Black snow  

"Anaxagoras, it will be remembered, maintained that snow is black, and this I have since found to be the case." --Loss of Breath", Edgar Allan Poe

"Anaxagoras maintained that snow is black, but no one believed him. The social psychologists of the future will have a number of classes of school children on whom they will try different methods of producing an unshakable conviction that snow is black. Various results will soon be arrived at. First, that the influence of home is obstructive. Second, that not much can be done unless indoctrination begins before the age of ten. Third, that verses set to music and repeatedly intoned are very effective. Fourth, that the opinion that snow is white must be held to show a morbid taste for eccentricity. But I anticipate. It is for future scientists to make these maxims precise and discover exactly how much it costs per head to make children believe that snow is black, and how much less it would cost to make them believe it is dark gray."--The Impact of Science on Society (1952), Bertrand Russell