More on John Woodburn Davies:
http://www.lva.virginia.gov/public/dvb/bio.asp?b=Davies_John_Woodburn
The article says that "In 1864 Davies photographed Robert Edward Lee, about the same time he published 'The Lee Schottisch.' His single bust-length image of Lee, showing a weary general with heavy bags under his eyes, a closely cropped beard, loose-fitting shirt collar, and sloppy, drooping tie, proved wildly popular." Not sure if this is a description of the OP photo or another one. Maybe there are competing claims as to who photographed it?