Hubble and Shapley - Two Early Giants of Observational Cosmology
  Observational cosmology of the first decades of the Twentieth Century was dominated by two giants: Edwin Hubble and Harlow Shapley. Hubble's major contributions were to the study and classification of individual galaxies with large telescopes, whereas Shapley is best remembered for his work on groups and clusters of galaxies using telescopes of more modest aperture. 
Sidney van den Bergh
physics.hist-ph astro-ph.CO