Valentijn, E.A. 1990, Nature 346, 153.
The photographic images of a carefully selected sample of about 16,000 galaxies have been digitized, and an extensive set of photometric parameters has been determined. These data allow a reanalysis of the effective transparency using the measured surface brightness profiles for much larger, more strictly defined samples. Several tests provide evidence that the major parts of many spiral disks are opaque and that in many cases perhaps only the outer layer of stars is observable. This invalidates most determinations of mass-to-light ratios. Various photometric properties point to an obscuring component with a larger exponential scale length than obscuring component with a larger exponential scale length than that of the stars, possibly composed of cool compact opaque clouds. This result diminishes the evidence for weak haloes around spirals, as inferred from rotation curves.