Proposition XV/Theorem XII in Book Two of Newton's Principia deals with the spiral path of a body attracted by an inverse-square force toward a fixed center and retarded by the medium in which it travels. This article examines the argument offered by Newton as proof of the proposition/theorem and finds it fallacious. Also presented here are accounts of how Newton's purported proof is dealt with in each of three late-20th-century publications-none of which reports detection of the fallacy. (C) 1998 Academic Press.