Sir, Dr William Reville need not have bothered us with that complicated and perplexing explanation (January 29th) as to why the sky gets dark at night. An elegant solution was already available from de Selby, theologist and physist he later featured in some books by the late Flann O'Brien who concluded that night is "an insanitary condition of the atmosphere due to accretions of black air". Yours, etc Mapas Road, Dalkey, Co Dublin.