Suzon, according to the recollections of Manet's friends: a young woman who worked at the Folies-Bergère, one of the great Parisian cafés-concerts , a kind of beer hall with music, circus acts and other entertainment. Distinguishing features: Suzon stands alone in a crowded room. The look on her face is detached, melancholy, distracted from her job serving at the bar in the vast crowded room reflected in the glass behind her. There is a locket around her neck that is a token of another life, a love a long way from this job. This is an unusual portrait because it is of someone at work, and someone who to our eyes is defined by her work and is profoundly unhappy with it. She is alienated from her surroundings, as if there is a glass pane between her and everyone else in the room - the drinkers, chatters-up, lovers, liars, thieves and businessmen. Manet conveys Suzon's estrangement from her world by the fact that she is the only person in this painting who is not reflected in glass. Everyone else in the painting is seen in the big bar mirror: the quickly painted, harshly reflected faces and bodies, a woman in gloves with her lover or client, someone else looking at the scene with binoculars. They are objects she is looking at - but at one remove, through a glass darkly. The only solid realities are the marble bar top and the bottles - crème de menthe, champagne, beer - a bowl of oranges, two flowers delicately placed in a vase. She has both hands firmly on the bar as if she needs to touch something solid, in case she should be carried away by the vortex of light and shapes reflected in the mirror. This is not a realistic painting of the Folies-Bergère. Suzon did work there, but she posed for the painting in Manet's studio, behind a table laden with bottles. He merged this image with rapid painted sketches he made at the Folies-Bergère. There is no attempt to make the image cohere: there is, as contemporary critics pointed out, an inconsistency to the relationship between the reflections in the mirror and the real things. The man in the top hat approaching Suzon in a sinister way in the top right hand corner of the mirror would in reality have to be standing with his back to us in front of the bar, and Suzon herself should be reflected in an entirely different place. The dislocation of Suzon's world is deliberate. Paris is a hall of mirrors where Suzon floats helplessly, clinging to her bar. The flowers are a touching attempt to preserve a little humanity, as are her neat blue clothes and whole demeanour. It's amazing that contemporary critics saw her as a prostitute. And who are you? The top-hatted stranger, of course, the Jack the Ripper whose ghostly reflection approaches her with such menace in the mirror. Manet captures the coolness, cruelty and glamour of modern life. This is one of the keystones of modern art. Inspirations and influences: A Bar at the Folies-Bergère is a modern version of Velazquez's Las Meninas (1656-7), the most profound meditation on the portrait. In Las Meninas, ostensibly a picture of the royal Infanta and her retinue of children, pets and dwarf, Velazquez includes the king and queen reflected in a mirror at the back of this palace apartment. He himself stands painting them on a vast canvas looking at us, and we are the royal eye, looking back at the world that exists for our regal gaze. Manet worshipped Velazquez, and transferred this aesthetic of reflection to modern times, to create a world that only exists in mirrors; this turns the viewer into a spectral, disturbing presence, part of the crowd that Suzon looks at with such disillusion. The 20th-century painter whose portraits owe most to Manet was the flatly ironic Andy Warhol. Where is it? Courtauld Institute Gallery, London WC2 (020-7873 2526).