The series relations for the arc spectrum of cobalt[324] have been published by Walters. Cobalt lines are frequent in the solar spectrum, but as the strongest of them lie near 3500, they cannot be traced in the spectra of stars. The following multiplets are certainly identified in the spectrum of the sun: , , , , , , , , , , and . The incompletely observed multiplet is apparently absent from the solar spectrum. NICKEL (28) The series relations for nickel are as yet unpublished. The lines appear in great numbers in the solar spectrum, but they are not strong enough to be conspicuous in the spectra of the stars. The line 5476 appears to have a maximum at , indicating either that it is an enhanced line of nickel, or that it is blended with the enhanced line of some other element. The lines 5081, 4714 are strengthened in low temperature stars, and are probably due to neutral nickel. From the solar behavior of the lines of this element,[325] the ionization potential seems to be of the same order as that for cobalt, probably about 8 volts. COPPER (29) Copper is represented in the solar spectrum by the ultimate doublet 3273, 3247 (), which is strong. The pair 5700, 5782 () is probably also present. The former lines are too far in the ultra-violet to have been studied in the stars, and the latter are too faint. ZINC (30) The principal singlet is at 2138, and has therefore not been observed in stellar spectra. The lines at 4722, 4810, are seen in the stellar sequence, where they appear at , and have a maximum[326] at . [Pg 81] Two unclassified lines of ionized zinc are mentioned in Fowler’s Report as lying at 5894, 6214. Neither of these lines can be traced in solar or stellar spectra. GALLIUM (31) The occurrence of gallium in stellar spectra is confined to the identification of two solar lines by Hartley and Ramage.[327] The lines in question are at 4033, 4172, and are the ultimate lines of the element (). They are too faint to be studied in the stars. RUBIDIUM (37) The ultimate lines of rubidium have been detected in the sunspot spectrum,[328] but they are not found in the spectra of the sun or stars. STRONTIUM (38) The element strontium is of great astrophysical importance, owing to the use of its enhanced lines in the estimation of absolute magnitudes. The neutral atom is represented in the sun and stars by the ultimate line () 4607, which is first clearly seen[329] at , and increases progressively in strength with advancing type. It varies with absolute magnitude, being weakened in stars of high luminosity later than . Estimates for the intensity of this line are difficult with small dispersions, as it is blended in cool stars. IONIZED STRONTIUM