In a dimly lit section of the mail at Tysons Corner, antique dealer Howard Smith lifted a piece of clear plastic to reveal a manuscript Bible from 1410. He hadn't brought to the antique dealers show his 12th century, hand-letered Bible, or his "Book of Hours," Published in Paris in 1500 and illustrated with woodcuts on every page. But on the shelves could be found a 1714 collection of Shakespeare and eight original volumes of The Spectator, Addison and Steele's two-year newspaper venture critiquing the lifestles of 18th century England.

"Nothing shows the power of the press like The Spectator. It started the wheel forward from Stuart libertarianism to the Victorian Age," Smith said as he rested his hand on the leather-bound volumes.

He read in Latin from Bartholomeus' thoughts on natural history, published during Columbus' lifetime. He explained the importance of Flavius Josephus' "Antiquities of the Jews," while holding the first Greek edition, published in 1544. "Josephus wrote during the first century A.D., and he is the first historian to actually say that Christ existed."

"A battered 1544 copy of Josephus or a worn first edition of Adam Smith's 'Wealth of Nations' means more to me than any beautifully bound, veilumcovered book," Smith said. For Howard and Mabel Smith the importance of rare books is calculated in terms of the ideas expressed, "the breakthrough of man's mental evolution," not the beauty of the encasings.

Smith began seriously collecting rare, technical books in 1932 when he was a young chemical engineer. He gradually expanded into natural history, philosophy, theology and literature, seeking the "earlier works of man's thinking" and confining his library to books published before 1880.

At first the collection was restricted to rare books in English, but eventually it grew to include Latin, French, German and Spanish - not to mention the early English works in Anglo-Saxon. In the years before they began selling their collection, the Smiths read all of their rare books, savoring the "record of man's mental evolution." Smith retired from his position as a consulting engineer in 1957 and the couple then spent a number of years living in Europe, buying and selling books informally.

While browsing in a bookshop in London, Smith made his one, accidental "find" in rare books. He noticed several large leather folics supporting the furniture in the shop, the remainder scattered over the floor. After a quick glance he asked the owners if they would sell the entire set of 24 volumes. COnsidering themselves lucky to fetch five pounds apiece for the tomes, they immediately agreed. What Smith acquired was a history of Italy, embellished with magnificient engravings, worth more than $5,000.

Living in Tennessee, Florida, and now Manassas, the Smiths have been regularly collecting and selling books at antique shows all over the country for the past seven years. Although they specialize in books from the 15th to 18th century, they maintain a selection of works on Americana, particularly Virginia history.

"The South is still fascinated with the Civil War," according to Smith, so a large number of his history books deal with that era.

Their greatest joy in selling rare books is "the enthusiasm of young collectors," the Smiths said.

"There is less of sense of loss, more a feeling of continuity when we sell our books to young collectors," Mabel Smith said.

The decline of rare books in the open market has become a saddening reality. In Smith's opinion, more books are being locked away in institutions, out of the reach of ordinary collectors. Coupled with this diminishing market, the enormous increase in the price of rare books is an obstacle to potential young collectors. Within the last decade, Smith has seen increases up to 20 times the original cost at auction sales.

But at the various antique fairs they frequent, the Smiths still encounter young collectors who will purchase rare books, particularly in specialized fields such as law and medicine, and repay the cost in installments over a year or two.

"The contagious enthusiasm of the 25 to 25-years-olds makes it all worthwhile," Mrs. Smith added.