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.