Time is a medium of photography, the way canvas is a medium of painting. A photograph isn't just paper and chemicals, it's also time, the instant and visible sense of something that happened in a moment Back Then--not a vision of the lost past as much as a vision of a lost and antique present.

That lost present is almost as antique as photography itself in a new show at the National Portrait Gallery called "A Durable Memento: Portraits by Augustus Washington, African American Daguerreotypist."

There are 33 of them--most of them 3 1/4 by 2 3/4 inches--in the brass frames and velvet cases that Washington sold along with his portraits in Hartford, Conn., and in Liberia, where he immigrated in late 1853.

The Hartford pictures look like biopsies of slightly awkward moments in the lives of the middle and upper classes.

Does this specimen quality derive from the occasional spots of tarnish on the silver coating of the plates that Louis Daguerre had invented in France only a few years before? Or from the rigid, seated poses that make couples look a little silly, like old folks riding a roller coaster with their grandchildren? From Victorian pomposity? From the fact that after the Virginian aristocracy faded from public life, few Americans had public faces that were at ease with fame and authority?

Here is the distinguished Eliphalet Adams Bulkeley in a circa-1853 portrait, looking like a man listening to a magazine salesman at his door. His wife, Lydia, has a preoccupied wariness about her. Son Charles may well have spent an hour practicing his look in front of a mirror. Daughter Sarah is most graceful of all, with her hair pulled straight and shiny as the silk on Christmas tree balls, and ribbons on each wrist. Talk about moments in time: She later sailed with her husband, a clipper ship captain, to China, where the ship was dismasted and fetched up on an island of pirates who killed them both and scuttled the ship, or so newspapers speculated at the time.

Perhaps the awkwardness of these and so many other daguerreotypes comes from the fact that people didn't know how to pose for the camera. They hadn't trained their faces to bounce light toward lenses with preferred shadows and angles. Now, we start our training as babies. Smile. Smile for Daddy. Just a li'l tiny smile.

Unlike painters, daguerreotypists couldn't create the proper pose on the canvas when the subject failed to provide it. Daguerreotypists couldn't light their subjects' way to nobility or ease, because they had to use windows or skylights for illumination--no bounce flashes or fills. And no darkroom manipulation. Daguerreotypes had no negatives. The plates they were shot on were the plates that were delivered to customers. And the exposures took up to 15 seconds--a long time to hold still any face but a tensed one.

Of course, a pose is just another attempt to defeat time, to express some eternal virtue, moral or aesthetic.

Washington knew this. The exhibition has at its center one of the small jewels of the Portrait Gallery's collection--the earliest known photograph of abolitionist John Brown, circa 1846 or '47.

In his left hand, Brown appears to be holding what may be the banner of his "Subterranean Pass Way," his plan for a more violent version of the Underground Railroad. He holds his right hand at shoulder height in a pose of vow and defiance. In fact, it was his left hand he held up. Daguerreotypes were mirror images: Right and left were reversed. A moment's study reveals the manipulation--his vest buttons appear on the left side. But the public pose and rhetorical gesturing took precedence over literal reality.

Or did the posing problem come from differences in white and black culture?

The portraits from Liberia show a group of Western-clad expatriates from America--wise, alert, thoughtful, admonitory, eager, confident. . . . Perhaps by the late 1850s, when these pictures were made, people had seen enough photographic portraits that they had taught themselves how to look in front of a camera. Perhaps these immigrant Liberians were confident enough in their authority, virtue and beauty that they could face down Washington's camera with ease, and forestall time with their certainties and nobilities. They succeed--these pictures are much less pieces of time than the Bulkeley portraits. (It was this sort of success that made popular what we used to call "candid" photography, which tried to defeat the poses that defeated time.)

Washington, the son of a former-slave father and a mother who was a "native of South Asia," was born in Trenton, N.J., in 1820 or 1821. He attended largely white schools until the early 1830s, when the rising outcry of abolitionists such as William Lloyd Garrison provoked a backlash that closed many schools to blacks. He fought his way through an education in various schools in New York state.

In 1841, Washington used his connections in the abolitionist community to get into Dartmouth College in Hanover, N.H. He didn't have the money to stay there. After a search for teaching jobs failed, he learned the new Daguerrean art, as it was known. He went back to Dartmouth, but found he couldn't both study and do portraits.

In 1844, he arrived in Hartford, where abolitionism had strong supporters, and began making portraits. His advertisements promised low prices, "distinctness, softness of light and shade," and so on. His pictures cost as little as 50 cents apiece.

Like other free blacks, he worried not only that white laws would enslave them, but also that African Americans would never be accepted by European Americans. He considered resettlement in Canada, the West Indies or South America before resigning himself to Liberia, which had recently become an independent republic.

He took his camera with him, and made portraits along the West African coast.

He also became a prosperous farmer and a government official, and established a family whose importance lasted until recent political upheavals. He died in 1875, in Liberia. There is no known portrait of him.

The show runs through Jan. 2.