I know this is going to sound crazy, but six years ago I saw that same photograph and sort of "fell in love" with the man holding the cards. I then set out to learn more about his life and ended up finding out a lot through all these random coincidences. He is in several more photographs online and there was a book written about the experience at Libby prison when he was there. A sketch of him playing cards with the writer of the book is on the cover if you can get ahold of a copy.
He is in picture number 4, 10, 11, 12, and 13 here:
http://www.loc.gov/pictures/collection/ ... 114th&sp=1
Here's the book on Libby Prison:
Libby life: experience of a prisoner of war in Richmond, Va. 1863-64 / by Lieut. Colonel F.F. Cavada, u.s.v
If you go to this link and enter "70" in the page field or just page to 71 in the book you will see a sketch of Cavada and Harry playing cards. You can tell it is him because of his face and also because the trunk he's sitting on has his name on it "H.E.Rulon".
Good luck and I would love to hear any more stories you have about him.
Oh! And you probably already found this but there is a picture of him, very young, shortly after enlisting in the book Collis' Zouaves.
Take care!
Liz
