No period drama, it seems, is complete these days without a scene in the bath. Conversations about sex, swearing, and a great deal of ooh-Matron naughtiness get included; they are not, nor likely to be, in any 19th-century author.To be fair, Thackeray seems to offer encouragement in this respect, with the famous passage in Henry Esmond in which he regrets that a 19th-century author cannot write about an 18th-century man in the way that Fielding could have done. What is more pervasively damaging is the way that the directors insist on including things that could never have occurred to the authors of the novels. The characters are not exactly of the gratin, but they are admitted to society, and thrive in it.

Talking like that, Becky Sharp would hardly have got a job as a housemaid.But this is all par for the course. It is certainly true that the BBC has not, as yet, plumbed the depths of the Hollywood adaptation. In the Forties Pride and Prejudice with Olivier, the invitation to Mr Bingley's party includes the instruction "Please bring this card with you"; I had not thought that gatecrashing was a popular pursuit in rural England in the early 1800s.And things have not much improved since. I nearly fell off my chair when Gwyneth Paltrow, in Emma, asked Mr Knightley whether he knew that "Jane Fairfax and Frank Churchill had become a couple".Sometimes it is a cherishable naivety. Martin Scorsese's quite good film of The Age of Innocence has a fabulous misinterpretation of one of Edith Wharton's snobbish asides: when she remarks that one of the characters lives in the "wastelands" above the Park, he produces a shot of a house which appears, quite literally, to be in smoking badlands.These are all rather sweet errors, which a bit more determined research, or the employment of more intelligent directors, would have put right. I know, no one knows precisely how anyone spoke at the beginning of the 19th century, but these accents will still give anyone over the age of 30 a jolt.

Becky Sharp may be a schemer on a fabulous scale, but she is certainly not a kleptomaniac, and the idea that she would bother stealing trinkets from Amelia Sedley is absurd and damaging.And, having employed someone to ensure that Natasha Little's hair was of the right period, I wonder why they did not trouble to ask anyone whether her vowels were credible; most of the cast speak in the familiar Actress Estuary which was invented about 10 years ago, with their final l's replaced by w's, and glottal stops in every sentence. But, having paid researchers to specify these heritage-industry details, it never seems to occur to anyone to get anything else right.The music is appallingly wrong; the orchestra seemed to be playing a tango by Gottschalk at Vauxhall Gardens, and the effect was as ugly and moronic as if Amelia Sedley were wearing blue eyeshadow, or as if George Osborne had lit up a joint.It is perfectly incredible that, even in a household as shambolic as Pitt Crawley's, the universal ritual of "taking in" the ladies to dinner would have been neglected. As usual, months of research have gone into the costumes, the hair, the sets and the food, and the physical details are as exact as money can make them. It is a truly awful traduction of a novel, a tale told, apparently, by an idiot. Viewers, it is thought, love adaptations of classic novels, but not all the time; and it seems utterly beyond the wit of the drama department of the BBC to guess what will strike home.It would be nice to think that this Vanity Fair will sink without trace, and the BBC turn away from its obsession with making money out of these macabre exercises. One lady, having booked her holiday to coincide with the last episode, is reputed to have telephoned the BBC begging them to tell her whether Darcy married Elizabeth Bennet in the end, or not.On the other hand, a pounds 9m adaptation of Nostromo, no less great a novel, no less thrilling a plot, sank to barely 2 million viewers by the second episode. His Pride and Prejudice was such a popular series that it seemed to reach people barely conscious that it had ever been a book at all.