  A crucial difficulty in understanding the nature of the putative accretion
disk in AGNs is that some of its key intrinsic spectral signatures cannot be
observed directly. The strong emissions from the broad-line region (BLR) and
the obscuring torus, which are generally yet to be spatially resolved,
essentially 'bury' such signatures. Here we argue that we can actually isolate
the disk emission spectrum by using optical and near-infrared polarization of
quasars and uncover the important spectral signatures. In these quasars, the
polarization is considered to originate from electron scattering interior to
the BLR, so that the polarized flux shows the disk spectrum with all the
emissions from the BLR and torus eliminated. The polarized flux observations
have now revealed a Balmer edge feature in absorption and a blue near-infrared
spectral shape consistent with a specific and robust theoretical prediction.
These results critically verify the long-standing picture of an optically thick
and locally heated disk in AGNs.
