When leading spectroscopists in Europe and America were engaged, during 1897, in exploring the recently-discovered Zeeman Effect, they were overtaken by a relatively obscure phsicist working in Dublin. Thomas Preston had previously been known only for his excellent textbooks. His achievement in discovering the Anomalous Zeeman Effect was immediately recognized, but his untimely death has deprived posterity until now of a full account of his life and qualities.