MLA
Russell, George William, 1867-1935. Letter from George Russell (AE) (Dublin) to Augusta Gregory - April 28, 1899. April 28, 1899. Gregory Family Papers, 1770-1956. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/805fbg79t4-cor.
APA, 6th edition
Russell, George William, 1867-1935. (April 28, 1899)[Letter from George Russell (AE) (Dublin) to Augusta Gregory - April 28, 1899]. Gregory Family Papers, 1770-1956, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/805fbg79t4-cor.
Chicago
Russell, George William, 1867-1935, Letter from George Russell (AE) (Dublin) to Augusta Gregory - April 28, 1899, April 28, 1899, Gregory Family Papers, 1770-1956, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/805fbg79t4-cor.