“A Bar at the Folies-Bergre” is one of those paintings that, when you see it for real, exerts a bewitching effect. It was Edouard Manet’s last major work, and is regarded by many as his masterpiece. Completed in 1882, a year before he died, it has a sombre tone considering its setting. Perhaps the mournful expression of the woman amid the gaiety of the surroundings was Manet’s commentary on the transient nature of happiness and life itself. Perhaps that is her figure reflected in the mirror on the right. But if that’s her, is the moustachioed man talking to her supposed to be the viewer? The more you try to make sense of this curious picture the more puzzling it becomes.

The curators at the Courtauld must have realised this because they have given it pride of place in the main room of the collection and installed a bench facing it, from which you can contemplate its complexities.

And the longer you look at it, the more you notice, like the subtlety of its colours, the shades of light and dark and details such as the perfectly rendered cut glass bowl of oranges, the blurred faces on the balcony and the trapeze artist’s legs in the top left corner. Or as one 10-year-old visitor described the painting: “I like it because it has so much detail.

It’s the kind of picture I could look at all day.”