Claude Monet is said to have called him "the master of light" and in 1909 he was one of the most celebrated painters in the world. Today, outside his own country, most people have never heard his name... He was Joaquín Sorolla, and that year he arrived in New York for his first American exhibition. The city fell for him completely: of the 356 paintings on show, 195 sold. He stayed five months and painted the President of the United States. Then he went home to the sea he grew up beside, and that summer, on the beach at Valencia, he painted this. He first called it The White Horse. A boy leads the animal out of the water, both still streaming wet, the sun running along the boy's bare shoulder and the horse's flank, then breaking across the shallows until the sand shines like glass. The canvas is over two meters wide, and up close the brushstrokes look fast, loose, almost careless. Step back, and they turn into some of the most convincing sunlight ever put into paint. Eleven years later, working in his garden, he had a stroke and never painted again. He died in 1923. By then the art world had moved on to Modernism, and beauty like this had started to look old-fashioned. Within a generation, the master of light had slipped into the shadows...