‘Close your bodily eye, so that you may see your picture first with your spiritual eye,’ he wrote, ‘then bring to the light of day that which you have seen in the darkness so that it may react on others from the outside inwards.’ Friedrich began with topographical drawings in pencil and sepia wash and did not take up oil painting until 1807. One of his first works in the new medium, The Cross in the Mountains (1808, Staatliche Kunstsammlungen, Dresden), caused great controversy because it was painted as an altarpiece, and to use a landscape in this unprecedented way was considered sacrilege by some critics. His choice of subjects often broke new ground and he discovered aspects of nature so far unseen: an infinite stretch of sea or mountains; snow-covered or fog-bound plains in the strange light of sunrise, dusk, or moonlight. Even when he does not use any overt religious symbolism, his landscapes convey a sense of haunting spirituality. When he included human figures, they are often shown from behind, so that—faceless—they serve as ‘everyman’ (Wanderer above a Sea of Fog, c.1818, Kunsthalle, Hamburg). Friedrich had a severe stroke in 1835 and returned to his small sepias. Although he enjoyed considerable success and renown earlier in his career, by the time of his death he was virtually forgotten and his immediate influence was confined to members of his circle in Dresden, notably G. F. Kersting, who sometimes painted the figures in Friedrich's work. It was only at the end of the 19th century, with the rise of Symbolism, that his greatness began to be widely recognized. Most of his work is still in Germany; Winter Landscape (c.1811), acquired by the National Gallery, London, in 1988, is the first Friedrich oil to enter a British public collection.