Reddening may be determined by finding pairs of lines which are far apart in wavelength but have line ratios fixed by atomic physics. The best-known set is the ratio of [S II] lines in a quartet at λ 10287-10370 to a pair at λ 4069,4076, discussed by Miller 1968 (ApJLett 154, L57) and Allen 1979 (MNRAS 186, 1P). More (weaker) lines that may be so used are listed by Draine and Bahcall 1981 (ApJ 250, 579). Hydrogen or helium recombination lines are also frequently used to measure reddening; if the temperature is known (and a one-zone model is not too bad) they may be measured over an enormous wavelength range, but are frequently dominated by the tiny BLR or have highly velocity-composite profiles. Ionization parameter . The ionization balance of a species emitting collisionally-excited lines is set by the local ionizing-radiation field (since the electrons are freed by recombination). However, the intensity of the lines (below N cr is set by N e , abundance of the emitting species and
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