  X-ray irradiation in X-ray binaries is thought to control the behavior at the
outer disc, which is observable mainly at optical wavelengths. It is generally
parameterized phenomenologically, but it can also be predicted from theoretical
models of irradiated discs and their coronae/winds. We test these models using
five multi-wavelength Hubble Space Telescope (HST) and quasi simultaneous Rossi
X-Ray Timing Explorer (RXTE) datasets from the black-hole binary XTE
J1859$+$226. These show how the reprocessed optical emission changes during
outburst as the source fades from the very high/intermediate state at
$\sim$0.4$L_{\rm Edd}$ down through the high/soft state towards the transition
to the hard state at $\sim$0.02$L_{\rm Edd}$. The models are able to reproduce
the small change in reprocessing efficiency as the source flux decreases by a
factor of two, and the spectrum softens from the very high/intermediate state
to the bright high/soft state. However, the low luminosity high/soft state as
well as the transition spectrum show more complex behaviour that is not well
described by current models. We suggest the disc geometry has changed
drastically during the outburst, probably due to tidal forces, and that the
disc is no longer in steady state at the late stage of the outburst.
