Niépce's correspondence with his brother Claude has preserved the fact that his first real success in using bitumen to create a permanent photograph of the image in a camera obscura came in 1824. That photograph, made on the surface of a lithographic stone, was later effaced. In 1826 or 1827 he again photographed the same scene, the view from a window in his house, on a sheet of bitumen-coated pewter. The result has survived and is now the oldest known camera photograph still in existence. The historic image had seemingly been lost early in the 20th century, but photography historian Helmut Gernsheim succeeded in tracking it down in 1952. The exposure time required to make it is usually said to have been eight or nine hours, but that is a mid-20th century assumption based largely on the fact that the sun lights the buildings on opposite sides, as if from an arc across the sky, indicating an essentially day-long exposure. A later researcher who used Niépce's notes and historically correct materials to recreate his processes found that in fact "several days" of exposure in the camera were needed to adequately capture such an image on a bitumen-coated plate.