Graph showing wavelengths of the diffuse series of sodium plotted against N^-2 (inverse square) making assumptions of different starting point of n. Blue diamond starts with n=2, red square starts with n=3, green triangle starts with n=4, violet X starts with n=5. Only with starting n of 3 is a straight line achieved. Based on idea from "A Procedure to Obtain the Effective Nuclear Charge from the Atomic Spectrum of Sodium" by O. Sala , Koiti Araki and L. K. Noda, in Journal of Chemical Education 76 (9), p 1269 Sept 1999, doi=10.1021/ed076p1269