Angeles, and see how the traffic lights run liquid through its arteries; how its blocks, grids, circles and buildings are outlined as though some secret skeleton, some forgotten plan, is showing itself beneath the sprawl.

Like the city itself, its inhabitants change, too, under the spell of lights. By lighting up, cities also invite us to throw off night's natural somnolence and become fully awake.

When we walk the city at night, knowing the streets are more dangerous, we feel our pulse quicken from both fear and excitement. Suddenly all our senses, like those of other nighttime creatures, are alert.

In San Francisco, for example, along a narrow street amid a deep canyon of high-rises, where the edges of buildings were lighted up like the outlines of a Christmas tree, I walked with my friend the photographer, feeling a sense of magic, a rush of excitement. But then, as a drunken stranger suddenly roused himself from a shadow on the sidewalk next to us, there was that rush of fear.

So we went on. To Chinatown where the gaudy, jumping lights at night seem to blot out the seediness and the pungent smells of the daytime streets, and promise an endless carnival.

And further, we climbed the coiled streets of Telegraph Hill, getting a different view. Looking down toward the glittering spans of the Bay Bridge, we watched the bold pyramids and towers of light, the lights of commerce, grow dim in a foreground of softer, interior lights, those in houses. Night lets us see how these images, these neighborhoods where people live and where they work, cascade into each other. And how the city would not be the city without all of them.

The anonymity of night light may give us cover; it may also give us courage. It beckons or even dares us to go somewhere or be someone new, someone older or younger than our daytime selves. It may be time to try a new dance, taste a new dish, have a new drink, take a chance.

We found all those impulses and more in the window of North Beach's famed City Lights Bookstore. The best of civilization - the wisdom of its books - is revealed inside.

But by night the window reflects another truth of the city back to itself. The neons of a tawdry strip of discos and burlesque shows beat across the glass with a kind of discordant, seductive tempo, as alive as the real jazz that floated out of the club across the street.

With night vision, we could see both faces of the city for ourselves.