a) Absorption spectrum for the C iii* 1175 Å multiplet. The 2013 COS spectrum shows clear and relatively unblended individual troughs from the J = 2 and J = 0 levels, but no contribution from the J = 1 level that is populated at higher densities (see panel c)). b) Absorption spectrum for the Si iii* 1298 Å multiplet (similar in level structure to the C iii* 1175 Å multiplet). The 2013 COS spectrum shows shallow but highly significant individual troughs from the J = 2 and J = 0 levels, but again no contribution from the J = 1 level that is populated at higher densities. c) C iii* and Si iii* level population ratios, theory and measurements. The computed populations for the J = 2/J = 0 and J = 1/J = 0 are plotted as a function of electron number density for both ions (see text for elaboration). The crosses show the measured ratios for the J = 2/J = 0 ratio of both ions. From the C iii* ratios we infer log (n_e) = 4.8 ± 0.1 cm^-3, where the error includes both statistical and systematic effects. This value is fully consistent with the one inferred from the Si iii* ratio, where in the case of Si iii* the statistical error is larger since the troughs are much shallower.