This small picture is one of the most celebrated paintings in European art. This is partly because its title – used by Monet to avoid a name that might suggest it was a reliable description of a place – was adopted by critics to label the work not only of Monet himself but of his colleagues in the show: they became the ‘Impressionists’, and although it began as a derisory joke, the name stuck.

In spite of its atmospheric generalisation, the picture does indeed represent a particular place: the port at Le Havre at the mouth of the Seine on the north coast of France, the town of Monet’s birth. This is not an exotic location, rather a very familiar one seen in conditions that render it strange and intriguing. We glimpse the hulls and masts of ships, and distant factory chimneys sending their smoke to combine with the mist, all swallowed up in an early-morning haze, with the sun an occluded red disc reflected in water that shimmers in the half-light. One or two small boats are rather more clearly discernible against the luminosity of the water, but nearly everything is implied rather than explicitly stated: an impression indeed.

Because this exhibition launched, and gave a name to, a group of artists who changed the way painting was performed, it is taken as the starting-point of something utterly new – indeed of ‘Modernism’ itself.  The leading figures apart from Monet were Edgar Degas, Camille Pissarro, Auguste Renoir and Alfred Sisley. They have been celebrated ever since as originators, though of course they were really picking up and taking a stage further a movement that had been in progress for the last half-century or more. The Impressionists were based in and around Paris, as their predecessors in the School of Fontainebleau had been in the 1850s. (Fontainebleau artists, too, had been called ‘impressionists’ in their day.) Much of their practice was derived from ideas that had been demonstrated by British artists as early as the 1820s and 1830s: John Constable, David Cox and Richard Parked Bonington, who spent most of his tragically short active life in France. Some of Cox’s late oil paintings, executed in the 1850s, were, like Constable’s, known in Paris and had a measurable influence on the French.

The most innovative thing about Monet’s picture is that, spontaneous sketch as it is, the artist signed and dated it, and submitted it to a public exhibition. It was not a private meditation or preliminary study but a product of the painter’s very personal interaction with the world that he was happy to share with anyone who derived pleasure from simply looking at what he had done.  His first audiences were not used to being given such intimate glimpses of an artist’s working procedures, and were duly shocked by the informality, but Monet’s work established itself as exemplifying a new language of painting.

Because he and his colleagues were immensely prolific (Monet went on painting till his death at the age of eighty-six) that new language could be seen in the many places, notably the United States, where it was appreciated and collected. In England, the New English Art Club, established in 1885, took up the ideas of the Impressionists and their colleagues  (some of them also recognizing the contribution of Constable and his contemporaries in Britain a little earlier) and in course of time – though not without many battles on the way – the spontaneity and frankness of Impressionism, with its predominantly high-toned palette and recognizable everyday subjects, won not only the admiration but also the affection of the public, as well as of artists everywhere.  It was a sweeping triumph, the effects of which we still palpably feel a century and a half after Monet’s sun rose over the murky horizon at Le Havre.