Because old photographs are often dull, old portraits often duller. "Facing the Light: Historic American Portraits "Daguerreotype" at the National POrtrait Gallery might have been a yawn. But it's a magical exhibit. It starts out as a lesson and turns into a seance.

Wizards used enchanted gems to summon shades and ghosts. Daguerreotypists worked with instruments of rosewood, mercury, and silver, fumes, and salts of gold - the alchemy is different, but the wonder is the same.

Look into these casket-frames, lined with plush, embossed with gold. The icon-likenesses inside them confront us with our dead.

Here is Henry James, aged 12; Caesar who was born a slave in 17371 Stonewall Jackson, Poe, Mathew Brady, Daniel Webster, Emerson, Thoreau. We know their words and deeds. Here, if we hold still, we meet the level glance of their patient eyes.

Daguerreotypes aren't snapshots. They are fragile, exquisite images on metal, older and more beautiful than photographs on paper. There are no two alike.And their colors are not black and white, but tones of gold ad silver, mercurial greens and blues. Move your head a little bit, and these visions seem to vanish in a flare of metal light.

Oliver Wendeil Holmes, a prudent man, saw in them a "miracle" worked by "the Lord of Light." Samuel F. B. Morse, himself no mean inventor, looked upon "the sunbeam art" as one of the most beautiful discoveries of the age."

The year was 1839. The government of France announced L.J.M. Daguerre's invention in mid-August New York saw the marvel before the year was out. "How greatly ashamed of their ignorance," wrote New Yorker Philip Hone, "the by-gone generations of mankind ought to be."

What started as a novelty soon became a business. "It is just what the people in this country like, namely, something new," wrote Bostonian Geoge Fuller in April, 1840. "I think anyone would give $7.00 for their perfect likeness.

Fuller figured he could take 50 "in one day." Others, as ambitious, had the same idea. "If our children and our children's children . . . are not in possession of portraits of their ancestors, it will be no fault of the Daguerreotypists," wrote Timothy S. Arthur less than 10 years later. "From little Bess, the baby, up to great-great-grandpa, all must now have their likenesses."

The novelty became a fad; the fad, as fads will do, soon became a bore. "It is no uncommon saying by our first men, that they wish there was no such discovery as Daguerre's" observed the "Daguerrean Journal," "for it is impossible to go to New York, Boston, or Philadelphia, without being tormented by a dozen invitations to sit for a daguerreotypes."

The process thrived for 20 years, until new technologies drove it out of fashion. The period 1840-1860 is the salami-slice of history covered by this show.

Old daguerreotypes may still be found in junk shops. Most have small market value, but prices ranging to $40,000 have been quote for daguerreotype portraits of the famous.

The portraits on display here are much more than nice old pictures. Harold Francis Pfister, the 30-year-old scholar who wrote the superb catalogue, has tied each one to history.

That schoolgirl posing primly is poet Emily Dickinson. We have no other portrait. Had this daguerreotype been lost, we would not have a face to place behind her voice.

These images cast ripples. Edwin Booth, the actor, glares into the camera. "Before I was 18 I was a drunkard, at 20 a libertine," he wrote. In his stare we seem to sense something of the fury that led his brother to shoot Lincoln. William Lloyd Garrison, the abolitionist, had fire in his voice."I will be harsh as truth," he said. "I do not wish to think, or speak, or write, with moderation . . . I am in earnest - I will not equivocate - I will not excuse - I will not retreat a single inch - AND I WILL BE HEARD." And yet his face is mild, his mouth is prim, his head bald, his spectacles thick.

Here is Stephen Douglas, who debated with Lincoln, and Sen. Charles Sumner, the man who labeled Douglas "a noisome, squat, and nameless animal." In payment for such insults, Sumner was then caned, and almost killed, by Rep. Preston Brooks on the Senate floor. When old John Brown, in Kansas, heard of the attack, he went "carzy-CRAZY," and started sharpening his sabers. We still see his anger in his pale eyes. Douglas, Sumner, Brown, their faces all are here.

Here is Thomas Cole, the painter in cloak and baroque hairdo, looking as theatrical as any of his paintings. Here is Horatio Alger, at 5-foot-2 the shortest student in the Harvard class of 1852; 14 years would pass before his homosexuality forced him from the ministry in (temporary) disgrace.

Glowing in each image is a sense of eerie slowness. It took 30 seconds, often more, to sit for a daguerreotype. We see each of these sitters gathering his calm; in each face is a blend of tension and repose.

Artists knew the sunbeam art would change the role of painting. Morse was optimistic, so was Oliver Wendell Holmes, who felt the new invention would help demean "the wandering thugs of art." Cole affected skepticism: "If you believe everthing the newspapers say (which, by-the-by, would require an enormous bump of marvellousness), you would be led to suppose the poor craft of painting was knocked in the head . . . Newspapers are great Hars."

"Now the shopkeeper who wants his portrait has only to go the photographer," observed Pierre Renoir. "So much the worse for us, but so much better for the art of painting."

"All kinds of people are not equally fortunate in being well taken by the daguerreotype," wrote Nathaniel Parker Willis in 1859. The world already was dividing into those who are, and those who are not "photogenic." Emerson, a little vain, disliked all of his pictures. He thought the process "asinizing," and described the torture: "Every muscle becoming every moment more ridig; the brows contracted . . . and the eyes fixed as they are in a fit, in madness, or in death."

The National Portrait Gallery - which will be 10 years old Oct. 5 - was, for its first eight years, told to stay away from photographs. Until Congress saw the light in 1976, that preposterous prohibition was written into law. The loan show now on view in thus the Gallery's first serious research effort in the field of photography. Marvin Sadik, its director, and William F. Stapp, its curator of photographs, and particularly Pfister, are the scholars most responsible. "Facing the Light: Historic American Portrait Daguerreotypes" brings credit to them all. It will remain on view at the Portrait Gallery, 8th and F Streets NW, through Feb. 4.