If you drop down to the Twin Bridges Marriott this weekend, you can pick up an antique stereoscope--which looks like a pair of binoculars attached to a miniature music stand--for $25 or more. Since there are no non-antique stereoscopes, this is just about a mandatory investment for the would-be collector of stereographs, the name for the twin-imaged cards you put into the back of the stereoscope.

But John Waldsmith has another way. It's free. In fact, it's called "free-viewing." He taught it to himself, and so (he insists) can you.

His instructions: Gaze into the distance, as far into the distance as possible, and then, just when your eyes are least expecting it, pop a stereograph in front of them. If you try not to focus on the stereograph at all, your eyes, unassisted by any foreign appliance, will see a single, miraculous 3-D picture. Or so, at any rate, says Waldsmith. It may not last long the first time, he concedes, but if you keep at it, "you develop the muscles in your eyes to toe out just a little bit, so that you see absolutely parallel to infinity."

Unfortunately, he adds, his own eye muscles are so well trained in this regard that "if I'm reading a magazine and there are two unrelated pictures side by side--boom!--I'm trying to make them into a stereograph."

Waldsmith spelled all this out in his hotel room yesterday, between sessions of the eighth annual convention of the National Stereoscopic Association, surrounded by the 16,000 stereographs he had brought here from Columbus, Ohio, in boxes with index-card-style dividers marked (among other things) "Caves," "Ghosts," "Children with animals" and "Bulgaria."

Ten years ago, Waldsmith was one of the founders of the association, and the first editor of its bi-monthly journal, which he was going to call "Stereo Review" until someone told him a magazine of that name already existed, for people interested in stereo sound.

"Although why anybody should be interested in anything like that, I don't know," Waldsmith said.

About 100 stereography buffs showed up for yesterday's opening ceremonies, but heavier attendance was anticipated for the rest of the weekend, when the speeches and slide shows would give way to the the opportunity to buy and sell old pictures.

The scholars in the midst of yesterday's crowd noted that stereography has been snubbed by critics, museums and historians, so many people nowadays don't even know what stereographs are. But in their heyday, "They were a gentleman's or a lady's way of becoming acquainted with the world," said William C. Darrah, retired professor of biology at Gettysburg College, and the author of the only known book on the subject, "The World of Stereographs," which he published in 1977.

During his research, Darrah said he interviewed one old woman who recalled girlhood "stereo parties" at which the guests would sit around the living room trading cards. "She mentioned that this was the closest you could get to your boyfriend unless you were out in the carriage."

According to former CIA agent Bill Duggan, who now works as a stereo photo consultant and a realtor out of the same office in McLean, the history of stereography dates back to the invention of photography. Even earlier, he said, English artists produced stereographic drawings designed to be seen through a device called the Wheatstone Viewer.

In 1860, Oliver Wendell Holmes Sr., the essayist and father of the Supreme Court Justice, invented the Holmes Stereoscope. It had an easy handgrip and a shrouded pair of lenses mounted on a bar. Refusing to apply for a patent, he presented his device as a gift to the American people, who accepted it eagerly. (Later tinkerers were less munificent. Between 1853 and 1873, 60 patents were issued for assorted variations on the theme.)

Holmes may have been a little overoptimistic about the use of his invention. "The time will come," he said, "when if a man wishes to see any object natural or artificial, he will go to the Imperial, National or City Stereographic Library."

But Charles Dickens didn't gauge the success of stereography any more accurately when he wrote: "The application of photography to the stereoscope produces an extremely pretty toy that is of no use except as an elegant and valuable illustration of a chain of scientific reasoning."

In America, the market grew so fast that the federal government tapped into it between 1864 to 1866, requiring all stereographs to bear a revenue stamp for the financial benefit of the Civil War.

After the war, "Stereoscopic Emporiums" thrived in major cities, where customers could buy "Wilson's Scottish Scenery" or "Frith's Egyptian Views." Door-to-door salesmen peddled boxed sets of 50 or 100 cards, plus a guidebook. They became so popular that Quaker Oats put inexpensive stereographs in its cereal boxes as a giveaway.

They served the functions later filled by newsreels, post cards and slide shows, according to Tex Treadwell, current president of the National Stereoscopic Association.

"You have to remember that at that time there was really very little other chance of parlor amusement," explained Treadwell, who has a collection of about 100,000 cards, many from the 1850s and '60s.

Through most of the latter 19th century, "The great majority of photographs were taken in stereo," said Treadwell, "because it gave the photographers the option of going either way--they could sell stereos or they could publish a book made up of halves."

Typically, cards cost 15 or 25 cents apiece, he said, although "fine imported English views" sold for as much as 75 cents. "Unfortunately, it was not a diversion that was available to lower class people," said Treadwell.

Darrah said the stereoscope was used in grade school when he was a student in Bethlehem, Pa., during the late teens and early '20s. The teacher would use them to illustrate industries, foreign peoples, the songbirds of the United States. One stereoscope for each row of eight students. "It was a problem maintaining order," he added.

The decline of the stereoscope began around the turn of the century, but views of World War I battlefronts brought about a revival. World War II had a more complicated effect. Public appeals for scrap paper and cardboard encouraged the destruction of huge quantities of stereographs. At the same time, aerial stereography was heavily used as an intelligence tool--to estimate the contours of invasion sites, for example.

Darrah discovered the existence of stereography in 1944. While researching a biography of the geologist John Wesley Powell, he came across some of the thousands of stereographs taken on Powell's Rocky Mountain travels in the 1870s.

He began buying cards at tag sales and estate sales. A few years ago, however, he sold his entire collection--about 100,000 views in all--to a New York gallery. These days, prices are so high that "I don't go and bid anymore," he said. "When I was first collecting, I could buy them in bulk for $2 or $2.50 a hundred."

A collector who started out now, Darrah said, would be well advised to specialize, and to stay away from subjects like the Civil War or Indians or western mining towns--"very rare material at very high prices."

Bill Duggan makes stereographs in addition to collecting them and heads the Potomac Society of Stereo Photographers. He belongs to the new, post-World War II generation of stereographers who, relying on polarized-lens technology, produce single-image slides rather than dual-image prints.

He takes his pictures with two separate 35mm cameras. That way, he can switch from wide angle to telephoto, and "I don't have to put the lenses 2 1/2 inches apart"--the normal distance between human eyes or between the lenses of a stereoscopic camera. By putting them closer together, he explained, he can dig a hole in the ground and photograph a mushroom from the perspective of a beetle "and make it look like the Empire State Building.