 Much of his later work was closely linked to the Mount Wilson Observatory, which he visited for the first time in 1920-21, and where he spent the summers from 1922 to 1949 as a volunteer.There he took numerous photographs of galaxies and nebulae, explored the extent and filaments of the Crab Nebula, and discovered three variable stars in the Triangulum Nebula.His textbook on astronomy was republished several times, from the first edition in 1926 through the fifth edition in 1955, and an abridged 1947 edition.