Officers of Battery C, 3rd U.S. Artillery. L-R: Meinell, Gibson, Pendleton, and Fuller. (names from Wikipedia)
The Horse Artillery Brigade of the Army of the Potomac was a brigade of various batteries of horse artillery during the American Civil War.
From Library of Congress:
Title: Fair Oaks, Va., vicinity. Capt. Horatio G. Gibson (second from left) and officers of his battery
June 1862
Photographed by James F. Gibson
Photograph from the main eastern theater of war, the Peninsular Campaign
Civil War glass negative collection. Library of Congress Prints and Photographs Division Washington, D.C. 20540 USA
Record page for this image: http://www.loc.gov/pictures/item/cwp2003000081/PP/
