1854 - 1861
Modern eyeglasses should be replaced as quickly as possible. When in formation, drilling, or serving on the piece in the view of spectators, if you have modern eyeglasses on, you may be asked to remove them. There are several sutlers who specialize in making period eyewear and duplicating your lens prescription, and you may find one or more optical stores in your area that either make reproduction period eyeglasses or will sell you period frames for which they can make lenses for your prescription. If you have a pair of frames that are period, you can take them to almost any optical store and have lenses made for them.
Modern eyeglasses will temporarily - TEMPORARILY - pass for period eyeglasses so long as they meet certain general guidelines. Gold or silver rims are acceptable so long as the frames and lenses are oval or round; round frames were relatively rare, but they certainly were around. Black rims, if they're metal, are acceptable; many period eyeglass frames were made of iron.
Be careful in your choice of frames, as the temple pieces may be decorative in a way impossible or never used by optometrists of that day. By the same token, avoid frames with bridge pieces that are of an irregular or complex shape, as that is not accurate for the period. Plastic-covered rests on the bridge piece cannot be avoided in modern glasses, so look yours over to be sure that the pads are relatively small and reasonably inconspicuous.
You can often get period frames for less than $40 on e-bay on the Internet, at flea markets, or at some optical stores, or you can purchase reproduction frames from some sutlers for less than $20 or $30. Others will make you reproduction frames that are fantastic, and the price could range from $40 on up for the frames alone. Whatever course you take, examine the frames carefully to be sure that they are in reasonably good shape. It is always advisable to place a drop or two of clear fingernail polish on the heads of the screws which hold the temple pieces to the frames in order to keep the screws from backing out, and this should be repeated every three or four months.
Keep in mind that older metal frames tend to have extremely high copper or nickel content, so folks with metal allergies may break out within minutes or hours of putting on original period glasses. You can combat this by coating the metal with a good quality brass coat / sealer, or even clear fingernail polish after you thoroughly clean the frames and temple pieces. The drawback to using fingernail polish is that, as the metal corrodes, it will tend to cause the fingernail polish to flake and break up from within, sometimes causing the polish to peel off in strips. Using clear fingernail enamel (versus polish) should give better and longer service, but it will also eventually come off because of the corrosion.