 With colleague Frances Woodworth Wright, he published two widely used atlases of the Magellanic Clouds.He was the first to study the structure of the Local Group dwarf galaxies  and carried out the first large-scale survey of star-forming regions (HII regions) in spiral galaxies, in which he and his students mapped a total of 13011 of these objects.He and his former student, K. Krienke, discovered 652 star clusters in the Andromeda Galaxy (M31).