In a story of lost and stolen books and scrupulous detective work across continents, a Caltech historian and his former student have unearthed previously uncounted copies of Isaac Newton's groundbreaking science book Philosophiae Naturalis Principia Mathematica, known more colloquially as the Principia. The new census more than doubles the number of known copies of the famous first edition, published in 1687. The last census of this kind, published in 1953, had identified 187 copies, while the new Caltech survey finds 386 copies. Up to 200 additional copies, according to the study authors, likely still exist undocumented in public and private collections.