Finally, the nebulae themselves appear to be of a definite order of absolute luminosity, exhibiting a range of four or five magnitudes about an average value M (visual) = - 15.2.1 The application of this statistical average to individual cases can rarely be used to advantage, but where considerable numbers are involved, and especially in the various clusters
of nebulae, mean apparent luminosities of the nebulae themselves offer reliable estimates of the mean distances.