Figure: Arp's (1965) plot quantifying the effects sky brightness has on limiting the range of galaxies which we can view. The plot shows absolute magnitude vs. log(diameter) for numerous galaxies. The line on the left is for objects with 1" apparent diameter at apparent magnitude 18.0. The line on the right represents a surface brightness of 25.5 mag arcsec . 3C273 and 3C48 are the galaxies Arp found which prompted the diagram.