  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.
