They sit there, secure and comfortable in their climate-controlled glass cases, the ancient royalty of the kingdom of letters.

Sir Isaac Newton's "Principia," Montaingne's "Essais" and Milton's "Areopagitica" - all first editions acquired by the Folger Library within the last 10 years - rest in an entourage of some 80 humbler volumes, equally old and devoted to such diverse subjects as dogs, apes and mushrooms, mining and navigation, and the natural superiority of women to men.

The new exhibit, "A Decade of Folger Acquisitions," on display until April, is whata the library's director, O.B. Hardison Jr., calls "the tip of the tip of the iceberg" - approximately 1 percent of the more than 8,000 old and rare books and manuscripts acquired by the Folger since 1968 - in addition to the new publications poured out regularly by the international Shakespeare industry.

A number of the books in the collection are virtually impossible to find anywhere else. One of those on display, for example - Samuel Bufford's "A Discourse Agains Unequal Marriage, Viz. Against Old Persons marrying with Young. Against Persons marrying without the Parents and Friends Consent. Against Persons marrying without their own Consent" (London, 1696) - exists in only one other copy. And the Great Coffee Controversy thas raged 300 years ago can still be followed at the Folger. One booklet in the collection has a women's petition against the drinking of coffee, claiming that this "enfeebling liquor" makes men less ardent lovers. On display among the recent acquisitions is 'The Mens answer to the womens petition against coffee" (London, 1674), which replies that coffee is the national drink of Turkey, and "no part of the world can boast more able or eager performers."

Hardison, whose tenure as Folger director coincides almost exactly with the decade of acquisitions, modestly disclaimed responsibility for the impressive array of new old books during the ceremony opening the exhibit Wednesday night. "Without Elizabeth Niemyer's insatiable avarice for rare books," he said, "the collection would not be what it is."

Niemyer, the Folger's acquisitions librarian, smiled enigmat ically when asked how much money she had spent on books in the last 10 years and changed the subject: "We aren't buying as many as we used to; the prices have gone up tenfold since 1968, and we have had to slow down. People used to take their old books to a book-seller, but now they're more sophisticated and put them on sale at auctions. We can't usually afford to buy books at auction.

She said that books are preferable to microfilms for scholarly work "because you can see more on the printed page than on the microfilm," and most of the people who come to the library "prefer to handle real books when it is possible. They like the feel of it, the physical presence, the civilized joy of turning the pages rather than winding film on a spool."

Across the exhibition hall from the recent acquisitions, in a glass case of its own, is one of the cornerstones of the collection: a Shakespeare First Folio, one of 79 which were in the collection when the library was founded and which remain there still.

"The closer you get to the heart of the collection, the more meaningless the word 'duplicate' becomes," said Hardison, not at all apologetic that his library has nearly one-third of the 240 First Folios known to be in existence. The Folger would acquire more if it could, but they are not easy to come by: "One went for $120,000 on the market last month."

The reason so many copies are needed, he said, is that the text differs from one copy to another. The pages were proofread during the press run, and when a reader found an error, the press would be stopped, a correction made and the printing would continue. Some pages were corrected as many as three or four times and since the pages were stacked on top of one another, the ones at the top of the pile would have a much better text than those at the bottom. But the printers didn't discard the pages at the bottom that were full of errors.

So when the Folger published a facsimile edition of the First Folio, he said, 36 different copies from the library were used to get the best version of various pages. "It's still not perfect, but it's a far better edition of the First Folio than anyone could have bought in the 17th century."

Someone suggested that the books in the collection, three or four centuries old, were in better condition than a lot of volumes published 10 years ago, and Hardison agreed. "They used rag paper then. They didn't know how to make cheap paper."

One of the scholars currently studying at the Folger, Joan Hartwig of the University of Kentucky, leaned close to a glass case examining the horses in an engraving of a royal procession.

"I'm studying horse imagery in 'Richard II' at the moment. Last year I was into Rosemary." She said. "In Shakespeare study, one thing leads to another - there's a whole background of knowledge and received ideas that was in Shakespeare's mind but is not in ours. We have to work hard to reconstruct even a part of this background - what was in a well-stocked Elizabethan mind like Shakespeare's."

Beginning around April, scholars like Hartwig will have to say goodbye - probably for about a year - temporarily to their happy hunting ground. There is now an enormous hole in back of the library, and by next spring it will be transformed into new stacks to house the library's collection.

Then the books will be moved, and the library will be closed to scholarly work (though not for exhibits or theatrical performances) for about a year while the building is renovated to conform with the present building code. The Folger has been raising funds for this work and is within $900,000 of the $3.6 million it will cost - but then will have to raise another $1 million to make more room for the growing number of scholars (and actors and scene and costume designers) who want to study.

"We've never turned away a qualified reader yet," said Hardison, "but sometimes we nearly have them sitting on one another's laps."

Despite the Library's impressive resources, Hardison admits that there are still books it has not been able to acquire. He has a mental list that he can run through without consulting notes:

"We don't have a first edition of Boyle's "Skeptical Chemist.' We had an opportunity to buy a first edition of Copernicus' "De Revolutionibus" with the handwritten annotations of his teacher, Rheticus, but we couldn't afford it; prices on Copernicus went sky-high when they held the Copernican Year.

"We would like a copy of Hooke's 'Microcosmographic,' which contains the first descriptions of observations through a microscope - lice magnified to the size of elephants. We need a Tyndall Bible, and we don't have the revised 1552 and 1559 editions of the 'Book of Common Prayer.' We have the first edition, 1549, but the changes made in the later editions are very significant."