Extra-Galactic Nebulae in The Astrophysical Journal, LXIV, 64, 1926, pp. 321-369 [HUBBLE'S SEMINAL CLASSIFICATION OF GALAXIES & ESTIMATES OF THEIR DENSITIES, 3 Plates]
FIRST EDITION OF THE SEMINAL PAPER IN WHICH HUBBLE PRESENTS HIS CLASSIFICATION OF GALAXIES, ESTIMATES THEIR MEAN DENSITIES, & DERIVES FOR THE FIRST TIME THE MEAN MASS DENSITY IN GALAXIES IN THE UNIVERSE AS A WHOLE. THE PAPER IS “A MORE OR LESS COMPLETE DESCRIPTION OF GALAXIES AS EXTRAGALACTIC SYSTEMS…..... More