attempt has been made to correct for contamination of the line measurement by the [*narrow-line*]{} component of , which is of course expected to be constant.

We have also measured the flux in the 686 line. Only set B spectra are suitable for this measurement, since set A spectra do not extend shortward far enough to provide a suitable short-wavelength continuum point. The  flux was measured by adopting a linear underlying continuum between $\sim4447$Åand $\sim4775$Å, and integrating the flux above this continuum between 4613Å and 4772Å.

We then compare the independent light curves from the two sets of data to identify small systematic flux differences between the sets, as we have done in many previous experiments (see Peterson et al. 1999 and references therein). We attribute these small relative flux offsets to aperture effects, although the procedure we use also corrects for other unidentified systematic differences between data sets. We define a point-source correction factor $\varphi$ by the equation $$\label{eq:defphi}
F(\Hbeta)_{\rm true} = \varphi F(\Hbeta)_{\rm observed}.$$ This factor accounts for the fact that different apertures result in different amounts of light loss for the point-spread function (which describes the surface-brightness distribution of both the broad lines and the AGN continuum 