An extraordinary black-and-white photograph of a young black boy taken on the Isle of Wight by Julia Margaret Cameron in 1868 shows him in exotic clothes and a heavy silver-bead necklace, like a chain-of-office or a prisoner’s collar. He looks so sad, reminding me of the caged lions in London Zoo, his eyes heavy-laden, his listless body lacking the restless energy you would expect of a seven-year-old.