Théo van Rysselberghe, 'Coastal Scene', about 1892

This cool, elegant painting explores the quality of light on water – how it moves, how it reflects and, towards the base of the picture, how it glows. The clouds are hardly reflected in the water: it seems as if the light has slipped under them to illuminate the vast, still stretch of almost tideless sea. The spits of land that reach out to pierce the water are also still, unpeopled and deserted. Their outlines are so stark that they appear like stage scenery, until subtle changes in the depths of the blue within the silhouettes slowly reveal banks and hollows that give them form and mass. Close to us, the water also begins to reveal depth, but here it is less certain than on the land. The six poles might be visible down to where they are buried in the seabed, or they might simply be reflected on the surface.

As a young man, Théo van Rysselberghe was influenced by the French Impressionists and by his friend James Abbott McNeill Whistler. He was at the centre of the influential Brussels society Les Vingt, a group of 20 artists, organising their avant-garde exhibitions and showing his own work. For a short period in the 1880s, together with the critic Octave Maus and the poet Emile Verhaeren (also directors of the society), they made the Belgian capital the vanguard of innovation and ideas on poetry and painting in Europe.

Widely travelled and in touch with many artists across the continent, van Rysselberghe arrived in Paris in 1886 to visit the eighth and final Impressionist Exhibition. There he saw the work of Georges Seurat for the first time. The monumental figures and subtle but arresting colours of his Sunday Afternoon on the Ile de la Grande Jatte (now in the Art Institute of Chicago) astounded van Rysselberghe. He immediately realised the importance of Seurat’s new and startling technique known as peinture optique or pointillism – using myriad tiny dots of paint in complementary colours. He persuaded Seurat to show his works, including La Grande Jatte, at a Les Vingt exhibition. Seurat’s closest follower, Paul Signac, also contributed work to these exhibitions. He became a close friend of van Rysselberghe and remained so for some years after Seurat’s death, exchanging information and ideas on pointillism and pointillist painters. Van Rysselberghe began to experiment with the pointillist technique himself, becoming the leader of the Belgian pointillists, the most important group outside France to take up the innovations of Seurat and Signac.

The light shimmers and dances across the water in Coastal Scene, almost making it appear like a glittering sheet of ice. Van Rysselberghe achieved this by placing pure white dots among the many shades of blue. He varied and augmented the blues in intensity and depth by adding complementary colours among them: orange-brown close to blue, and green with red and pink. In the sky, the dots are painted in swirling shapes, making the clouds the only moving things in the picture. By contrast, Seurat’s The Channel of Gravelines, Grand Fort-Phillippe is peopled with tiny stylised figures in the distance. The sand glows softly in the early morning light, without any brilliant white to give it the sparkle of van Rysselberghe’s picture – Seurat has used softer yellows, creams, gold and ochre with blue. But like van Rysselberghe, he has made the wide expanse of a single element into a living image full of atmosphere and emotion.

In Coastal Scene, the overall impression of blue and yet more blue and the absence of trees or buildings make it appear a cold, distant northern seascape, but it’s thought to have been painted when van Rysselberghe was in the south of France, on the shores of the Mediterranean. He travelled there with Signac and painted several deceptively simple landscapes, possibly including this undated picture. For 60 years it lay unrecognised in an English country house. Rediscovered in 2000, unlined and unvarnished but in remarkably good condition, it was purchased by the National Gallery, the only example of van Rysselberghe’s work in the collection.