Life takes many forms in late Degas - so many forms that you often have the sense that even the painter himself cannot quite believe the riches he has stumbled upon, so late on - but the life in question is almost invariably female. Dancer with Bouquet, from the Chrysler Museum of Art in Norfolk, Virginia, is one of the exhibition's curtain-raisers. In the painting, the curtain has just fallen on the ballerina's performance and she is enjoying her applause. Done sometime in the 1890s, when Degas entered his sixties, it is one of his subtlest essays on the touching awkwardness of human beings.
The dancer, seen from the side, is in mid-curtsy. Her face has been slightly smeared by the artist as he painted it, especially about the lips and above the right eye - perhaps because a certain type of striving for verisimilitude had come to bore Degas; perhaps because he wanted to convey movement. Her expression is set in the type of puzzling, inadvertent rictus that hastily taken photographs often catch. Her dress blends in with the painted theatrical backdrop seen blurrily behind her - some Arcadian scene of woods, mountains and lakes at sunset. She has her back to the illusion of which she has just been part and two bouquets of roses the colour of strawberries lie before her.
It is a picture that goes to the centre of Degas's preoccupations. He was an artist famously obsessed by the ballet. Yet he was fascinated by all of its aspects except the performance itself. He painted the rehearsal and the preparations for performance; he painted dancers in the wings, waiting to go on and participate in the performance; he painted the aftermath of performance. The dancer, once the dance is over, ceases to be a sprite or nymph and becomes again a woman. Degas enjoys the transition immensely. He cherishes her return from the ethereal world of ballet back to the mundane, ordinary world, where people are unpoised, where their limbs stick out at odd angles and curious expressions, quite unplanned, cross their faces.
Degas went to the ballet and hung around ballet-dancers not because he loved their art but because he was looking for the places where, and the moment when, the art would stop, or fall away. That is why he painted them so often at their exercises, stretching, bending and otherwise toughening themselves for the nightly acts of gravity-defiance required of them.
Degas was fascinated by the way people are when they think no one is watching them. But he was under no illusions about the degree of artfulness required of the painter who, like him, sought to get this edgy vitality into his work. In this sense he was not only the observer, eternally apart, of his legend. He was also like the dancers whom he painted: someone who had to rehearse, almost endlessly, to get things right. The exhibition is particularly revealing of just how arduous his quest for off-the-cuff effects could be.
"A picture is something that requires as much knavery, trickery and deceit as the perpetration of a crime." That was Degas's slightly sinister way of pointing out that artlessness is always an artful illusion. Playing variations on a theme was an essential method, because he knew that the effect of instantaneity required planning, and much trial and error. We see sketch after sketch of the same woman washing herself. The contour flickers and changes from each drawing to the next, and you sense Degas feeling that the sequence might actually be a totality, as a film. The story goes that Degas once entertained Walter Richard Sickert with the flickering shadows that were cast on a wall when he placed a candle behind his small maquettes of dancers. Here again, Degas was playing with the possibility of an image that might change, minutely but constantly, as living creatures do. He had many ways of chasing vitality into his art.
Looking and thinking, painting and playing around, making drawings and making sculpture and lighting a candle to his own work in the darkened studio - all things become equal to Degas. Age can make good artists pompous, or it can give them the necessary arrogance to realise that they no longer need the rules that they have spent their lives constrained by. In Degas's case, the latter was true. In his old age, he broke with convention so constantly that profound originality became a habit.
Degas spent much of his later years in the semi-dark of his studio. He did not like to go out, partly because bright light hurt his eyes. Yet late Degas does not fit the mould of depressive recluse into which some have tried to confine him. The art in this show is defiant and joyful, and filled with a youthful sense of discovery (proof positive, if any were needed, that old people have a tremendous capacity for feeling young). The colour in Degas's late art is vibrant to the point of incandescence. Wallpaper buzzes green. Curtains become hypnotic veils of mixed colours. Painting the stage scenery in the incomparable Blue Dancers, from the Musee d'Orsay, Degas created a patchwork of bright, shimmering dabs and dots that make a mockery of Seurat's laborious pointillism.
Portents are everywhere in this show. Bonnard learnt from the elderly Degas how walls or floors can become the very raison d'etre of a painting, the excuse for a transfiguring, redeeming flood of colour. Degas taught Bonnard how to bless his paintings with brightness and living texture. Matisse received lessons, too. The red room in Degas's After the Bath, Woman Drying Herself, from the Philadelphia Museum of Art, is surely the antechamber to Matisse's own The Red Studio. The colour in Degas's later pictures, so radiant, so brilliantly disconnected from literal description, is the glorious counterpart of his undeceived realism.
In our rhetorically unconventional age, Degas's genuine radicalism is difficult to appreciate, but the pastels of the nude to which he devoted so much of his old age were unprecedented in painting at the time. To Degas, educated within the great French tradition of academic, classical figurative art, the discovery that the human figure in art can be placed in any attitude under the sun was a breakthrough and a great liberation. His life had, in a sense, been dedicated to making that discovery, and in his later works he was gathering in its fruit.
The women, naked, wash themselves, sprawl, rub their arms and backs vigorously with towels. Except us, no one is watching them and they do not know that we are in the room. They are extremely tender and quiet pictures, but Degas sometimes seems to have felt that he was carrying out a form of violation when he painted them - not a violation of any one woman in particular but, rather, a violation of the generic image of Woman. "Women can never forgive me. They hate me, they feel that I am disarming them. I show them without their coquetry."
There is certainly no coquetry in the pastels, and there are hints in places that Degas regarded the human condition, as he sought nakedly to reveal it, as a fallen, imperfect, shameful state. The so-called statue and drawing of A Woman Taken Unawares combines observations with biblical overtones. The woman clutching at her genitals to hide them is Degas's vision of Masaccio's Eve, rushing tearfully from paradise covering her nakedness. Yet she is also related, surely, to Rembrandt's Susannah, spied on by the elders. Degas's greatness, like that of Rembrandt, was in the end inseparable from his humanity.
The late pastels of the nude are not lifelike, exactly, because Degas was little concerned with the academic articulation of human anatomy that besets the life class: legs are impossibly long, torsos over-extended. But the pastels are like life, which is a different and far more impressive achievement. Degas's anatomical inaccuracies are ways of emphasising those parts of the body (the nape of a neck, or the long slope of the back) that touched him at the time. They are painted with feeling rather than fear of incorrectness. The women stoop and scrub and squat, entirely absorbed by their own physicality. They are, as Degas once said, like animals. But there is no cruelty in the revelation. Looking at them, we recognise that the condition they describe is our own.
n `Degas: Beyond Impressionism', National Gallery, London WC2, until 26 August (advance booking: 0171-420 0000)