Saint-Tropez, the Green Ray

That green streak slicing down through the pink sky is not a decorative flourish. Paul Signac was chasing the "green ray," a real optical event in which the last sliver of the setting sun flashes emerald for a second or two as it drops below the horizon. Jules Verne had written a whole novel about it in 1882, complete with the legend that anyone who catches the flash can read their own heart and the hearts of others. Signac, a passionate sailor who had spent years watching the sun go down over water, painted his version around 1895 from the shore at Saint-Tropez, with a boat's bare mast rising against the sky and its sail hanging loose in the windless evening.

The technique is Neo-Impressionism, sometimes called pointillism or divisionism, built up from thousands of separate dabs of unmixed color. Signac had adopted it alongside Georges Seurat in the 1880s and kept refining it long after Seurat's early death, gradually loosening the tiny dots into the broader, mosaic-like touches you see here. The water is a good place to see it work: the pinks, oranges and blues sit side by side rather than blended, so the surface seems to shimmer as your eye does the mixing. Signac had sailed into Saint-Tropez in 1892 and bought a house there, and his enthusiasm for the fishing village drew other painters south, including a young Henri Matisse, who arrived in 1904 and left with his colors permanently changed.