On the Hammersmith and City line of the London underground, between Baker and Liverpool Streets, I watched a young woman who was so evenly black and white that she was neither black nor white. She was quite beautiful, but her features kept shifting from black to white so readily and so regularly that looking at her was almost disconcerting, albeit fascinating. She reminded me of one of those foreground-background images associated with Gestalt psychology. I have not yet experienced such an unsettling combination of the two sets of racial features, though. I did not even suspect such an experience possible.