  Disc spectra from highly luminous black hole binaries are observed to be
rather simple, despite theoretical predictions to the contrary. We collate the
disc-dominated spectra from multiple observations of 10 separate sources, and
show that these overwhelmingly follow a multi-temperature disc blackbody
spectrum with luminosity proportional to T^4, and that such discs are stable.
These results are in conflict with standard Shakura-Sunyaev alpha-discs
predictions, including proposed modifications such as additional energy loss in
a jet, wind or corona. Accretion disc spectra can be useful, reliable guides to
the disc structure, and should be used to test the next generation of accretion
disc models, in which the disc viscosity is calculated self-consistently from
the magnetically generated turbulent dynamo.
