  We show how wavelength-dependent time delays between continuum flux
variations of AGN can be used to test the standard black hole-accretion disc
paradigm, by measuring the temperature structure $T(R)$ of the gaseous material
surrounding the purported black hole. Reprocessing of high energy radiation in
a steady-state blackbody accretion disc with $T \propto R^{-3/4}$ incurs a
wavelength-dependent light travel time delay $\tau \propto \lambda ^{4/3}$. The
International AGN Watch multiwavelength monitoring campaign on NGC 7469 showed
optical continuum variations lagging behind those in the UV by about 1 day at
4800\AA and about 2 days at 7500\AA. These UV/optical continuum lags imply a
radial temperature structure $T \propto R^{-3/4}$, consistent with the
classical accretion disc model, and hence strongly supports the existence of a
disc in this system. We assume that the observed time delays are indeed due to
a classical accretion-disc structure, and derive a redshift independent
luminosity distance to NGC 7469. The luminosity distance allows us to estimate
a Hubble constant of $H_{0} (\cos i / 0.7)^{1/2} = 42\pm9 \Hubble$. The
interpretation of the observed time delays and spectral energy distribution in
the context of an accretion disc structure requires further validation. At the
same time, efforts to minimize the systematic uncertainties in our method to
derive a more accurate measurement of $H_{0}$, e.g by obtaining an independent
accurate determination of the disc inclination $i$ or statistical average of a
moderate sample of active galaxies, are required. However, this remains a
promising new method of determining redshift-independent distances to AGNs.
