Dependence of galaxy properties on viewing angle.

It is shown here that most previous attempts to derive the inclination dependence of luminosity and diameter for spiral galaxies could have been afflicted by severe sample selection effects. In the absence of distance information, analysis of a diameter-selected sample will reach one conclusion, and the same kind of analysis of a luminosity-selected sample will reach the opposite conclusion. It is shown that, when distances are used, well-selected samples of spirals can yield reliable inclination corrections. When distance-related tests are applied to diameter-limited samples of the Uppsala General Catalogue of Galaxies, it is found that apparent diameter is independent of inclination for the spirals. This implies that the luminosities of spirals are inclination-dependent, becoming much fainter as they are viewed more edge-on.