  We analyze the photometric variability of 4,611 active galactic nuclei (AGNs)
from the Sloan Digital Sky Survey Stripe 82. We recover the spectral energy
distribution (SED) of the variable flux as a function of wavelength. For rest
wavelengths longer than $\sim \lambda2200$ we find that the SED of the variable
component of the bluest AGNs is consistent with the $F_{\nu} \propto
\nu^{+1/3}$ spectrum predicted for an externally-illuminated accretion disc. We
confirm there is some residual variable emission corresponding to the "small
blue bump" and other broad-line region variability. We interpret steeper
optical spectra of the variable component as being due to intrinsic reddening.
This is supported by the correlation of the Balmer decrement with the colour
excess of the variable component. We find the median internal reddening of SDSS
AGNs in Stripe 82 with $z < 0.4$ to be $E(B-V) \thickapprox 0.10$ in agreement
with the reddening derived from the Balmer decrement. Individual AGNs in the
sample can have $E(B-V) > 0.4$
