@other{https://thesis.library.caltech.edu/id/eprint/1581,
    authors = "Abell, George Ogden",
    title = "The Distribution of Rich Clusters of Galaxies",
    year = "1957",
    url = "https://resolver.caltech.edu/CaltechETD:etd-05022007-092134",
    id = "https://thesis.library.caltech.edu/id/eprint/1581",
    doi = "10.7907/99X5-ZM38"
}


@other{https://thesis.library.caltech.edu/id/eprint/2493,
    authors = "Mathis, John Samuel",
    title = "The Abundance of Helium Relative to Hydrogen in the Orion Nebula",
    year = "1956",
    url = "https://resolver.caltech.edu/CaltechETD:etd-06072004-163340",
    id = "https://thesis.library.caltech.edu/id/eprint/2493",
    doi = "10.7907/CB4D-4026"
}