Cornell University Library has a rich history of science collection - both print and online. These include both published books and journals as well as the personal correspondence of many scientists and engineers. Much of this material is classified as 'primary source' or 'archival' material: information recorded for the very first time by the author, even written by hand. Most of CUL's history of science collections are administered by the Division of Rare and Manuscript Collections, housed in the Kroch Library.
Below are links to some of CUL's specific primary-source collections on the history of science and technology, with at least some of their content available online. Note the other tab that links to digital collections available from other institutions.