A portrait of a lady

National Gallery of Scotland, Edinburgh ****

She was young and she was beautiful. She was newly married and very much in love. Gainsborough, the greatest painter of the day, was himself half in love with her too. So he turned the Hon Mary Graham into an icon that has preserved her image in the popular imagination to this day.

Leaning with one elbow on the base of a column against a wild romantic landscape, her glance from beneath her marvellous plumed hat is directed away from us, over our heads and to one side, giving her, in the words of one Victorian critic, an air that is both haughty and vulnerable. She invites admiration, but she does not deign to acknowledge those who admire her. Her hat is beaded with the same pearls that are wound round her bosom in a long string. The string is pinned by a huge ruby in her brooch that picks up the pink in her cheeks and lips. Her skin is pearly in tone and her white satin gown with its fantastic lace collar is the same pearly white.

Yet its silvery surface also shimmers with the reflected colours of the landscape and the stormy sky behind her. Her crimson underskirt is shot silk and flashes pink and red. The whole thing is painted with such bravura that its very surface seems alive. Gainsborough himself said of the picture that it was "the completest" he had ever painted.

Since the National Gallery of Scotland opened in 1859, Mrs Graham’s portrait has been one of its most popular pictures. But for all the universal currency of this marvellous image, few people, I imagine, pause to ask, who was the magical Mrs Graham we know so well? To answer that question, the National Gallery has put on a small but choice exhibition that tells her sad and romantic story.

Born Mary Cathcart, Graham was the daughter of the distinguished soldier Baron Cathcart. Charles Cathcart and his wife are both present here in portraits by Sir Joshua Reynolds. He wears a black silk patch on his cheek to cover the scar of a sword wound. His wife, meanwhile, sits with a baby (Mary’s elder sister, Jane) on her knee.

The Cathcarts’ family seat was near Alloa, and before Gainsborough they were patrons of the painter David Allan. A family group done by Allan before he went to Italy shows them at home with their six children. The earliest portrait of Mary herself is also by Allan, painted when she was about 16.

Allan was not Gainsborough, but in his painting the sitter is recognisably the same person that the more famous artist painted and the picture still has a great charm. Punning on the nursery rhyme, Mary is painted stringing a garland of flowers round the neck of her pet lamb.

Mary Cathcart married Thomas Graham of Balgowan, later Lord Lynedoch, in December 1774, aged just 17. It was a double wedding with her sister Jane, who married the Duke of Atholl. Mary's portrait was painted a few weeks later, before she returned to Scotland. (Gainsborough also painted her brother, William, a few years later.)

He in fact painted her not once, but three times. There is a lovely half-length picture of her that has been brought back from the US for this show. In it she is without a hat and wearing more or less contemporary dress. This more conventional portrait may have been the picture originally commissioned. It seems however that Gainsborough was so taken by her that he went on to paint the full-length version, dressing her in a Van Dyck costume and exhibiting the picture to great acclaim in 1777.

There is also a half-length portrait here of Lady Margaret Fordyce wearing much the same costume, though in a different colour. It is a little disappointing to learn that Gainsborough used the costume that Graham made so much her own for several other ladies. Nevertheless, he did not forget Mrs Graham for she appears again as a housemaid in a lovely, sketchy full-length that he painted ten years later.

The Cathcarts were tragically prone to TB, or consumption as it was known at the time. Mary's mother and father both fell victim to the disease. So did her sister Jane and her brother Charles, who died while he was travelling on an embassy to China. His funeral in Java was recorded by the artist travelling with him, Julius Caesar Ibbetson.

Mary herself was never strong. Her husband nursed her lovingly until, in 1792, braving the upheavals of the French Revolution, he took her via Paris to the Mediterranean in search of a better climate for her health. They reached the sea, but she died in June aged just 32.

Cathcart was heartbroken and wore her wedding ring until the day of his own death more than fifty years later. It is here, worn thin and bent with wear. You can see it on his finger in his magnificent full-length portrait by Thomas Lawrence that hangs next to his wife’s picture. He sought solace from her death in a military career and fought with distinction in the Peninsular War. Lawrence paints him every inch a soldier, standing sword in hand against a landscape of smoking ruin.

Thomas Graham even preserved the last dress his wife wore and it gives us a vivid impression of her slender figure. After her death, however, he could not bear to see her portrait and it languished in store for 50 years until his death in 1843. So Gainsborough’s masterpiece was forgotten until it made a sensation when it was exhibited in 1857 in Manchester.

Two years later the picture was bequeathed to the National Gallery by Robert Graham, Thomas Graham’s heir, with the condition that it should never leave Scotland. The picture’s popularity has been such that it has spawned a whole genre of kitsch, from tea trays and figurines to jigsaw puzzles. There is a selection here.

The most startling testimonial to the picture’s universal currency is a Maidenform Bra advertisement from 1956. A powerfully built lady in a longline bra stands in the familiar pose above the caption: "I dreamed I was a work of art in my Maidenform Bra."

Until 22 June