Series spectra of three-valence-electron atoms of P, S and Cl.—By the methods of the preceding paper, 21 lines of PIII have been identified, 20 lines of SIV and 10 lines of ClV, including a pp′ group for each; also 10 term values of PIII have been determined and 10 term values of SIV, including the x level. Screening constants for one, two, and three valence electron systems are collected in a table for Na to Cl. The values increase regularly with the number of valence electrons and decrease with the atomic number, the difference decreasing as the atomic number increases.