An improved δ-ray method for determining the nature of heavy nudei is obtaincl img application of a seini-empirical formula giing;; the δ-ray density per 100 μ of the track in function of the remaining range R; N=C ZpR-b with p=1.54. b=0.46 and C depending on the nature of the emulsion and the conditions of measurement.Discussions arc given on the "thin-down effect" and the relation between the ihin-down length and the atomic charge.