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I give analytic expressions for the relative number, luminosity, and massdensity of disc galaxies as a function of surface brightness. These surfacebrightness distributions are asymmetric, with long tails to lower surfacebrightnesses. This asymmetry induces systematic errors in most determinationsof the galaxy luminosity function. Galaxies of low surface brightness exist inlarge numbers, but the additional contribution to the integrated luminositydensity is modest, probably 10 -- 30%.

The number, luminosity and mass density of spiral galaxies as a function of surface brightness