We present new H \alpha and H \beta images of the HH 1/2 system , and we find that the H \alpha /H \beta ratio has high values in ridges along the leading edges of the HH 1 bow shock and of the brighter condensations of HH 2 .
These ridges have H \alpha /H \beta = 4 \to 6 , which is consistent with collisional excitation from the n = 1 to the n = 3 and 4 levels of hydrogen in a gas of temperatures T = 1.5 \to 10 \times 10 ^ { 4 } K. This is therefore the first direct proof that the collisional excitation/ionization region of hydrogen right behind Herbig-Haro shock fronts is detected .