There is at least one advantage in a classification of galaxies based on their spectra: the absorption and emission features, despite their composite nature, contain significant information bearing on the stellar populations of the systems. Such a classification, however, should be regarded as complementary to the classical Hubble sequence based on form and degree of resolution. The reason is that the spectral characteristics considered here seem to correlate rather well with the appearance of spiral structure and central concentration, which are two properties of galaxies that show progressive changes in Hubble's scheme. Thus the classification proposed here will incorporate additional information that did not, in general, enter into Hubble's assignments of nebular types.
The procedure of classification was very simple: accurate estimates of spectral type (and of luminosity class, when possible) were made for a number of the brightest galaxies. Special attention was given to the wavelength region used and to the particular parts of the galaxies to which the spectra referred. The observational material consists of spectrograms obtained principally by Humason at the Mount Wilson and Palomar Observatories and by Mayall at the Lick Observatory. 1 For some objects new plates were taken with the nebular spectrograph of the 36-inch Crossley