the emission line and exclude constant components (such as contamination from the NLR), we measure the relevant line widths in the [*rms*]{} spectrum constructed from all the set B spectra in subset 1. The mean spectrum is constructed by averaging all $N (= 18)$ set B spectra in subset 1, i.e. $$\bar{F}(\lambda) = \frac{1}{N}
\sum^{N}_{i=1} F_{i}(\lambda),$$ where $F_{i}(\lambda)$ is the flux density (in ) at wavelength $\lambda$ in the $i$th spectrum. The rms spectrum is similarly constructed as $$\sigma(\lambda) = \left[ \left( \frac{1}{N-1} \right)
\sum^{N}_{i=1} 
\left( F_{i}(\lambda) - \bar{F}(\lambda)  \right)^{2}
\right]^{1/2}.$$ The mean and rms spectra are shown as the top two panels in Fig. 5. The widths of the  and 686 emission lines (full-width at half maximum, ) are given in Table 4, as Doppler widths in the rest frame of NGC 4051. A virial mass is then computed as described by Wandel, Peterson, & Malkan (1999). On the basis of the  variations, a mass of $1.1^{+0.8}_{-0.5} \times 10^6\,\Msun$ is inferred; unfortunately however, the extremely large uncertainty in the 686 lag renders the virial mass obtained from it not very enlightening, but it is consistent with the  result.

It is also important to keep in mind that because of 