MLA

Ritchie, Alexander Hay, 1822-1895, Sigourney, L. H. (Lydia Howard), 1791-1865. Letters to my pupils : with narrative and biographical sketches / by Mrs. L.H. Sigourney.. 1853. https://collections.library.yale.edu/catalog/10521847.

APA, 6th edition

Ritchie, Alexander Hay, 1822-1895, & Sigourney, L. H. (Lydia Howard), 1791-1865. (1853)[Letters to my pupils : with narrative and biographical sketches / by Mrs. L.H. Sigourney.]. https://collections.library.yale.edu/catalog/10521847.