An old man with streaming white hair crouches inside a blazing disc, reaching down into the darkness with an enormous compass, as if measuring out the universe.

William Blake — poet, painter and printmaker — made this as the frontispiece to his book Europe a Prophecy in 1794. The figure is usually identified as Urizen, a character from Blake's own mythology who represents reason, law and restriction. For Blake, that was not entirely a compliment: he distrusted any system that tried to measure everything.

Blake printed his books himself, using a technique of his own invention, and coloured each copy by hand, so every one is different. According to an early account, he was colouring a copy of this image in the last days of his life, in 1827, and said it was the best he had ever done.