Sotheby's autograph expert Roy Davids was recently offered a letter signed by the poet Robert Burns. He politely turned it down. One look convinced him it was the work of Alexander "Antique" Smith, one of the most talented 19th-century forgers. However, he has just bought some delightful and genuine letters from Lord Nelson to Lady Hamilton.

The buying and selling of autograph letters, historical documents and manuscripts is booming. In the London Sotheby's 1979-1980 auction season autographed letters and manuscripts realized fl,600,000 or about $3,760,000 (fl equals $2.35 on today's exchange). Even the reported forgery of rare items fails to check the demand. At two recent Sotheby's and Christie's autograph letter auctions, prices from f20 (George Bernard Shaw writing to a Dr. Collins about vaccination:" . . . Are all leaders of your profession simpletons?") to f650 (Jefferson asking Washington on an address to Congress) reflected up to a fourfold increase in value over two years.

Christie's 1979-80 sales of manuscripts and letters amounted to $8,340.167. The Leonardo da Vinci Leicester Codex sold in December for $5.1 million to Armand Hammer, the most spectacular manuscript sale of our times. The only known scientific manuscript of da Vinci's in private hands, the papers, an assembleage of things written at different times, dealt with da Vinci's experiments on water, the moon and geologic phenomona. The Codex belonged to the Earl of Leicester, and was sold by Lord Coke, his son.

Sarah Ward, David's counterpart at Christie's, says that the lure of a good investment partly accounts for the large sums. At a recent Christie's auction, a letter from Mozart to his wife, dated Leipzig, 1789, brought f25,000.

Ward said, "Kipling's letters sold for a mere f20 two years ago. Since then three biographies have been published, and now a good Kipling letter costs a f100 or more."

Davids said, "Letters in the person's own handwriting are obviously more valuable than dictated or typed letters, but of paramount is the content of the letter." A routine typed letter signed by Winston Churchill may be worth only f50, but one giving his views about Lloyd George's conduct of the first world war recently brought f800.

Davids, a quiet, cultivated man, has read about half a million of other people's letters since he joined Sotheby's seven years ago, making his way through antiquated language, obsolete calligraphy or impossible handwriting.

"Packages of exciting letters," Davids said, "turn up surprisingly often." A shopkeeper from Carlisle recently brought in a box of dozens of Wordsworth letters which were unpublished and which no one had known existed, as well as a number of Wordsworth poems, many different from the published versions. The owner had found them in his attic. The name Wordsworth meant nothing to him, but a friend told him of a poet by that name. The letters brought f35,000 at auction.

The world of autographs has always appealed to forgers but, as autograph collecting became fashionable in England in the late 18th and 19th centuries, forgers became more active.

Among the most successful hoaxes, was William Henry Ireland, who forged Shakespeare documents to impress his father. Ireland was finally exposed when he put on a "recently discovered" Shakespeare play that was laughed off the stage.

Others included Alexander Smith (a Scottish clerk whose counterfeits included Burns, Mary Queen of Scots,Thackeray, Stuart kings and Oliver Cromwell) and Maj. George Gordon Byron, copyist of original letters by Shelley and Byron. Claiming to be the illegitimate son of Lord Byron, the major executed letters, complete with fraudulent postmark and seals, which were bought by the poet's own publisher.

"Forgeries are a problem of everyone dealing with autograph letters," said Davids, who deals with 50,000 autograph letters and documents a year and gets an average of a forgery a week.

Love letters of the famous do not often appear on the market because their recipients usually destroy them. However, Sotheby's has sold the love letters of Field Marshal Sir John French, and of Dylan Thomas to his wife. One from Napoleon to Josephine fetched f4,100, but Napoleon is said to have left 22,000 letters, and a routine one can be picked up for f50).

A few weeks ago, a woman brought in a Bonaparte love letter that Davids dismissed on four counts: the shape of the "B" was wrong, the bowl of the "a" was closed (Bonaparte usually left it open), the "n" had a curious loop and the whole was underlined three times although Napoleon usually only underlined twice. "Until we can find a proven example of his signature which matches this one, we can't sell it," Davids said.

Yet, he said, people's handwriting can differ at various times. "Pepys had several hands as did Voltaire, Churchill, Napoleon and Nixon.

"It's not necessary to do with acquiring status -- illness or old age can account for the change -- although it's said that Nixon's signature is an example of the effect power can have. It became almost a straight line during his most powerful years."

Davids, who can cope with letters in French, Latin, German, Italian, Spanish or Russian, also has a vast knowledge of period paper, watermarks and inks.

"Not long ago we were offered what appeared to be a Charles Dickens letter. The writing was remarkably similar to Dickens' own, the letter formations were the same, so was the color and consistency of the ink. The letter turned out to be by a novelist called Charles James, whose books were even published by Dickins' publisher."

James, writing to a friend, mentioned that the printers had pointed out that his hand resembled Dickens' "like a ghost," and added: "But here comes the really curious point in connection with the resemblance -- whenever a reviewer gets to work on one of my books, 9 times out of 10 there is a reference to Dickens in his critique -- sometimes complimentary to me, sometimes very much the reverse."

"The point is," Davids said, "that Dickens used so many pseudonyms that he might have used Charles James, but, in this case, the paper bore a watermark that was not introduced until 20 years after Dickens' death."

Davids also recently dismissed two purported Renaissance letters. They were written with a steel nib, which replaced the quill only in the late 19th century.

"There are other pitfalls. The French kings Louis XIV, XV and XVI would often have their secretaries imitate their signatures, and these are very good indeed. Fortunately there are reference books containing samples of the king's own signatures so we can compare the writing."

English monarchs, with the exception of Henry VIII (who used a rubber stamp), used to sign every minor document themselves. Then, during World War I, with thousands of officers being commissioned, George V began to use a facsimile.

Most facsimile signatures are easy to spot. Generally, the ink looks dull and is all one color and thickness and the characters under a microscope broken in places because the ink hasn't been absorbed by the paper.

But the biggest hazard, Davids said, is the autopen: an automatic signing machine used by U.S. presidents since Eisenhower and which produces a signature almost indistinguishable from the real thing.

"Compared with the difficulties posed by such devices, spotting the Robert Burns forgery was easy," Davids said. "Not only was Burns' own handwriting more sprightly, but Antique Smith, greedy for money, had stained the paper with tea to simulate age and altered the ink -- strategems which, carelessly executed, eventually did him in."

On Feb. 6, in New York, Christie's plans to sell books, letters and manuscripts of Oscar Wilde, Mark Twain, Jack London, James Barry, Washington Irving, Walt Whitman and Conan Doyle. On March 26, Phillips in New York has a books and manuscript sale. CAPTION: Picture 1, 19th-century whirligig, Washington Antiques Show; Picture 2, 17th-18th century manuscript from India, Sloan's; Picture 3, Tiffany spider-web lamp, Weschler's; Picture 4, Slant-front desk, Sloan's; Picture 5, "1930" by Louis Icart, Phillips.; Picture 6, Diamond cocktail ring, Christie's; Picture 7, Dingyao-type Meiping vase, Sotheby's; Picture 8, "Man With Pickaxe": 19th-20th century bronze by M. Jamboge; from C. G. Sloan & Company.