2.1. Humason's Spectral Classification System
For galaxies having absorption-line spectra, Humason estimated spectral types by comparison with standard spectrograms of M 32 (NGC 221), for which Adams, Joy, and Humason had determined a type of dG3; the dwarf designation resulted from the relative intensities of luminosity-sensitive pairs of lines. It was noted, however, that the hydrogen lines indicated an earlier type, while CaI 4227 and the G band indicated a later.
The range in spectral type for the galaxies observed by Humason is from A5 to G8 in his most recent discussion. 1 He found a loose but definite correlation between these types and Hubble's nebular classifications. On the average, the earliest spectral types are associated with Sc and SBc systems; at the other extreme, the latest spectral types are associated, in general, with elliptical systems. But the spectrum-form relationship shows a large scatter; in particular, the Sc or SBc systems show a range in Humason's spectral types from A5 to G5 - that is, over almost the whole spectral range found for all kinds of galaxies observed.
It should be emphasized that these determinations of spectral type generally represent data obtained as by-products of the principal program for which the plates were taken: the determination of redshifts. In addition the best spectrograms obtainable in earlier years do not compare favorably with those taken at the present time.