Library of Congress Civil War Photographs

The Library of Congress posted a series of Civil War photographs on Flickr, many of them of unidentified soldiers. I skimmed through them wondering if I could identify any of them.

I came across this one:

and wondered if it might be possible that this was my Bagley ancestor; these are the pictures I've got:

This one:

also seemed like a possible match, although maybe less likely.

Have I identified an unknown soldier in the Library of Congress photo collection?

There must be people who are well-versed in -- what, forensic photograpy? -- who know what to look for as far as ruling out a possible match. But how would one ever know for sure?