Then, around 1826, the first photograph was made. The rest, as they say, well, you get it.

For more than 150 years, photography has had an extraordinary impact on the way we live. From the start, it changed the way people saw themselves, and it launched innovations in nearly every field. The sciences, the arts, politics, history–all were transformed. Suddenly, too, the world must have seemed a much smaller place. So much of it could be seen from wherever you were. Space, geographical difference–all were up for grabs. It even created a rift in time, between the pictured world and the world before the photograph. Indeed, the invention of photography was in itself a defining moment, a time when the world shifted, tectonically–and irrevocably.

The idea of photography had been around for a while. Centuries before, people had figured out that the sun had the power to tan their skin and fade their fabrics. They also noticed that if a dark room had a little hole in a wall that let in light, an inverted image of the scene outside could be projected against the opposite wall. Scientists, artists, and dabblers dreamed of making these images permanent. By the end of the 18th century, they were getting close. Some Germans had experimented with silver salts and ammonia, with promising results. In England, Thomas Wedgwood (son of the famous potter) made some images by placing an object on treated paper and exposing it to the sun. But to preserve the images, he had to keep the papers in a darkened room–which rather defeated the purpose.

There is much debate about who invented photography; several people were working on the idea around the same time. The first fixed images were most likely produced by Nicéphore Niépce in Gras, France, in 1826 or a little earlier. But photography would not be a going concern until Niépce teamed up with Louis-Jacques-Mandé Daguerre, who gave his name to the invention that would soon be everywhere. Daguerreotypomania hit hard, especially in the United States, where people were wild for the extraordinary detail of the images. Once the daguerreotypists got going, there was a steady stream of new technology for that century's "early adapters"–and a torrent of arcane journal articles for newly minted photography geeks. Pewter plates or silver-plated copper? Iodine vapor or mercury vapor? Calotypes, ambrotypes, ferrotypes–the names are as suggestive as the pictures.

For the general public, much of the appeal of the first photographic images came from their perfection and detail. No matter how good the draftsmanship–and by the 1850s, some of it was very fine indeed–a painting is not a photograph. Writing in his diary on Dec. 4, 1839, Philip Hone described a visit to see some daguerreotypes. "Every object," he wrote, "... is a perfect transcript of the thing itself; the hair of the human head, the gravel on the roadside, the texture of a silk curtain ... are all imprinted as carefully as nature or art has created them. ..." Making a perfect copy of nature was the goal of the photographer, but also the bane; too many people weren't photogenic. A painter could leave out blemishes or minimize noses; the daguerreotypist showed people, more or less, as they really were.

This was also a delight. "Ah! What tales might those pictures tell if their mute lips had the power of speech! How romance then, would be infinitely outdone by fact," wrote Walt Whitman after a visit in 1846 to a New York gallery displaying daguerreotype portraits. In the unmediated expression of faces (even faces held frozen for the early camera's long exposure times) there was a kind of modern truth. Whether capturing science experiments or erotic boudoir shots (and yes, photographic pornography is probably nearly as old as the daguerreotype itself), photos could communicate something never before expressed.

So now, say it is 1852, and you're about to have your photograph taken for the first time. You might as well, since Main Street has been virtually taken over by competing photography studios, which have replaced the daguerreotypists. You walk down the street and stop at a studio to glance at a row of sample photos, including some of your friends and neighbors. You push open the door and ascend a narrow stair, and then another, past the dance studio, and the dressmaker's, until you get to the top floor of the building. You step into a room with the biggest glass window you've ever seen: the photographer's skylight, maybe 20 feet across. And then the odor hits: oil of lavender, ether, a faint whiff of cyanide–things you've never smelled before.

The photographer greets you and invites you to get into position. He, or sometimes she, pulls up the cast-iron brace that will keep you from moving during the exposure, then goes back into the lab to prepare the glass plate. There is a pouring of chemicals, then a slotting of plates when he returns to the room, focusing his camera on you one last time before making the exposure. You can blink, he tells you, as long as you keep your gaze fixed on a single point–if you move your eyes from side to side, you will look like a zombie in the final print, since your pupils will disappear.

A few tense minutes and it's over. The photographer pulls out the plate and takes it back into the lab. If he shows you the image that emerges on it, you will gasp: Why is your skin that funny color? What happened to your hair? Is this some sort of joke? In 1852, you won't have seen a negative before, and it is a shock. Which is why he probably won't show you the image until he has printed the positive, a few days later, depending on how the weather is, if the sun comes out.

That's how Mark Osterman describes it, and he should know. Osterman, a historian based at the George Eastman House International Museum of Photography and Film in Rochester, N.Y., is also a photographer who works in the 19th-century wet-plate collodion process. Osterman notes that there is a movement among photographers today–including Sally Mann and Chuck Close–to return to the early processes. What they no doubt learn is that 19th-century photographers had a lot of variables to worry about. Andrew Robb, senior photograph conservator at the Library of Congress, likens the process to cooking "a soufflé, or a meringue–it's very dependent on environmental factors." But it was worth it; those early photographers had the opportunity to capture many things for the first time ever.

For the first time–and forever. Part of what amazed was photography's independence from the laws of time. Alice Liddell was already a much photographed young woman by the time Julia Margaret Cameron made this portrait (opposite page) in 1872. But somehow she is still modern to us. Liddell was also the Alice of Alice in Wonderland, and a fixation of Charles Dodgson (Lewis Carroll). But in this picture she is something more: The fairy-nymph Alice is fading into (or emerging from) Nature, her hair and the leaves blending into one organic mass. Can we imagine what pain, sorrow, boredom she must have endured? Does the jut of her elbow signal attitude, or the sitter's fatigue? More than a century later, when Sally Mann began photographing her children, she showed us a wisdom on their faces that is no more or less complex–or fascinating. The context is modern, but the look is timeless.

Cameron, who came to photography relatively late in life, had enormous expectations for the technology. "My aspirations are to ennoble photography and to secure for it the ... real & Ideal and sacrificing nothing of Truth by all possible devotion to Poetry and beauty ..." Truth and beauty: a tall order. Because as much as photography has always been about the depiction of reality, it has also always been a kind of deception. A picture looks so real we call it an exact copy, but it's basically not. Photographers show us what they want to, framing the image depending on what story they want to tell. But there is something about the immediacy of a photograph that allows us to suspend our doubts and accept it as unmediated reality.

Taking a picture today? Well, you would certainly go about it very differently. Everything is photographed now, and everything has been photographed. Already, in fact, we are moving beyond the stage of believing photography, questioning whether a photograph is real–there have been too many years of picture manipulation, digital and otherwise, and we are no longer innocent observers. But somehow photography–through whatever laws of optics and chemical properties–can still transport us to places we've never been before and, perhaps, never dreamed of.