What an awesome photo from the Civil War era. You’re looking up what is Independence Ave. today.
View of the Smithsonian Institution Building looking east up B Street (Independence Ave) towards the U.S. Capitol. Grounds are landscaped following a plan laid out by Andrew Jackson Downing, with the Castle nestled among trees. Visible amidst the trees is the Magnetic Observatory, built in 1853. People stand along the fence installed along the sidewalk on B Street In this image, the central roof of the Castle is intact and the small towers have the caps on them. In January of 1865, a fire destroyed the central roof and the caps on the towers, so this photograph had to have been taken prior to January 1865 and is most likely April 1863. The Capitol dome, seen in the distance, is not yet completed
Source: Smithsonian Archives