Whistler is contrasting. His 'Nocturnes' are a couple of ugly paintings! There are some paintings that aren't really pretty or beautiful, but that are still great works (the best example for me would be Goya's 'Saturn Devouring His Son'), but Whistler's 'Nocturnes' are just ugly for me. However, his 'Peacock Room' is REALLY impressive, almost perfect, precise details. It seems that we're actually standing in there!
