Graph showing six emission line ratios as a function of Hα/[O I] 6300 for several regions of CRL 618 and HH objects (from the compilation of Raga, Böhm & Cantó 1996). Filled circles correspond to features “a1” and “b1” (faint lobe) and crosses correspond to “b2”, “c2” and “d2”. The points corresponding to HH objects are shown with open symbols: the high excitation spectra are shown as open squares, the intermediate excitation spectra are the open circles and the low excitation spectra are the open triangles.