Patrick,
"Civil War era" can be a pretty broad term, when applied to optical instruments. The simple (non-prismatic) Galilean "field glass" (or marine glass) changed little between, say, the 1850s and the WW II period. Sounds as if what you have is what is often called an "opera glass," with magnification usually on the order of just two or three times. These were used, much as the modern monoculars you may have seen, of comparable magnification. In neither case were they intended for long range use or fine-grained magnification. They were, however, used during the Civil War for a "quick look." There are some characteristics of the war-time period one can look for -- usually the bridge between the two "tubes" is straight, rather than curved for the nose, or adjustable in the center for eye-spacing. I'd be happy to take a look if you'd like to e-mail me, (dwgaddy at verizon dot net), and I can suggest other links for info.