Slave pen, Alexandria, Virginia / Negative by Brady & Co., Washington.

Stereograph shows the interior of a military prison with a Union soldier standing guard; prior to its capture by the Union Army in 1861, the facility had been used by Price, Birch & Co. slave dealership to hold slaves before they auctioned. (Source: The Center for Civil War Photography, http://www.civilwarphotography.org/index.php/3-d-photographs-exhibit/70)

Corresponding glass negatives: LC-B811-2299B, left, and LC-B811-2299A, right. E. & H.T. Anthony & Co. acquired the negatives from the studio of Mathew Brady in exchange for photographic supplies.
Purchase; Russell Norton; 2012; (DLC/PP-2012: 069).