The Third Spectrum of Copper (Cu iii).

An analysis of Cu iii based on observations from 500 to 6900 Å is presented. The low structures 3d ⁹ and 3d ⁸4s are complete, including the rarely, if ever before, found 3d ⁸(¹S)4s ²S. The 3d ⁷4s ² includes ⁴F, ²F, ²G, and ²H but the ⁴P, ²P, a ²D, b ²D have eluded all attempts to find them. The ionization potential calculated from 4s, 5s, 6s ⁴ ${}^4{\text{F}}_{4^{1/2}}$ is 296 980 cm_-1 but by a comparison with Ni II which has a longer series an approximate value of 297 140 can be estimated. The 3d ⁸4d group is complete, except for one level, as is 5dbased on ³F and ¹G, the other 5d groups being incomplete. 3d ⁸(³F)4f is complete and 26 levels based on ¹D, ³P, ¹G are known. 3d ⁸(³F)5g is incomplete and a few levels based on ¹D and ¹G have been found. A discussion of the validity of the analysis of Cu iv by J. F. Schröder and Th. A. M. Van Kleef is given.