is normalized to unity at the same wavelength. Comparison of the spectra in these six figures, all of which have excellent signal to noise ratios (at least 20:1 per pixel), shows that the region is densely populated with numerous weak absorption features and a few stronger lines. We will show that the fine-scale spectral structure is real and repeatable, and that it is mainly attributable to FeH or H$_{2}$O. The strongest atomic features are the doublets of K I that occur in orders 61 and 65. These lines persist from M6-T4.5 but clearly change their character with spectral type. In later sections the K I lines will be singled out for closer inspection. For reference, Table 3 summarizes the main spectral transitions observed in the $J$-band over the spectral type range from M6-T4.5, including the energy levels of the atomic transitions.

As shown in Figure 1, the M6 dwarf Wolf 359 has at least one distinguishing feature in each order. Atomic lines of Al I at 1.31270 and 1.31543 $\mu$m appear in order 58. There is a moderately strong line of Mn I at 1.29033 $\mu$m in order 59, plus some weaker lines of Ti I at 1.28349 and 1.28505 