Two sets of sulphur lines, differing in astrophysical behavior, were noted by Lockyer[272] at 4163, 4174, 4815, and at 4253, 4285, 4295. These lines have been attributed by the writer,[273] and by Fowler and Milne,[274] to S+ and S++ respectively. The S+ lines appear to be in pairs, and the S++ lines suggest a triplet, although one of the three lines is extremely faint in stellar spectra, and it would be expected that the once and twice ionized spectra of sulphur would display even and odd multiplicities respectively. The two series have maxima at and at , but the stellar intensities of the lines are small. An amplification of our knowledge of stellar sulphur is greatly to be desired. POTASSIUM (19) The ultimate lines[275] of potassium () are at 7664 and 7699, and have been traced in the solar spectrum, although they are very faint. They appear to be absent from the flash spectrum.[276] Russell[277] expresses the opinion that they persist, with rising temperature, as far as in the stellar sequence. CALCIUM (20) The element calcium is extensively represented in stellar spectra. The ultimate line of the neutral atom is at 4227 () and appears at . The line increases in strength in all cooler stars, in accordance with theory, and has a distinct variation with [Pg 71] absolute magnitude. The , and multiplets[278] are satisfactorily identified in the solar spectrum and can be traced with certainty in the spectra of stars cooler than . The , , , and lines appear to be present with the appropriate intensities in the sun, but are too faint to be seen with small dispersion. Thus all the classified lines of calcium which are strong in laboratory spectra have been traced in the spectra of the sun and stars. IONIZED CALCIUM The and lines of ionized calcium are seen throughout the stellar sequence, and reach a maximum within the type,[279] where their intensity is greater than that attained by any other line in any class. They vary with absolute magnitude.[280] In the sun the lines are doubly reversed, and they are probably singly reversed[281] in 61 Cygni.