In European painting, the second half of the 19th century saw great change. The painted surface got fractured and broken up as never before as Impressionism came into being in France, followed by Pointillism and, in Italy, its variant form, Divisionism. This revolution in Italian painting - which would lead to the birth of Futurism - was bound up with political change in a way that didn't happen in France, as the first major new show of work by Italian Divisionist painters (1891-1910), opening today at the National Gallery, demonstrates.

Several of the Divisionists - Angelo Morbelli, Giovanni Segantini, Giuseppe Pelizza and Emilio Longoni, for example - were political activists too, voracious readers of Marx and Engels. And the way they painted, all this strangely luminous fracturing, added an urgency to the political message of their paintings. Take Emilio Longoni's huge, turbulent canvas, The Orator of the Strike (1890-01). A young mason-turned-political orator harangues a huge crowd from a scaffolding. He leans out, fist clenched. Beyond him, the tiny, blurry, ever receding cityscape of seething strikers unfolds before our eyes. The painted surface - like the scene itself - is in a state of immense agitation.

Radical technique goes hand in hand with an urgent political message. In his urge to depict the impoverished Milanese underclass with as much fidelity as possible, Longoni occasionally used local criminals as models. In Reflections of a Hungry Man (1894), a boy stares through a restaurant window as a rich couple tuck into a meal; in real life the boy was a local thief.

Go up close to any painting here and you see how it differs from more conventional painting. The colours are kept separate, each applied beside the other, in tiny, pure, thread-like runs. The surfaces look heavily textured. The application is painstaking and meticulous. The result is an intense luminosity.

There is one aspect of this show which seems to turn away from the political: a group of Alpine scenes of an almost other-worldly tranquillity. Spring in the Alps (1897) is by Segantini. He specialised in scenes of peasant life in remote Alpine villages, energised by this same brilliant, pure light. For Segantini, the use of Divisionist technique was a way of achieving pictorial effects which incline in mood towards the pantheistic.

This show unfolds its arguments, and moves through its themes, pleasingly and judiciously, showing us how Divisionism - an art in the service of light - led ultimately to a celebration of the great artificial light of electricity. The Futurist Giacomo Balla celebrates this proof of the triumph of modernity in the final room, in a painting called Street Light.

Box office: 020-7747 2885, to Sep 7

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