  We have used the LONGSP spectrometer on the 1.5-m TIRGO telescope to obtain
long slit spectra in the J, H, and K wavelength bands towards two positions
along the Orion bar. These data have been supplemented with images made using
the ARNICA camera mounted on TIRGO as well as with an ESO NTT observation
carried out by Dr A. Moorwood. We detect a variety of transitions of hydrogen,
helium, OI, FeII, FeIII, and H_2 . From our molecular hydrogen data, we
conclude that densities are moderate (3-6 10^4 cm^-3) in the layer responsible
for the molecular hydrogen emission and give no evidence for the presence of
dense neutral clumps. We also find that the molecular hydrogen bar is likely to
be tilted by ~10 degrees relative to the line of sight. We discuss the relative
merits of several models of the structure of the bar and conclude that it may
be split into two structures separated by 0.2-0.3 parsec along the line of
sight. It also seems likely to us that in both structures, density increases
along a line perpendicular to the ionization front which penetrates into the
neutral gas.
  We have used the 1.317um OI line to estimate the FUV radiation field incident
at the ionization front and find values of 1-3x10^4 greater than the average
interstellar field. From [FeII] line measurements, we conclude that the
electron density in the ionized layer associated with the ionization front is
of order 10^4 cm^-3. Finally, our analysis of the helium and hydrogen
recombination lines implies essential coincidence of the helium and hydrogen
Stromgren spheres.
