Primary sources
Research focuses on a primary source of evidence. Depending on the academic discipline, a primary source is generally a text, image, artifact or data (archeological, survey, experimental). Librarians describe the different types of primary sources as "formats." Notice that the Catalog's default display highlights these formats in the "Limit your search column":
When searching for primary sources, begin by limiting your search by Format, then add other limits that target your source. For example, see here for the results of a search that limits by:
Format: Manuscript / Publication year: 1000-1500 / Language: Ancient Greek
Notice that you can also limit by Library and by Online (digitized). Many of our most unique primary sources are held in Special Collections, and many of those have been digitized.
Also, peruse Digital PUL, which is Princeton's repository of specialized digital collections, many focused on "ephemera," i.e., posters, pamphlets, post cards and other minor publications.