FeH (Cushing et al. 2003). These FeH lines strengthen slightly from M6 to M9 (Figures 1 and 2), become much broader in the L dwarfs (Figures 3 and 4) and then vanish completely in the T dwarfs (Figures 5 and 6) to leave, once again, a remarkably smooth continuum between the K I lines.

Comparing orders 62 and 63 in Figures 1-6, the known FeH features in these spectral bands strengthen from M6 to M9, broaden markedly at L0 and remain strong and broad through L5 before becoming weaker in the T0 and T4.5. As with the atomic lines, the individual FeH lines seem to broaden significantly at the transition from spectral types M9 to L0. Evidence of weak FeH absorption is still present around 1.222 $\mu$m at spectral type T0, and possibly even at T4.5, as shown in Figures 5 and 6, but this molecular species is clearly not dominant in T dwarfs.

For the M6 dwarf (Figure 1), order 64 is characterized by a pair of strong lines of Fe I at 1.18861 and 1.18873 $\mu$m that are easily resolved, and another Fe I line at 1.19763 $\mu$m that is blended with FeH. These features remain strong at 