Each field of study has its own world of sources, conventions, and vocabularies. Ask your subject librarian for assistance finding primary sources in your field.
Following are links to guides for primary sources in selected subjects.
Labor & the Left (Tamiment Library)
Image: Original Source: From Scientific American. (New York : Munn Co., 1845-). Source: Mid-Manhattan Picture Collection / New York City -- New York Public Library. Catalog Call #: PC NEW YC-New.