MLA

Gibbs, J. Willard (Josiah Willard), 1839-1903. Lorentz, Hendrik Antoon (1853-1928) - Autograph postcard, signed, and autograph letter, signed, to Josiah Willard Gibbs. Leiden.. Box 3, folder 87. 1884 April 8, 1902 April 1. https://collections.library.yale.edu/catalog/2020782.

APA, 6th edition

Gibbs, J. Willard (Josiah Willard), 1839-1903. (["Box 3, folder 87"])(1884 April 8, 1902 April 1)[Lorentz, Hendrik Antoon (1853-1928) - Autograph postcard, signed, and autograph letter, signed, to Josiah Willard Gibbs. Leiden.]. https://collections.library.yale.edu/catalog/2020782.