I stopped by one of my favorite gift shops today (which is an old converted mill) that specializes in antique items, namely currency and military artifacts from 18th and 19th century America. As I was perusing their current collection I saw a pair of "sunglasses" labeled as such, but underneath they were identified as Civil War sharpshooter glasses. There was about a dime sized area in the middle that looked kind of like a bifocal would (with an obvious lense change from the perimeter of the glasses). Can someone crack an egg of knowledge on me about what these are, who used them/how widespread their use was, how they came about, what they're made of/how they made them, and how effective they were at targeting and shooting over distance vs. a naked eye?
Your antique store is mislabeling. Based on your description you seem to mean the "sunglasses" (usually orange, but it could be grey or blue) with a center that is transparent in order to aid in shooting.
There were no Civil War "sharpshooter glasses" of that nature even though they show up (as you discovered) in various collections labeled that way; there are no primary source references to them and the only examples with confirmed dates being sold post-date the war.
We do have a few references that are early enough but they're clearly of prototype nature and/or with different designs. From example, Max Hilb in 1851 (an optician in Philadelophia) mentions some "shooting spectacles" that are "to be had by inventor only"; that is, just one prototype model, without details as to its working.
A 1861 patent for "Shooting Spectacles" by J. Broham of England mentions
...a mode of adapting to spectacles additional glasses, which are capable of being brought over the eyes or removed therefrom with facility, without removing the spectacles from the head.
with separate "eye discs" that are attached via a swivel mount. We have no evidence of this being used during the actual Civil War.
The first verified "sunglasses target" style glasses as you're thinking of are from a catalog in the 1870s; all known models date to those years or later. We don't have any evidence who actually invented this style. While there was plenty of use of glasses during the Civil War (General Meade famously had spectacles and he was apparently once called a "four-eyed son of a bitch") including colored ones (usually green, blue, or neutral smoky -- there was frankly unorganized argument over which was better) there is little doubt that any "sharpshooter glasses" you might find in Civil War antiques come from the 1870s or later (especially if the color is orange, which didn't appear until that decade).
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McBrayer, A. & Valenza, T. (2012). Spectacle Styles Before, During and After The Civil War, 1835-1870. Newsletter of the Ophthalmic Antiques International Collectors Club.