Smart women have always had a problem. If you're clever and plain you can be dismissed as a bluestocking; high-minded but unable to get a man. If you're clever and sexy you're ten times worse, as the authors Mary Wollstonecraft and Catharine Macaulay found out in the 18th century. Horace Walpole abused Wollstonecraft, calling her a "hyena in petticoats", while Macaulay's unconventional love life - she had a close friendship with a clergyman almost 30 years her senior and then married a man half her age - made her an easy target for every self-appointed moralist and misogynist. No wonder these early bluestockings were intensely conscious of their own image, sending careful signals about their femininity as well as their intellectual qualifications.
The female intellectual's never-ending dilemma - what type of disguise to adopt, or whether to adopt one at all - is the subject of a new exhibition at the National Portrait Gallery in London. Entitled "Brilliant Women: 18th-Century Bluestockings" and curated by Elizabeth Eger and Lucy Peltz, it presents the leading female thinkers and writers of the day and shows how they chose to have themselves portrayed. Some played the game, pushing gently at gender boundaries, while others challenged them and were cruelly lampooned as a result.
If few of the names and faces are familiar, leaving aside the obvious stars such as Wollstonecraft, it is evidence of the neglect into which even well-known women quickly fall. The 18th-century bluestockings were at least fortunate in leaving a substantial legacy in the shape of their own writings and other people's representations, sitting for artists as celebrated as George Romney and Élisabeth Vigée LeBrun. To some, these daughters of the Enlightenment were muses, recalling the Greek goddesses who presided over the arts and providing an honourable if spurious pedigree. To a later, more conservative century, they were the harpies portrayed in Thomas Rowlandson's savage 1815 etching entitled Breaking Up of the Blue Stocking Club. Rowlandson's image of scratching, brawling women is a classic misogynist fantasy, a cat fight that supposedly exposes the harsh truth of female rivalry.
The Bluestocking Circle or "blues" became a cultural force in the middle of the 18th century. One centre was the splendid Hill Street house of Elizabeth Montagu, a great hostess who owned coal mines in the north of England and was married to a mathematician. Montagu seated her guests in a wide semi-circle to encourage general discussion, whereas her contemporary Elizabeth Vesey - known to friends as "the sylph" - turned her apartments into what sounds like an obstacle course. One celebrated guest, the novelist Fanny Burney, recalled that Vesey "pushed all the small sofas, as well as chairs, pell-mell about the apartments, so as to leave not even a zigzag of communication free from impediment".
These opulent salons attracted not just women, but also men - among them Dr Johnson, Joshua Reynolds and the actor-manager David Garrick. The term "bluestocking", which had been employed to abuse Cromwell's Puritans a century earlier, was revived in 1756 when the poet and botanist Benjamin Stillingfleet turned up at Montagu's house wearing blue worsted stockings instead of the fashionable white silk. The event is recorded in Boswell's Life of Johnson, in which the author observes that Stilling fleet's conversation was so sparkling that in his absence people declared: "We can do nothing without the blue stockings."
It is a curious origin for a word that came to be so closely associated with intellectual women, but the term's history - quickly becoming a mark of approbation, then one of abuse - is just as singular. During the conservative backlash against the French Revolution, it became associated with women's striving for sexual freedom, personified by Wollstonecraft's unconventional private life - she had a child outside marriage with an American, and then married the atheist philosopher William Godwin after becoming pregnant with his child. Only later did the label acquire connotations of sexlessness and asceticism.
The bluestockings were acceptable, in other words, as long as they clothed their intellectual accomplishments in the trappings of conventional femininity. Montagu had all the advantages of social position, great wealth and latterly, a magnificent new house in Portman Square, but she still felt it necessary to state her dislike of women who made the same intellectual claims as men. Her portraits advertise her wealth, serenity and love of fashion, especially in Allan Ramsay's 1762 painting: Montagu wears a sumptuous rose silk gown trimmed with lace and her eyes are modestly averted. Her literary fame rested on an essay about Voltaire's critique of Shakespeare; it appeared anonymously in 1769, her authorship becoming known only by accident. Montagu believed that there was "a general prejudice against female Authors especially if they invade those regions of literature which the Men are desirous to reserve to themselves", and she was reluctant to challenge it overtly.
She however encouraged other women writers - but only up to a point, reserving her scorn for genuinely radical and transgressive individuals such as the historian Catharine Macaulay. Macaulay was a formidable republican thinker whose eight-volume History of England was initially welcomed as a Whig riposte to David Hume's Tory History of Great Britain, but her unconventional private life turned public opinion against her. Painted by Robert Edge Pine around the year 1775, Macaulay opted for a classicising pose that recalled her reputation as the "female Cicero" or "fair historian"; she wears vaguely Roman clothes and her hairstyle might be that of a Roman matron, associating her with the great days of the Republic. But Macaulay was an unashamed self-publicist and could not resist being painted in a senator's sash, with her right arm resting on a pile of her own books.
Two or three years later, Macaulay's sudden marriage to the 21-year-old brother of her doctor made her a laughing stock. She was lampooned in an engraving that shows the 47-year-old historian - barely older than Montagu in her rose silk - as a vain, elderly woman painting her face while a hearse pulled by plumed horses traverses her preposterously inflated hair arrangement. Walpole held up Macaulay as an example of how "sense may be led astray by the senses", and her fellow radical John Wilkes denounced her as a "monster". Elizabeth Montagu could not resist joining the tormenters: "All this has happened from her adopting masculine opinions and masculine manners," she wrote. "I hate a woman's mind in men's cloaths . . . I always look'd upon Mrs Macaulay as rather belonging to the lads . . . than as one of the gentle sex. Indeed she was always a strange fellow."
Female solidarity counted for only so much, and Montagu was no kinder to her protégeé Ann Yearsley, a self-taught poet who, after delivering milk door-to-door, achieved fame as the Bristol Milkwoman. Montagu met Yearsley through the dour evangelical writer Hannah More and duly exclaimed over her working-class genius. A popular engraving of 1785 shows More introducing Montagu to Yearsley, who is suitably attired in a proletarian hat and shawl, but two years later she was posing for a little-known artist, Sarah Shiells, with a confident stare. The painting is known through a mezzotint by Joseph Grozer, and shows Yearsley seated at a table, quill pen in hand as a badge of her status as a writer - an exceptionally bold statement for a woman of her background. Her involvement with the bluestockings ended badly when she had the temerity to accuse More of financial fraud, after which she sensibly took over managing her own literary career, with great commercial success.
The role of class in the history of this intellectual movement cannot be overstated. The founders of the Bluestocking Circle were willing to acknowledge and even encourage working-class women, but only as long as they knew their place. In a century of radical ideas, galvanised by the French Revolution, it was inevitable that the founders would in time be eclipsed by a more radical group in which Wollstonecraft was pre-eminent. In John Opie's famous portrait, Wollstonecraft turns her head towards the viewer, as though momentarily distracted from her reading; she is fashionably though not ostentatiously dressed, in an image that seems intended to endorse the compatibility of intellect with femininity. Even so, she was attacked in a 1798 verse diatribe by Reverend Richard Polwhele, who included her in a group of bluestocking women - the authors Mary Hays, Anna Laetitia Barbauld, Ann Jebb, Helen Maria Williams, Mary Robinson and Charlotte Smith - who had supposedly abandoned "natural" feminine modesty.
The title of Polwhele's bilious production - "The Unsex'd Females" - helped to create a climate in which generations of women lived in fear of the label "bluestocking", which became an insult. In the 19th century, the Brontë sisters and Mary Ann Evans (George Eliot) felt it necessary to hide their genius under male pseudonyms; in the 20th century the word conjured up images of desiccated female dons. The original "blues" were much more various than this stereotype suggests, and their dilemmas about intellect, fashion and femininity are still with us today. "Brilliant Women" restores them to their rightful place as our foremothers, the missing link in an unbroken chain of female creativity. The struggle for the right to be clever, sexy and feminine is still going on.
"Brilliant Women: 18th-Century Bluestockings" is at the National Portrait Gallery, London WC2, until 15 June. www.npg.org.uk