A forgotten but significant chapter in the history of art forms the subject of the major exhibition at the National Gallery next month. In their heyday - the 1890s - the Milan-based Divisionists produced work that shocked the art establishment of Italy. But why, well over a century later, should we still care about these painters - a product of their time and place if ever an art movement has been?

A haunting oil painting of three old men in a Milanese almshouse suggests the beginnings of an answer. It captures much of what the Divisionists were about. Anyone familiar with the world of care homes will recognise the isolation and inwardness of these figures, conveyed here with astonishing poignancy by the artist Angelo Morbelli.

In this canvas, warm sunlight beams on to cold walls. For old age and the other social concerns that they were so committed to depicting, the Divisionists claimed to offer hope, symbolised by light. Morbelli's epoch in the north of Italy was one of immense idealism alongside dreadful social problems. For him and those of his Divisionist ilk, to turn a painting - literally - towards the light was to hold out the promise of progress for the working men and women who people their canvases.

In the 1890s Milan was already a hive of modernity. Its newly rich bourgeoisie had taken up the work ethic with a Protestant zeal. But there were losers as well as winners. Faced by the hardships of the factory workers and peasants, such as the bent-double women who harvested the rice for Milanese risotto, some Divisionists became political radicals.

The obsession with the depiction of light was what gave Divisionism its name. The word describes a laborious technique of painting loosely related to French Pointillism. This point (as it were) is a touchy one in Italian quarters. “Never say Divisionism derived from Pointillism,” Morbelli supposedly protested. Italian painters felt keenly the loss of artistic supremacy to Paris.

Italian Divisionism was probably a concept coined by the svengali of the movement, a Hungarian baron called Vittore Grubicy de Dragon. A dealer and gallery owner in Milan, he encouraged newcomers to adopt the Divisionist technique, staging shows of their work, including one in London in 1888. An accomplished artist himself, the aristocratic Grubicy steered clear, however, of social realism.

Indeed, Divisionism and business made uncomfortable bedfellows. A Grubicy contract might entail the right to pass off a painting as the work of a better-known artist in the dealer's stable. Grubicy's most bankable signature was that of Giovanni Segantini, whose name he once substituted for that of the unknown Emilio Longoni. Disillusioned, Longoni promptly left the gallery and was soon reading Marx. Five or so years later, in 1890-91, he painted The Orator of the Strike, one of the most powerful images of its kind.

But Grubicy's Divisionists were not just a concept. They were real artists joined by friendship. Morbelli and the artist Giuseppe Pellizza corresponded sometimes twice a day. The surviving letters reveal intense discussions of artistic, not personal, matters. Hence Morbelli was deeply shocked when his friend hanged himself after his wife's death in childbirth. This was in 1907. Division by now looked outdated. Key figures had died young. The rising generation of the Italian avant-garde had discovered Futurism. The world was moving on. These paintings, however, including works of great artistry and power, remain as a legacy of a moving moment in Italy when art aspired to change the world.

Radical Light: Italy's Divisionist Painters 1891-1910, National Gallery, London WC2 (www. nationalgallery.org.uk 020-7747 2885), Jun 18-Sept 7

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