On December 6, 1884, workers placed the 3,300 pound marble capstone on the Washington Monument, and topped it with a nine-inch pyramid of cast aluminum, completing construction of the 555-foot Egyptian obelisk.
The Library of Congress has more. That’s their photo (click to view larger version). NewMexiKen doesn’t even like being up that high on the inside.
[Update: Looking at that photo, I’m wondering where was the photographer standing? The photo is not from the completion in 1884.]