A recent addition to Ron Coddington's outstanding Flickr site.
Capt. de la Mesa, a native of Spain, was wounded late on the second day at Gettysburg, as he led his Company C of New York's "Garibaldi Guards," in recapturing Federal artillery lost to Mississippi troops near the Trostle House. He was discharged as disabled in September, but spent the remainder of the war in the VRC.
The New York Military Museum website has an excellent account of his service, as well as translations of some of his letters.