It's about sex.

I am, of course, referring to the peculiar and heretofore obscure Victorian phenomenon known as fairy painting, the subject of a show at the Frick that is a scaled-back version of a hugely popular exhibition in London a year ago.

It's not only about sex, because it's also about drugs, some of the pictures having possibly been painted under the influence of cocaine, morphine, absinthe or chloroform, all of which were readily available in Victorian Britain. Narcotics seem to be ''the stuff,'' for example, in ''The Stuff That Dreams Are Made Of,'' a picture by John Anster Fitzgerald, who is said to have sustained his imagination with a steady diet of laudanum, a preparation of opium, and who, so as to leave no doubt in this case, included tell-tale medicine bottles in a watercolor version of the painting, which shows a peaches-and-cream Victorian heroine in flimsy white dress conjuring up goblins, satyrs and other Boschian creatures at her bedside.

Lewis Carroll once counted 165 fairies in ''The Quarrel of Oberon and Titania'' by a Scottish artist named Joseph Noel Paton, much admired by Queen Victoria. That's 165 naked women in a single painting.

Yet, we are told, the British never acknowledged the erotic content of their fairy paintings, thereby demonstrating a form of collective sexual denial that is a root of the phrase there'll always be an England. For the Victorians, it seems, the un-Disneylike images were sufficiently couched in mythological terms to allow critics and the public to see the pictures as respectable, even ennobling variants of history paintings, the highest endeavors for painters at the time.

For contemporary sensibilities, the pictures are more simply about sex and drugs, and death, too. (Fitzgerald painted a fairy funeral, for example.) No wonder the exhibition has been so popular.

The show's cachet in London had partly to do, I think, with the current vogue for surrealism. The exhibition was on view there at the same time as ''Sensation,'' the much-talked-about survey of the new British shock art, and it made a useful point of comparison: the gap is very wide between young artists like Matthew Barney, an American, or the British Chapman brothers, and a fairy painter like, say, Daniel Maclise, whose ''The Faun and the Fairies'' shows a particularly malevolent faun among dozens of frisky, frolicking nymphs. But the divide isn't infinite, and some of the fairy paintings therefore look oddly up-to-date.

Up-to-date though not precisely a revelation. There are some very good pictures in the exhibition but only one great one, ''The Fairy Feller's Master Stroke'' by Richard Dadd. The paintings, roughly spanning the 1840's through the early 20th century, are mostly amusing, and above all they are instructive as byproducts, or symptoms, of a mix of 19th-century cultural phenomena, each of which is ultimately more significant than the art.

I am talking, for example, about what Charlotte Gere describes in the exhibition catalogue: ''As modern industrial progress engulfed the English countryside,'' she writes, ''the Victorians embraced belief in fairies as a reaction to the disenchantment of the world.'' Fairies provided an escape from a new Darwinian existence into an occult and magical kingdom that existed apart from urban society. Fairy paintings show a half-seen, pixie realm in nature.

Akin to that feeling of unease and the desire for escape in the mid-19th century was a taste for the paranormal, sparked by various spirit sightings, which made table-turning and seances popular British pastimes. The new spiritualism, as it was called, cut across society to include the educated classes, the most famous example being Arthur Conan Doyle, who, besides writing the Sherlock Holmes stories, was the son of a fairy painter named Charles Doyle.

From his father, he inherited a belief in the paranormal and became so convinced of the authenticity of several fairy or spirit photographs concocted by a pair of English girls in Cottingley, West Yorkshire, in 1917 that he even wrote a book defending them, ''The Coming of the Fairies.''

Never mind that the photographs now look patently fake. Fairy painting, overlapping with photography and eventually the early stages of cinema, provided visual evidence of an unseen universe to a British public (of which Doyle was a late representative) that longed to believe what it saw. Their faith in make-believe was probably less a matter of gullibility or collective delusion than simple nostalgic wishfulness or desire, which the undeniable brutality of World War I finally brought to an end.

There was also a good deal of discussion in the middle of the 19th century about the empire's importation of (read: dilution by) foreign cultures. Fairies, although they came partly from the German Grimm Brothers, derived from native folklore. The term folklore is itself Victorian.

What we see at the Frick exemplifies a kind of oblique jingoism, which can be described as another manifestation of social insecurity and longing. Fairies derived from Scottish, Celtic, Irish and Welsh stories and legends and from Shakespeare, whose fairy plays, ''A Midsummer Night's Dream'' and ''The Tempest,'' had revivals in 19th-century English theaters where the public also saw Gilbert and Sullivan's ''Iolanthe'' and ''La Sylphide.''