Until mass photographic reproduction of sex and nudity rendered it 'relatively harmless', erotic art titillated a small but devoted cache of curious thrill-seekers. Marèse Murphy peeps at a display of erotica on exhibit in Vienna

'There is nothing either good or bad, but thinking makes it so . . ." As usual, Hamlet got it right, and this sound principle may come in useful when viewing an exhibition of erotica at Fischer von Erlach's glorious baroque library in Vienna.

Spanning 2,000 years, it is called The Forbidden Glance, but eyes accustomed to the outrages so frequently perpetrated on contemporary screens and stages will encounter little to offend them. In an age when liberty is permitted in every arena of public spectacle, "what the butler saw" offers a relatively harmless diversion. It is demonstrated to delightful comic effect at the exhibition by an engaging study of three lascivious clerics watching eagerly as women emerge in various states of undress from the pages of a book that is clearly destined for the Index of Prohibited Books

In fact, much of the curiosa of earlier times reveals a charming naïvety, often humorous and sometimes lewd. Nowhere is the latter more explicit than in an item from Nora Joyce's library, featuring James Joyce and Sigmund Freud inspecting the upturned hind-quarters of a woman who can only be Molly Bloom in full soliloquy. The impact is strong and vulgar, though the salaciousness inherent in the design and fine drawing from Dr Leicht's "sexual" book goes deeper; eyebrows may well rise at the sight of a curvaceous reclining nude flinging up a leg to allow a sinister stick-figure to peer pruriently at her private parts. This is the "forbidden glance" with a vengeance, but none the less artful for that.

Prying is a major attribute of erotica, and is exercised tirelessly, through keyhole, window, treetop and bed curtain, in a set of 15th-century drawings for the Decameron. However, Boccaccio is better served by full-colour illustrations of the wanton nun, Isabetta, in bed with her lover. Veil and coif remain demurely in place, but bare breasts visible above the coverlet indicate that the remainder of the nunly habit has been shed. Pointing the finger of reproach, Isabetta's sisters in religion observe the goings-on from the wings; and in the next frame, the sinner stands meekly before them, fully clothed again, though sporting a pair of underpants on her head. She is obviously in line for severe reprimand, and most likely rigorous chastisement as well.

In chronological terms, the erotica collection starts with the ancient Egyptians. Unfortunately, the fragments of papyrus have suffered badly with time and only the keenest and best-informed eyes may discern their content. More immediately accessible in this section of the exhibition is a vivid and virile design for Europa and the Bull. It is worked in wool, and still retains bold, vigorous lines and vibrant, earthy colour.

With the passage of centuries, art and ambition become more complex and, in the Orient, find expression in a passion for elaborate decoration. It imparts a fairy-tale prettiness to an enchanting series of pictures, the most beguiling of a Prince Murad Bakhsch getting to work in his harem. The chosen female looks decidedly startled, even apprehensive, as His Highness moves in, while in the background two seductive attendants prepare the princely bed. Other concubines make music, or simply languish within convenient reach of a repast laid out on a sumptuous carpet featuring a multiplicity of flowers and tiny, scampering animals.

During the 17th and early 18th centuries, the proliferation of artistic techniques achieves an impressive level of sophistication and detail with masterly engravings from Philipp van Gust and Henrick Glotzius of Antwerp. The majority deal with classical subjects, in particular the metamorphoses of Ovid, and there is exceptional charm in a study of Venus and Adonis lounging amorously in a sylvan setting, while Eros sits beside them encouraging the friendly attentions of a large dog.

The permissive society comes into its own in the Age of Enlightenment with eroticism blatant in the hands of the Marquis de Sade. By our standards, the frontispiece of the Nouvelle Justine, showing one bound body being lowered on top of another, is bondage of an order far surpassed by the hideous reality of the following decade, when the mayor of Nantes took vicious pleasure in tying aristocratic victims of the Revolution back-to-back on barges to be despatched to the bottom of the Loire. However, there is jollier entertainment elsewhere in Justine, and an amusing challenge in trying to sort out the limbs of nudes romping cheerfully in the practice of group sex.

While this may be accounted mere slap-and-tickle stuff, there is graver and seriously relevant matter in the callous debauching of young innocence described by Charles Laclos. Les Liaisons Dangereuses has to be one of the most amoral and riveting books ever written, and the prospect of the Vicomte writing to pious Madame de Tourvel as he beds Solange evokes worried speculation about what he and his like might get up to in a children's chat room today.

Music is as much grist to the erotic mill as literature and the visual arts, and it is no surprise to find Salomé and Lulu in a prominent place at the exhibition, along with Mozart's randy Giovanni and even Orlando di Lassus. It is harder, though, to reconcile the romanticism of Chopin's Préludes with the concept of a woman lying on her back, naked except for knee-stockings and elbow-length gloves, and thrusting one foot between the breasts of a nude female musician. There is a distasteful and possibly unintended element of comedy in this, much less agreeable than the robust humour of the Lied vom Fensterln where a suitor taps on his intended's bedroom casement, calling out in dialect: "Girl, are you angry or don't you know me, or is this not your window?"

With the advent of the camera, cinema erotica becomes commonplace. Despite the contributions of several important artists - Klimt and Toulouse-Lautrec among them - constant repetition and universal distribution reduce a once highly-specialised area of expression to humdrum territory, with vamps two-a-penny and male nudity as freely available as female.

Apart from the exhibition's strange neglect of the vampire culture that originated in Bram Stoker's Dracula and is not yet exhausted, the decline of erotica derived from over-familiarity is thoroughly documented by the exhibition. It ranges from early photographs and movie posters to Andy Warhol's design of tightly-zippered jeans for the Rolling Stones' Sticky Fingers album cover, and beyond to the sex magazines current on book stalls today. If, like beauty, eroticism ultimately belongs in the eye of the beholder, you simply "pays your money and takes your chances".

The Forbidden Glance exhibition continues at the Austrian National Library in Vienna until October 3lst