[President Abraham Lincoln, Major General John A. McClernand (right), and E. J. Allen (Allan Pinkerton, left), Chief of the Secret Service of the United States, at Secret Service Department, Headquarters Army of the Potomac, near Antietam, Maryland]

Alexander Gardner · October 4, 1862 · Albumen silver print from glass negative

From the collection of The Metropolitan Museum of Art. Free to download, adapt and use — no permission needed.